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คำถามของคุณ เรย์ เรื่อง การสูญเสียในสายนำสัญญาณและวิธีการคำ นวน

คำถามของคุณเรย์ เรือง การสูญเสียในสายนำสัญญาณและวิธีการคำนวน

Ray, KA8SYX, asks:
If a piece of coaxial cable has a specified loss figure in dB per 100 feet at a given frequency, does that mean that the loss in a different length of the same cable that is a fraction of 100 feet long is the same fraction of loss? For example, I have a type of coax that has a loss of 6 dB per 100 feet at 150 MHz. I have a 15 foot length I want to use as a feed line for my 2 meter mobile SSB transceiver. Does that mean that my feed line would have a loss of about 0.9 dB (6 × 15 / 100), not including SWR and connector insertion losses? I am particularly interested in the loss in received signal. Is my math correct, or is there a different method to determine the amount of signal lost in a coaxial cable when the length is different from that for which the published loss figures are expressed?

คำตอบ วิธีการคำนวนที่คุณกล่าวมาถูกต้องแล้วครับ แต่บางครั้งเราอาจจะต้องคำนึงถึงตัวแปรบางอย่าง

• ข้อมูลเรื่องการสูญเสียภายในสายจะบอกไว้ในกรณีที่สายนำสัญญาณเป็นสายใหม่ ถ้าคุณใช้ภายในบ้าน มันก็คงสภาพเหมือนของใหม่ได้หลายปี แต่ถ้าน้ำ หรือความชื้นเข้าไป ก่อให้เกิดการกัดกร่อน ทำให้สายนำสัญญาณลดคุณภาพลง อัตราการสูญเสียก็มากขึ้น
• คุณพูดถูกครับ การสูญเสียในสายนำสัญญาณจะเพิ่มขึ้นถ้าค่า SWR มากกว่า 1:1 และในกรณีของการรับสัญญาณ เราหาค่า SWR ได้จาก input impedance ของเครื่องรับวิทยุไม่ใช่ impedanceของสายอากาศ

บางครั้งเครื่องรับวิทยุถูกออกแบบมาเพื่อให้มีสัญญาณรบกวนน้อยที่สุด อาจจะไม่ได้มีค่า input impedance 50 Ω เสมอไป ลองศึกษาได้จากคู่มือของเครื่องรับรุ่นนั้น ๆ

Your calculations are right on. That’s all there is to it. But do keep in mind just a few potential pitfalls:

• Published cable loss data is for new cable. If used indoors in a nonhostile environment, it will stay close to new for many years. If the jacket allows moisture, or moisture vapor, to penetrate, it can degrade from subsequent corrosion. I have been amazed to find that the copper in some old cables that have been used outdoors has turned black from corrosion.
• You are correct that the loss increases with an SWR higher than 1:1. For your receive case, keep in mind that the SWR is determined by the input impedance of the receiver — not the antenna impedance.

Sometimes receivers aligned for minimum noise figure do not have an impedance of 50 Ω. Check your receiver specs.

เมื่อต่อระบบดังรูปทำไมเวลาปรับ ATU แล้วอ่านค่า SWR จากภายในตัววิทยุ และ SWR ภายนอก ได้ค่าไม่เท่ากัน

คำถามจาก Mark เมื่อต่อระบบดังรูปทำไมเวลาปรับ ATU แล้วอ่านค่า SWR จากภายในตัววิทยุ และ SWR ภายนอก ได้ค่าไม่เท่ากัน

Mark, KG4UDL, asks: I run my 5 W HF transceiver through an external antenna tuner and a separate SWR and power meter as shown in Figure 3. The transceiver also has an internal SWR meter. Why is it that I can adjust the tuner for minimum SWR on the radio’s internal meter and read an SWR of 3:1 or more on the external meter? Also, when I obtain this reading I am showing a full 5 W output. I thought that high SWR and high power to the antenna were mutually exclusive — that high SWR indicates low antenna efficiency. I have had great success with the system, by the way, including multiple European contacts during the recent ARRL International DX Contest.

ตอบ To answer your more general question — SWR is just a reflection (pun intended) of the impedance match at the interfaces between the two systems where your measuring device is located. It really has nothing to do with antenna efficiency, and an antenna with a high SWR can radiate very well as long as you can couple power into it and you have low loss in the transmission line.

Many transceivers fold back or reduce power if the SWR is higher than a certain level, often 2:1. This is done to prevent excessive voltages or currents from damaging transmitter components. This is often a major issue — so even if the antenna could work fine, if the transmitter reduces power, your signal will be weaker.

The concept of reflected power is a mathematical way of considering the standing waves on a transmission line. If you have a 1:1 SWR and put 5 W into the line, you will measure 5 W forward power and 0 W reflected power. If you have a 3:1 SWR (in the usual 50 Ω system, that could be a 16.6 or 150 Ω resistive load, or an infinite combination of reactive loads, for example), you will have a reflected power of 25% of your forward power. If your transmitter could put out a full output into a 3:1 SWR, you would see a forward power of 6.67 W and a reflected power (at 25%) of 1.67 W. The net actual power going to your antenna would be the difference between the forward and reflected, 6.67 – 1.67 = 5 W.

Your wattmeter is in the right spot to read the same as the radio’s SWR meter — unless the coax between the two is not of 50 Ω (75 Ω RG-6 or RG-59 are two examples). I have found that many patch cables purchased at hamfests, for example, are not really 50 Ω. If other than 50 Ω, there will be an impedance transformation in them so that the impedance at each end (and thus the SWR) will be different.

Note that this really doesn’t cause any harm or significant loss — it just confuses the measurement process. The difference will be a function of electrical length and would be most significant with a cable that is a quarter wave long, around 6 feet on 10 meters, for example. If so if the load at one end is 50 Ω, the other end would see 100 Ω and vice versa. If this were the situation between your external meter and your radio, a 1:1 SWR at the external meter would result in a 2:1 reading at your radio.

One way to find out what’s happening is to put a good 50 Ω dummy load right at the transmitter and see what SWR it reads. Then move it to the end of the patch cord and repeat. If the cable is 50 Ω, the reading should be the same.

Another possibility is that the SWR/wattmeter is providing an erroneous reading. This is not uncommon, although most show 1:1 when matched. The readings away from being matched are not always as accurate. See any product review on wattmeters in QST. There was one in March 2009, for example.

If in doubt, tune with the meter in the radio. The SWR, as measured by that meter, is what determines the amount that the transmitter will “fold back” and reduce power. Thus, that’s the important place — if the transmitter ain’t happy, ain’t nobody happy! to borrow a phrase.

ทำไมความถี่ 14 MHz และสูงกว่าถึงใช้ระบบ SSB?

คำถาม :: จาก Robert ในกิจการวิทยุสมัครเล่นเวลาใช้ระบบเสียงพูดทำไมความถี่ 14 MHz และสูงกว่าถึงใช้ระบบ SSB แล้วความถี่ต่ำกว่าละทำไมถึงใช้ LSB ยกเว้นความถี่ 5 MHz นะที่ใช้ (USB)

Robert, KB5QN, asks: In amateur single sideband (SSB) voice operation, why is the upper sideband (USB) used on 20 meters and higher frequency bands while lower sideband (LSB) is used on the lower bands (except the 60 meter channels)?

ตอบครับ :: This goes back to the early days of SSB. At that time, the early 1950s, there was no 40 meter phone band (it was opened to voice on February 20, 1953), so the majority of SSB activity was on 75 and 20 meters. One common design configuration used an SSB generator that produced an upper sideband SSB signal at 9 MHz using a filter or phasing SSB generator. The 9 MHz signal was then heterodyned with a VFO covering 5 to 5.5 MHz, often made from a (then) $5 WWII surplus ARC-5 transmitter.

The additive (9 + 5 = 14, 9 + 5.5 = 14.5) translation to 20 meters maintained the upper sideband. The subtractive translation (9 – 5 = 4, 9 – 5.5 = 3.5) reversed the frequency relations (and the VFO tuning direction) to result in LSB. By just using those sidebands, they did not need to buy a second carrier oscillator crystal and worry about sideband filter symmetry, or put switching into their phasing rigs. It could have just as easily gone the other way, I guess.

It just went on from there. 40 meters went LSB and the upper bands went USB. The US military appears to have settled on USB on all HF frequencies, so the “green radio” guys with SSB-capable military surplus gear use USB, especially on 40 meters. Compatibility with the government protocol explains why we are required to use USB on the five 60 meter channels that we share with government users. Other than that requirement, there is no regulation specifying which sideband be used on any band.

ชั้นไอโอโนสเฟียร์ถูกค้นพบได้อย่างไร How the Ionosphere Was Discovered

Today we take it for granted, but it took the efforts of many to figure out what was happening. บทความโดย Robert H. Welsh, N3RW ใน QST ฉบับ December 2009

For decades, QST readers have studied articles describing the best methods for getting their signal from point A to point B. Past articles have taught us how to build or improve receivers, transmitters, filters and antennas (my favorite). From the 160 meter band through the HF spectrum to VHF, we are subject to one part of radio communications beyond our control. That part is the Earth’s ionosphere. Its discovery, aided in part by radio amateurs working with government and university scientists, makes an interesting story.

I suspect that most readers of QST recognize Guglielmo Marconi as one of the earliest radio “amateurs.” Marconi’s efforts using radio to cover long distances over water is well documented. There is still a question as to whether in 1901, he actually received the Morse code letter S transmitted from Poldu in Cornwall, England to St John’s, Newfoundland; nevertheless, it is accepted that his efforts led to radio as a viable means of long-distance communications.Keep in mind that prior to radio as a communications tool, wire telegraphy was the primary tool for long distance communications. Wires cannot easily span the surface of the world’s oceans, but radio waves can.

Laying the Ground Work

The physics community of the early 20th century investigated this new phenomenon.
Several theories were proposed by prominent physicists to explain how radio signals propagated over long distances. One of the several theories proposed was that the Earth’s upper atmosphere acted as some sort of a reflector. Another theory took into
account the optical phenomenon of superposition of waves. Another wave phenomenon was suggested — radio waves were diffracted just as light waves were diffracted, or bent, around an obstruction. Diffraction could not explain how these waves bent around the smooth curvature of the Earth.

Even the great Marconi attempted to explain how radio waves traveled. He observed the difference between day and night propagation of radio waves. His explanation was based on the presence or absence of sunlight; that is, during the day, the Sun’s rays falling on the antenna acted as a shield. Not a bad explanation for those amateurs active on 160 and 80 meters during daylight hours.

ในระหว่างที่มาร์โคนี่ทำการทดลองเรื่องการเดินทางของคลื่นวิทยุ เขาสังเกตเห็นความแตกต่างของการแพร่กระจายคลื่น ระหว่างกลางวันและกลางคืน การสังเกตของเขาใช้การมีและการไม่มีของแสงแดด เขาอธิบายว่าตอนกลางวันจะมีแสงแดด ซึ่งจะตกกระทบไปที่สายอากาศ มันทำหน้าที่ห่อหุ้มสายอากาศเอาไว้ เป็นการอธิบายที่ไม่เลวสำหรับความถี่ต่ำอย่างย่าน 160 และ 80 มิเตอร์ (1.8 MHz และ 3.5 MHz) ในเวลากลางวัน

According to the scientific method, a theory must be tested to confirm its validity. One of the equations developed to support a scientific theory proposed to show that as the wavelength decreased, the radio waves traveled over longer distances. This was the Austin-Cohen equation. Given this hypothesis, the shorter wavelengths were considered useless for long-distance radio communications and were relegated to those folks called radio amateurs. How fortuitous for us! The professional scientific community slowly began to take notice that radio amateurs using shorter wavelengths were having great success in communicating over long distances.




Figure 1 — Early pioneers in ionospheric research. Dr Merle Tuve and Dr Gregory Breit (at left) at the pulsed echo sounder for ionospheric research located at the Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC. The photo is dated February 14, 1927.

รูปที่ 1 ช่วงแรกในการทดลอง ค้นคว้าเกี่ยวกับชั้นไอโอโนสเฟียร์ โดยดอกเตอร์ทั้งสองส่งสัญญาณเป็นจังหวะสั้น ๆ ไปยังชั้นไอโอโนสเฟียร์


An Answer Comes into View

In 1902, two scientists independently suggested that radio waves were bent by a conducting layer in the Earth’s atmosphere. Those scientists were Arthur Edwin Kennelly in the US and Oliver Heaviside in England. The reflecting layer that we
now call the ionosphere became known as the Kennelly-Heaviside layer. It took two decades of experimenting before this hypothesis was proven to be the correct explanation for longdistance high frequency radio communications. As stated in the previous paragraph, the Kennelly-Heaviside layer hypothesis required experimental testing before it could be accepted.


Lee De Forest is well-known as the inventor of the triode vacuum tube. By inserting a third element between the cathode and the anode, this third element (the control grid) is able to control the flow of charge from cathode to anode, thus producing amplification. Not as well-known is De Forest’s work in radio propagation. During the period 1912 to 1914, De Forest and Dr Leonard F. Miller of the Federal Telegraph Company made the first crude measurements of the Kennelly-Heaviside
layer height using a spark transmitter. The spark transmitter delivered 1200 kW (that’s 1.2 MW) to an antenna at currents approaching 750 A. The wavelength of this transmitter was 3260 meters (a frequency of 92 kHz). Signals from this transmitter located in Los Angeles were received 350 miles north in San Francisco and 300 miles east in Phoenix. De Forest published the results of these tests in the journal London Electrician in 1912.

De Forest hypothesized that the main wave was returned by a reflecting layer whose heights were 17, 27 and 37 miles above Earth.De Forest makes an interesting comment regarding his experiments: “I know nothing about what goes on up above, and that attempts at exact explanation are silly.” Perhaps not the best attitude for an experimenter.

Development Continues

During the period of the 1920s, radio grew exponentially through increased commercial broadcasting and the use of shorter wavelengths for long distance communications. During that same period, there was an increased interest in the physics of subatomic particles. Much of the particle physics research was based on an increased understanding of the nature of matter. By the 1930s, this led to that branch
of physics known as quantum mechanics. Pulse techniques were developed to accelerate charged particles. The offshoot of this research for radio physics was the
introduction of high-power pulse generators. Hams think of these as CW transmitters. The two scientists recognized as the discoverers of the pulse technique for confirming the existence of the ionosphere were Merle Tuve and Gregory Breit.]

Tuve was an active amateur during his undergraduate days at the University of Minnesota. He spent considerable time operating the club station, 9NB. It is interesting to note that one of Tuve’s friends in Amateur Radio was Ernest Orlando Lawrence, for whom the Lawrence- Berkeley Laboratory at the University of California is named.

Lawrence eventually went into the field of nuclear physics. He used the accelerated charged-particle concept to construct the atom-smashing machine known as a cyclowas tron, for which Lawrence earned the Nobel Prize in Physics. Tuve received his undergraduate degree in electrical engineering.

Some time after graduation, Tuve received a letter from Lawrence wherein Lawrence suggested that the two of them start a business selling and installing radios.It is important to note here how young Amateur Radio operators set the stage for great work later in life within the physics community.

Two years later, Tuve earned a master’sdegree in physics. Following graduation, he cyclowas employed by the Carnegie Institution of Washington in a program to measure the height of the radio conducting layer.

Tuve worked under Gregory Breit, who held the position of Mathematical Physicist in the Department of Terrestrial Magnetism at the Carnegie Institution. This experiment would lead to Tuve’s doctoral dissertation. Breit began ionospheric measurements in 1924. He assumed a layer height of 62 miles; the same height suggested in a 1913 publication by De Forest. In addition, Breit was aware of propagation experiments by Albert Hoyt Taylor of the Naval Research Laboratories.

The ARRL Joins In

Albert Hoyt Taylor and his colleagues at NRL, in cooperation
with John Reinartz (1XAM and 1QP) of the ARRL and other radio amateurs, had discovered that high frequency radio waves could be transmitted to a distant receiver while being imperceptible at many points in between. Taylor labeled the gaps skip distances and conducted an in-depth investigation to determine their characteristics. He published detailed experimental measurements of skip distances in early 1925, including with them estimates of the height of the conducting layer previously suggested by Kennelly and Heaviside.

By 1925, Breit and Tuve designed an experiment to measure the height of the conducting layer by transmitting pulsed signals, then receiving the echoes and measuring the time lapse from transmitted pulse to received echo from both the sky wave and the ground wave. Considering the speed of radio waves is the same as the speed of light — that is, constant at 3 × 108 meters per second — Breit and Tuve used basic physics to calculate the distance traveled by the pulse. During a meeting in Washington, DC during November 1924, Breit and Tuve met with several leading radio experts. A plan was conceived in which several powerful radio transmitters would provide the source signal and the received signal would be at the Carnegie Institution. The transmitters were station KDKA in Pittsburgh, Pennsylvania, the National Bureau of Standards station WWV, the Naval Research Laboratories station NKF and coastal station WSC operated by the Radio Corporation of America in New Jersey. (See Figure 1.)



Figure 2 — Ray-path drawing representation of that from Tuve and Breit’s March 1925 article. LL is the postulated refl ecting layer. Ant 1 is the transmitting antenna location. Ant 2 is the receiving antenna location. The distance along the surface is L and the height of the layer is H.

The best results were received from the NRL transmitter, one of the earliest crystal controlled transmitters in use. Breit and Tuve used the new technique of oscillograph recording to analyze the received pulses. On July 28, 1925, they received the first conclusive results of ionospheric reflection at a frequency of 4.2 MHz from a 10 kW transmitter sending 200 μs pulses.

One of the most interesting aspects of their results was that the height of the reflecting layer varied from day to night. Their calculations indicated that the height ranged from 55 miles (88 km) during the day but rose to 130 miles (208 km) at night. They were not yet aware that the layers varied as a result of solar ultraviolet and X-ray emissions, which did not excite the atmosphere when the Sun was not visible. Today, hams recognize that the ionosphere behaves as if it were in several distinct layers at different heights. These are referred to as the D layer at a height of about 30 miles (50 km), the E layer at about 60 miles (100 km) and the F1 and F2 at heights ranging from about w180 miles (300 km)



Figure 3 — Illustration of the operation of a Digisonde that has the capability of portable operation. The system can measure seven observable parameters of refl ected or refracted signals as described in the text.


The method used by Breit and Tuve
to directly measure the ionosphere’s height
was to:

• Use directional loop antennas for receiving.
• Record the received pulses from both the sky-wave signal and ground-wave signal.
• Apply the time difference between the sky-wave signal and the ground-wave signal.
• From the difference, use their derived equation to measure the layer height.
  

The derived equation was a slight modification of the Euclidean geometry statement about right triangles known to legions of high school students as the Pythagorean theorem. (See Figure 2.) From this equation and comparing the time for the sky wave to reach the receiver compared to the time for the ground wave to reach the receiver, Breit and Tuve calculated the height of the reflecting layer for 4.2 MHz transmission.

From Tuve and Breit’s 1925 paper, the equation suggested that the time of arrival of the sky wave is given by Tsky wave = (2×H/C)(1 + (L/{2×H}))1/2 where Tsky wave is the arrival time, C is the speed of light, H is the height and L is the length over the surface of the Earth. Whereas the time of arrival of the ground wave is given by:

Tground wave = L/C.

From the difference in the two arrival times, the experimenters could arrive at a virtual height for the reflecting layer. Their results obtained from recording several different transmitting stations suggested a reflecting layer height of about 80 miles (128 km).

From their initial experiments in the 1920s, sounding the Earth’s ionosphere has developed into a remarkable analytical tool. Prior to the WWII, there were few ionospheric sounders in operation — those in England; Washington, DC; Peru; Australia, and the Soviet Union. During the war, the number increased to about 50 stations. By the time of the International Geophysical Year (1957-58), the numbers had increased to about 150.

The techniques used today are a direct offshoot of Breit and Tuve’s 1925 experiments. One of the most common systems is a vertical incidence sounding, referred to as an ionosonde. The ionosonde is basically a pulsed radar operating at a frequency range of about 1 MHz to 40 MHz. The measurement is based on the equation:

H = 0.5 × C × T

where C = the speed of light, T = is
the travel time of the pulse, and H is the layer height (also known as the virtual height).

An example of current ionospheric studies comes from the University of Massachusetts Lowell Center for Atmospheric Research. They developed a low power (300 W) ionosonde named Digisonde that has the capability of portable operation.

The system has the following capabilities: the simultaneous measurement (see Figure 3) of seven observable parameters of reflected, or in oblique incidence, refracted signals received from the ionosphere including frequency, range (or height for vertical incidence measurements), amplitude, phase, Doppler shift and spread, angle of arrival and wave polarization.

This is just one example of the on going research to understand the Earth’s ionosphere. As amateurs who use the HF and VHF regions of the electromagnetic spectrum, we owe much to past and present experimenters. They provide us with an understanding of how our signals propagate around our planet. As either a DXer or ragchewer, we rely on this information to better communicate via radio.

I would like to thank Dr Nelson Klein, Professor of Physics at Bucks County Community College for his valuable discussions while I researched this article.

Homeowners Insurance and Your Antenna System

Advice from an agent on financial backup for your station.

บทความโดย Ray Fallen, ND8L

Twenty two years ago, I was just like you. I’d buy an insurance policy to protect my home, my car, my life or my health. While the agent tried to explain it, my eyes would glaze over and my breathing would slow. I would nod my head knowingly, makeall of the appropriate “Why Yes, Mr Insurance Agent, your amazing explanation just couldn’t be clearer” noises, while in my head a voice was whispering the Novena of the Newly Insured: “If something happens, I sure hope it’s covered, for I have not one single clue what this guy is trying to tell me.”

Twenty one years ago, on February 1, 1988, I went over to the Dark Side. I became an insurance agent. This article shares some of what I’ve learned about homeowners insurance and your antenna system.

Caveats and Stuff

Like everything from bulldozers to your morning oatmeal this article comes with some disclaimers:

• Most homeowners insurance policies written in the US are based on standard language provided by the Insurance Services Office. Each company modifies that language to comply with individual State Insurance Statutes so coverage that applies to towers in Ohio, may not necessarily apply to towers in Florida.
• Coverage varies from company to company, state to state and country to country.
• Because of these variations, this article can’t tell you exactly how your antenna installation will be covered. It can, however, give you some talking points to follow when you visit with your insurance agent.

In the wake of Hurricane Ike, W5JON’s antennas looked like this. Can you fi nd the SteppIR in there?

And you are going to do that real soon, aren’t you?

An insurance policy is a legal contract between you (the insur-ed) and the company (the insur-er). The contract is quite specific in its definitions, coverage and the duties and responsibilities of both parties. For example:

• What’s covered in the policy is in the policy contract.
• What’s not covered in the policy is in the policy contract.
• Your duties following a loss are in the policy contract.
• How the loss will be paid is, you guessed it, in the policy contract.
  

If you’re starting to see a pattern here, go to the head of the class.

Here’s the problem: I said the policy was specific — I didn’t say it was always easily understood. Insurance policies are legal documents written in an obscure Old English dialect called Lawyer. You need to visit your agent, who can translate Lawyer into English. If you’re planning an installation, see your agent before you start. If your tower’s already up — what are you reading this for? Go see your insurance agent right now!

Property Coverage

Damage to towers, rotators, towermounted antennas and related cabling is covered by the Dwelling Extensions, Other Structures or Appurtenant Structures coverage listed on the Declaration Pages of your Homeowners Policy. (The specific name and amount of the coverage varies by company.) For example, State Farm’s Ohio Insurance Homeowners Policy defines Dwelling Extensions as “other structures on the residence premises, separated from the dwelling by clear space.” Hmmm, it’s starting to sound like Lawyer, again.

Dwelling extensions are permanently attached to or otherwise form a part of your property, but they are not physically attached to your home (dwelling). Dwelling Extensions are typically covered for 10% of the dwelling coverage amount. For us mere mortals, this may be more than adequate. At K3LR, K8AZ, W3LPL, KC1XX and other serious contest stations it may be a different story.

Remember, tower replacement costs should include professional help in removal,
repair or replacement, in addition to the replacement cost of the damaged gear. Also, consider other dwelling extension items: fences, sheds, pole barns, gazebos, detached garages, flagpoles and in-ground swimming pools, for example. Add up the replacement cost of those structures and if you need more coverage, buy it — now.

Wire antennas in trees and/or ground mounted verticals are generally not permanently affixed to your property (sometimes much less permanently than we’d like) and are considered personal property. The good news is personal property is covered for 50% to 75% of the dwelling amount; the bad news is the coverage is less broad than dwelling extensions, but wind, ice, vandalism and lightning damage are usually covered.

The distinction between dwelling (and dwelling extensions) and personal property is: If something’s “nailed down” and not easily moved, it’s a dwelling or dwelling extension.

If it’s easily moved (even if it’s “nailed down”), it’s personal property.

A Cushcraft R-7 vertical mounted on a piece of pipe pounded in the ground is personal property. Eighty feet of guyed Rohn 55G in 15 cubic yards of reinforced concrete is a dwelling extension. No claims adjuster will ever confuse the two.

One other thing, if you have towers and equipment on property you own or rent away from your primary residence make sure your agent knows. Hams who rent houses or apartments also need coverage. Some renter policies provide personal property coverage only, while others provide 10% of the contents coverage amount for dwelling extensions, like a homeowners policy. Again, see your agent for clarification. If you don’t have renter’s insurance, get some, especially if you have a tower, if for no other property, his insurance company may try to recover the monetary damage from you.

The good news is there’s some protection in your homeowners policy, typically $100,000 to $300,000 of personal liability coverage. The insurance company will hire an attorney to defend you and write a check for damages up to the policy limits.

The bad news is when an attorney parades a teenager in a wheelchair in front of a jury policy limits of $100,000 to $300,000 are just a down payment. (Your argument that this kid was trespassing will probably fall on the jury’s deaf ears.) You are responsible for the rest of the damages — if you’re collectible. You’re collectible if you have wages or other income, equity in your home or business, bank accounts, investments, retirement plans or any other assets that can be attached by court judgment. If you can afford to lose these things — fine. If you can’t, you need to purchase a Personal Liability Umbrella Policy (PLUP) with coverage of at least $1,000,000 per occurrence. It doesn’t cost a lot, typically less than a dollar a day, and will provide great peace of mind.

What’s the bottom line on justifying a PLUP purchase? If you’re sued for everything you earn or own, now and in the future and you lose who do you want to write that check, you or your insurance company? I thought so. Now, go back and read this section again. It’s very, very important.



Hurricane Ike sent this Comet GP3 fi berglass vertical right through the eave.

Filing an Insurance Claim (Honey, Did Your Tower Always Look That Way?)

It might have been a tornado or other windstorm, lightning strike, ice or vandalism but your tower and antennas are damaged. Aren’t we feeling better that we discussed things with our agent before a bad thing happened? It’s time to get busy.

Your primary duties after a loss are to protect your property and the property of others from further damage or loss, then make reasonable and necessary temporary repairs. You obviously want to prevent additional damage, but your insurance company doesn’t expect you to kill yourself doing so. You might want to take some pictures.

Notify your agent of the loss as soon as possible. Many agents and companies have 24/7 claims service, so if the loss happened on Sunday morning, make the call. Follow up with a short letter, fax or e-mail to your agent, to make sure the claim was filed correctly, requesting the claim number and the claim representative’s (adjuster) name and phone number.

After filing the claim, your agent’s involvement will probably be minimal, unless there’s some issue you and the claim representative can’t work out. When your claim is settled, an “attaboy” e-mail to the agent and the claim representative would be a welcome surprise.

Keep Your Claim Organized

Start a claim file, which should include:

• Notes on conversations with your agent and claim representative, including dates and times.
• Cost estimates on items to be repaired or replaced.
• Any time you, your friends or family spend on repairs or cleanup. Your claim by allowing you to offset it against your policy deductible. This is negotiable and should be discussed early in the process. representative may “pay” you for this time,
• Photographs and inventory of damaged or destroyed items. Do not throw anythingaway until the claim representative says to.
• Related claim data: the claim number, claim representative’s name, postal and e-mail addresses, office, cell and fax numbers.

Get prices from several vendors. If the items destroyed are no longer available, most insurance policies provide for replacement with items of “like kind and quality.” Document these items and discuss them with your claim representative.

Remember that your claim representative is probably way out of his element. Your damaged tower may be the first one he’s ever seen so he’ll ask lots of questions and need some documentation. That’s okay. Claim adjusters are trained to pay every penny the company owes, not a penny more, not a penny less. He will probably have to justify your settlement check to his supervisor. Make that job as easy for him as you can. He wants this claim off his desk just as much as you want to be back on the air

Your Dollars Have to Make Sense

Also, as my sainted grandmother used to say, “Pigs get fat, hogs get slaughtered.” Some guys think an insurance claim is like hitting the lottery. Not so, Bucko. If you can buy new gear at a great price, turn that price in not the list price. Claims people get real cranky (and rightfully so) if they feel a claim is being “padded.” In this day of Web sites and search engines, trust but verify is easily accomplished. Your claim representative may not know a Force-12 C31XR from next Tuesday but Google does. At best,“padding” slows the process and might leave you with egg on your face; at worst, you may find yourself facing felony insurance fraud charges.

Another thing to remember is that your damaged tower is not as high on the claim representative’s priority list as someone’s home with major structural or fire damage. When bad things happen, there are never enough resources to get things done as quickly as people would like. Be patient and be reasonable.

Most insurance companies write a check for the total damages (less your deductible) when the work is completed and all damaged/destroyed items are replaced. If you want a check prior to the start of work, the claim representative may reduce the payment for depreciation of the damaged property, paying the balance when the item is actually purchased or work is completed. This is consistent with contract language in most homeowners policies and keeps everybody honest.

Most homeowners policies settle losses on a replacement cost basis; meaning you get new stuff for old when the new stuff is purchased. Make sure you have a replacement cost policy before you have a loss — another good reason to see your agent real soon

Caveat Contractor

If you hire a contractor to repair or replace your tower, insist he provide you with a
current certificate of contractor’s liability insurance (his malpractice policy), a current certificate of Worker’s Compensation coverage and local references. If he’s a professional, this won’t be a problem. If he’s a fly-by-nighter, he’s probably not insured and you just don’t need those headaches. If the contractor tells you his employees are “subcontractors,” you’ll need to see the “subcontractor’s” insurance documentation. Do not even consider bending on this.

Your contractor may ask for some payment up front. It would be wise to include your claim representative in that discussion since he’ll be writing the checks.

Here’s the bottom line: Don’t rely on your ham radio buddies, your neighbor or your brother-in-law for advice on insurance matters. Call your agent right now, update him on your installation and ask hard questions. “If my tower and antennas are damaged, how will my policy cover the loss?” If your agent doesn’t know, make sure he finds out and then shows you in writing (in the policy) how losses will be covered or why they won’t be. If there are gaps in coverage, what will it cost for what you need? Photograph your installation and give your agent an accurate inventory of your gear.

Here’s a little secret. Everytime you pay your premium, your agent gets paid. (All right, I said it was a little secret.) If your agent can’t or won’t provide you with the service you deserve, he’s not earning his keep. It’s time to go shopping for a new company or a new agent with your old company.

Finally, and for the last time, please make it a priority to visit your agent and review
your policies. Insurance agents are just like you, we don’t like surprises and we don’t like conflict. We do feel pretty good when we take care of a client’s claim and put him back to where he was before the loss happened.

Baluns — What’s the Story?

สายอากาศหลากหลายชนิดสามารถทำงานได้ดีกว่าถ้าทำงานร่วมกับบาลัน คุณสามารถชื้อหามาใช้ แต่ !! ทำไมคุณไม่สร้างมันขึ้นมาเองละ?

Many antenna systems work better with a balun. You can buy them, but why not build your own?

บทความโดย Dick Arnold, AF8X จาก QST June 2008
 
If you like to experiment with antennas, sooner or later you will need a balun. The name balun mean balanced to unbalanced. Virtually all current radios are designed to load into an unbalanced feed line, typically coaxial cable.

ถ้าคุณเป็นคนที่ชอบทดลองกับสายอากาศ ไม่ช้าไม่นานคุณก็จะมีความจำเป็นต้องพบเจอกับเจ้าบาลัน ซึ่งบาลันย่อมาจาก balanced to unbalanced วิทยุสื่อสารในปัจจุบันล้วนแล้วแต่ออกแบบให้ต่อกับโหลดและสายนำสัญญาณแบบไม่สมดุล (unbalanced) อย่างเช่น สาย Coaxial

Enter the Balun

Many popular antennas, λ/2 dipoles for example, are inherently balanced. They will generally work in the expected manner if fed with a 1:1 balun at the center feed point where the coax feed line is connected. The dipole’s feed impedance is in the range 50 to 100 Ω, depending on the height above ground. That’s not a bad match to the radio’s 50 Ω expectation, but the antenna is a balanced load. The balun will convert the feed point to one more suitable for unbalanced coax and thus provide a radio friendly load.

สายอากาศยอดนิยมหลาย ๆ แบบยกตัวอย่างเช่น สายอากาศฮาฟเวฟไดโพล คุณสมบัติของมันจะเป็นสายอากาศแบบสมดุล มันสามารถทำงานได้อย่างดี ถ้าเราใช้บาลัน 1:1 เข้าร่วมตรงจุดที่บ้อนสัญญาณ impedance ของสายอากาศไดโพลอยู่ในช่วง 50 ถึง 100 โอห์ม ทั้งนี้ขึ้นอยู่กับความสูงเหนือพื้นผิวดิน และวัสดุไกล้เคียง ค่า impedance ประมาณนี้ไม่ทำให้การ Match ดูน่าเกลียดอะไร แต่ที่สำคัญคือสายอากาศไดโพลเป็นสายอากาศแบบสมดุล ตัวบาลันจะช่วยแปลงระบบที่ไม่สมดุลของวิทยุและสายนำสัญญาณให้เข้ากับระบบสมดุลของสายอากาศไดโพล

Other balanced antennas have different impedances and if it is around 200 Ω, a 4:1 balun can be used. Commercial baluns are available from many sources for about $15 to $100, depending on power rating, weatherproofing, frequency range and connector type. I found that baluns are easily homebrewed for much less cost than manufactured models.

สายอากาศแบบบาลันบางแบบมีค่า impedance เป็นอย่างอื่น เช่น 200 โอห์ม บาลัน 4:1 สามารถแก้ปัญหานี้ได้ บาลันที่มีขายกันอยู่ในท้องตลาดมีราคาตั้งแต่ประมาณ $15 ถึง $100 ขึ้นอยู่กับ อัตราการทนกำลังส่ง,การป้องกันน้ำ,ความกว้างของความถี่ใช้งาน และรูปแบบขั้วต่อเป็นต้น แต่ผมพบบาลันที่สามารถประกอบขึ้นได้เองได้อย่างง่าย ๆ และมีราคาถูกกว่าที่มีขายตามท้องตลาดทั่วไป

 
Figure 1 — At A a schematic of a homemade 4:1 balun. At B is a photo of the completed unit.

รูปที่ 1 ในรูป A แสดงแผนฝังทางไฟฟ้าของหม้อแปลงบาลัน 4:1 ส่วนรูป B เป็นรูปที่ประกอบขึ้นมาจริง ๆ

It should be pointed out that many hams connect coax directly to their dipoles without a balun and report satisfactory performance. Any resulting current due to unbalance will flow down the outside of the coax. The amount will depend on the coax length and the impedance to ground of the shield as it terminates at the radio or tuner. Those unbalanced currents can cause radiation that may distort antenna patterns or result in RF on equipment chassis, sometimes causing feedback and other problems. If you don’t have any of those problems, you can likely get away without a balun, but it almost never hurts to have one!

Rolling Your Own

Figure 1 shows a favorite 4:1 balun of mine that I found on the Internet. It uses an FT 114-43 toroid core wound with six turns of 16 gauge speaker wire. As indicated in the schematic, Figure 1A, the silver and copper color conductors are labeled A and B. The conductor A from one end and conductor B from the other end are spliced and connected to the shield side of a coax connector.


พันบาลันด้วยตัวเอง

รูปที่ 1 แสดงให้เห็นถึงบาลัน 4:1 ซึ่งผมพบมันในอินเทอร์เนทสามารถสร้างได้โดยใช้แกน Toroid เบอร์ FT 114-43 พันจำนวน 6 รอบด้วยสายลำโพงเบอร์ 16 ในรูป 1A สายตัวนำสีเงิน และสีทองแดง เชื่อมต่อกันที่จุด A B ปลายสายด้าน A ก็จะไปอีกฝั่ง ปลายสายด้าน B ก็จะแยกไปอีกฝั่ง ทำการปอกสายแล้วนำไปต่อกับขั้วต่อของสาย Coaxcial
 
The remaining conductors are the balanced line inputs. The coax connector (unbalanced output) is connected to the A conductor. The balanced line input is connected to two banana plugs; the unbalanced output is connected
to either a BNC or SO-239 type coax connector. The connectors are mounted in either a metal or plastic box and the toroid is secured to the box with hot glue. The completed unit is shown in Figure 1B.



The 1:1 current balun I use was made using another FT 114-43 toroid wound with 10 turns of RG-174 miniature coax. One end of the coax is connected to the unbalanced connector and the other end is connected to a dual banana jack as shown in Figure 2A.

บาลัน 1:1  ผมพันอีกแบบคือใช้สาย RG-174 พันบนแกนเบอร์ FT114-43 จำนวน 10 รอบ ปลายด้านหนึ่งต่อกับขั้วต่อที่ไม่สมดุล ปลายอีกด้านต่อกับ ขั้วต่อแบบบานาน่า (เป็นชื่อเฉพาะสำหรับเรียกขั้วต่อแบบนี้) ดังในรูป 2A

Again all this could be mounted in a suitable sized box. The unit with connectors shown in Figure 2B.

As you can see there is nothing too complicated in creating a homebrew balun.These two examples are only two of the many designs using smaller or larger toroids and wire. The toroids I used were purchased from www.kitsandparts.com, but there are many other suppliers.

This design is suitable for use with well matched antennas up to power levels around 100 W. For higher power, use larger diameter cores and shift to RG-58 coax.

ในการออกแบบของผมนี้เหมาะสำหรับการใช้งานได้ถึงประมาณ 100 วัตต์ ถ้าต้องการให้ทนกำลังส่งสูงกว่านี้ ต้องเพิ่มขนาดของแกน Toroid ให้ใหญ่ขึ้น สายนำสัญญาณที่นำมาพันก็ต้องใช้เบอร์ RG-58

 If you have an interest in learning more about baluns, check out Jerry Sevick’s book, which will provide designs for many different impedance ratios and power levels. 

ถ้าคุณสนใจศึกษาเรื่องบาลันให้มากขึ้นกว่านี้ ลองอ่านหนังสือของ Jerry Sevick ซึ่งเขาได้จัดทำบาลันไว้หลายรูปแบบ ทั้งอัตราส่วน Impedance และระดับกำลังที่ต่างกัน ให้เราได้อ่าน

 

Figure 2 — At A a schematic of a homemade 1:1 choke balun. At B is a photo of the completed unit with connectors attached.

 รูปที่ 2 ในรูป A แสดงแผนฝังทางไฟฟ้าของหม้อแปลงบาลัน 1:1 ส่วนรูป B เป็นรูปที่ประกอบขึ้นมาจริง ๆ พร้อมด้วยต่อขั้วต่อสาย

 ตัวอย่างหนังสือของ Jerry Sevick (สามารถหา Download ได้ลองค้นดูให้ดีๆครับ)

Hairpin Tuners for Matching Balanced Antenna Systems

Balanced Transmatch designs for 28 to 450 MHz. ATU สำหรับสายอากาศแบบบาลัน ความถี่ตั้งแต่ 28 ถึง 450 MHz

บทความโดย John Stanley, K4ERO

Once more, the advantages of ladder line for HF antennas were well presented in July 2008 QST.Before WWII surplus brought us cheap coax, balanced feeders were almost always used for VHF as well.

In the January 1942 QST reproduction that was included with The 2007 ARRL Handbook, we find that both home-brew and commercial VHF gear all used balanced
lines.2 For a given cost, open wire ladder line, window line or even TV twin lead can give you a lower loss installation than trying to buy large diameter coax in an effort to keep the losses to an acceptable level.

This is dramatically demonstrated by comparing losses in various line types.3 So, we wonder, why do so few present-day operators use ladder line or twin lead on the VHF frequencies? Might one reason be the lack of suitable antenna tuners (transmatches) for those bands? If balanced tuners were available, would ladder line be as popular at VHF as it has become at HF?

Balanced Tuners for HF Use

The ARRL Handbook, The ARRL Antenna Book and other ham publications have
always included designs for balanced tuners. Adam Nathanson, N4EKV, has one
of many good Web sites showing this type of tuner at www.n4ekv.com/tuners.asp. I have used tuners like this for years with good results. Figure 1 shows the one I use at up to 100 W on the HF bands.


Figure 1 — K4ERO balanced tuner for 60 through 15 meters.

รูปที่ 1 บาลันจูนเนอร์ย่านความถี่ 60 - 15 มิเตอร์ของ K4ERO

As noted in Volume 6 of The ARRL Antenna Compendium, I lean toward tuners
with a fixed link and tapped coil.The match is found by tuning the capacitor and
adjusting the output side to connect to a variable number of turns, keeping the taps equidistant from the coil ends. Other versions use the taps for coarse tune and a variable capacitor in series with the link for finer tuning.

Balanced Tuners for the Higher Frequencies

One of the problems with this type of tuner is that as the frequency goes higher, the number of turns on the coil goes down rather quickly. By the time you get to 28 MHz, there may be only four turns on the coil. This means that the adjustment steps available are very limited. You can tap to either two or four turns, or if you are willing to unbalance the tap positions, or can access the opposite side of the coil, you could use one or three turns. In either case, the operation is compromised. Another approach is to connect one tap to the bottom of the coil, instead of to the top, effectively using fractions of a turn rather than whole turns, but this often is not feasible from a mechanical standpoint.

In addition the symmetry and thus the balance are affected. If this type of tuner is to be used on 10 meters and higher, we need to rethink the design.

Figure 2(A) is a schematic of the conventional tapped link coupled tuner. Figure
2(B) is a representation of what I call a hairpin tuner. The electrical properties are essentially identical, but the physical layout of the hairpin type is optimum for the higher frequencies. By making the main inductor in the shape of a hairpin, or shorted transmission line, instead of a single layer solenoid coil, as is used in the conventional design, the tuner becomes much easier to build and adjust. The use of a short short-circuited transmission line section as an inductor is nothing new. It has been used for VHF/UHF circuits for many years.


Figure 2 — Balanced tuner confi gurations. At (A) conventional tapped coil based tuner, at (B) the hairpin equivalent.

Building Hairpin Tuners

For some time I have been using a hairpin inductor in a balanced tuner for 6 and 10 meters, and recently I built one for each of the 144, 220 and 432 MHz bands. The approach is the same on each frequency. A section of transmission line was used instead of the coil typically used on lower frequencies.

The capacitor was a split stator or butterfly design. The input link is a single turn inductor that overlaps a portion of the main transmission line inductor and the output taps were taken at whatever point of the hairpin that gives the best match.

The advantage of this layout is that moving the taps to any point on the hairpin is the same as tapping on different numbers of turns on a coil, except that with the hairpin, it is very convenient to make the adjustment in as fine a step as may be desired. In all of my designs, the coupling loop is held to the main hairpin by cable ties. This allows some adjustment of the coupling loop, but holds the loop sufficiently snug so that it is not likely to move around accidentally. Use less loop coupling for higher Q with more selectivity and more loop coupling for lowest loss.



Figure 3 — Hairpin tuner for 6 and 10 meters.


Figure 4 — 144 MHz version of a hairpin tuner.

Making them Play

The frequency tuning range percentage will depend on the capacitor used. For widest range, select one with a high minimum to maximum capacitance ratio. All of the capacitors I have tried have provided adequate range to cover the desired amateur
band. The range can be extended to cover a second band by putting fixed capacitors in parallel with the variable tuning capacitor.

For example, the 6 meter version, shown in Figure 3, works for 10 meters with the addition of a parallel 40 pF fixed ceramic capacitor, while the 220 MHz version, shown in Figure 5, works fine on 2 meters with the addition of a 12 pF ceramic. As it is, the 2 meter version just makes it to 222 MHz, so two bands are possible without switching caps. A single tuner could also work on 10 and 6 meters without switching by careful component selection

Capacitor Options

The most difficult component to find will likely be the split stator capacitor. There are ways of designing your own capacitor and it is made easier because of the relatively small capacitance required at these frequencies. For all of the designs shown here, I took capacitors from my well stocked junk box. You less well equipped folks will have to search at a hamfest or check out the basement of one of the local old timers.


Commercial capacitors are available, but the price may shock you. You could also choose to use a single section capacitor instead of the split stator. In that case, the capacitor shaft and frame will be “hot” and must be floated above ground. You will have to tune it via a long insulated shaft. And, of course, the balance will be somewhat compromised.

The method does work and a suitable single section capacitor may be easier to find, however . Alternatively you could use a pair of identical capacitors to ground if you adjusted them each to the same setting or worked out a common shaft arrangement. Fussy, but it could work. Target dimensions and component values for the bands in this range are shown in Table 1.



Table 1 Hairpin Tuner Component Values, Dimensions and Frequency Range

Inductor Choices

The length of the hairpin will depend on the value of your capacitor. The values below
represent tuners I have built and should give you a good starting point. Match the hairpin width to the spacing of the capacitor terminals, or bend the ends of the hairpin in or out at the capacitor end in order to make the connections. Spacing does affect the inductance value so keep it close to what you see in the photos.

My inductors are 1⁄8 inch diameter brass rod, but they could be soft copper tubing or wire in sizes from 12 gauge up to 1⁄4 inch. Brass welding rods from the hardware store could also be used. A smaller diameter means the hairpin should be shortened a bit as the inductance per inch will be higher. The links should be insulated wire, either enameled or PVC coated or, best of all, Teflon insulated. The 6 meter tuner in Figure 3 uses the shield of Teflon coax as the link. My links do not make electrical contact anywhere with the main hairpin. The coax shield and center of the hairpin could both be grounded to a chassis, if desired.



Figure 5 — Hairpin tuner for 144/222 MHz enclosed in its box.

Tuning Up

Tuning consists of setting the taps to an intermediate position and adjusting the capacitor for minimum SWR. If SWR is not as low as desired, move the taps a bit either towards or away from the capacitor and readjust the capacitor. You should be able to find tap settings that allow the SWR to be reduced to 1:1.

Safety Considerations

Note that neither of the tuners described above are shown installed within a chassis of any kind. This is to show the construction more clearly. Also, I must admit that during tests, I got away with this because I used low power and am very careful not to touch the hot parts of the circuit. I also have a nonmetallic operating desk. The open construction is useful during experiments.

I definitely do not recommend this approach for general use. You will want to put your tuner in a shielded box of some kind, probably with a hinged lid to allow you to move the taps as shown in Figure 5. You should never adjust the tap clips with
power applied. The tuning shaft should be brought out through the enclosure where an insulated knob should be installed for tuning even though with a split stator capacitor the shaft should be at ground potential. For high power, the use of a suitable enclosure is essential, especially if the tuner is to be installed close to the operating position. This is to prevent RF burns from contact as well as exposure to excess RF levels.

Remember that your body is more susceptible to pick up from VHF fields than is the case at HF. If you put the tuner well away from the operating position in a place that is protected from access by family members or pets, you may be able to use a somewhat more open construction as is sometimes done with conventional home brew tuners. Radiation from an unenclosed tuner of any type can be enough to cause interference with nearby electronics devices and could cause fires if anything flammable comes in contact with the hot parts of the circuit.


Figure 6 — 432 MHz version of a hairpin tuner.

These tuners have been tested with 100 W on 10 and 6 meters, 50 W on 2 meters and 20 W on 70 cm, the maximum output of my rig. For higher power, the designs can be scaled, remembering that the bigger it gets, the lower the frequency for the same geometry.

Thus, a design similar to that used here for 432 MHz, but three times larger, would probably work fine with a full kW, but on 144 MHz. Since my 20 W, 432 MHz design uses an inductor that is about as short as is practical (see Figure 6), getting up to 1 kW at 432 MHz, might prove difficult with this design. At the least, a different type of
capacitor would be required.

I hope that these simple to build and adjust tuners will start a trend towards greater use of balanced feeders on the higher frequencies just as ladder line has become the favorite for many on the lower bands.

A Simple Transformer to Measure Your Antenna Current

วงจรหม้อแปลงแบบง่าย ๆ สำหรับวัดกระแสในระบบสายอากาศของคุณ

SWR doesn’t give you the whole story you need an RF current meter.
บทความโดย Paul Danzer, N1II (จากหนังสือ QST ฉบับ September 2009)

In a recent QST article, Eric Nichols, KL7AJ, presented a good idea monitoring the RF current into your antenna system to insure optimum tuning. Actually, it was the second time it was mentioned to me — George Peters, K1EHW, suggested the same thing to me several months before. At the end of Eric’s article, he proposed using a current transformer to do the monitoring.

Making a Current Transformer

This could be as simple as a turn of wire through a ferrite core and several turns of
wire around the core to form a transformer. The output of the transformer would be proportional to the current through the wire.

Making it Happen

This seemed simple enough, and the result of one hour’s work is shown in Figure 1. The core used was a T37-6. T37 designates the size (0.37 inches OD), picked so the insulated center conductor of RG-58 (or RG-59) would fit comfortably through the core center. The 6 designation relates to the frequency application of the ferrite mix, in this case 2 to 50 MHz.

Searching the ARRL suppliers’ data base, it appears that Alstar Magnetics offers this core; an alternate would be a Palomar F37 with mix 61. There is no criticality here — if you want to try it, strip a core from any old source — perhaps from a junked PC power supply or computer cable. It may not be the most efficient RF transformer ever built, but if it works it will do the job.

Wrap 20 turns of 24 gauge enameled wire as the transformer secondary. The secondary is connected to half wave rectifier consisting of a silicon diode (1N914), a 10 kΩ resistor as the load and a 0.1 μF capacitor as a filter (see Figure 2). A high-impedance voltmeter (the $10 variety) is connected to the two pin jacks to serve as an indicator.

The unit shown was tested with a 100 W transmitter on all bands from 80 through 10 meters. Performance across each band was relatively uniform, considering the probable variation of SWR and power though the feed line as the frequency was varied.

Hook up the rest of the circuit as shown in Figure 2 and connect a meter to the terminals.



Figure 1 — View of the transformer and simple circuitry that make up the relative RF current meter.

รูปที่ 1 โครงสร้างประกอบด้วยหม้อแปลงและวงจรแบบง่าย ๆ ลักษณะเดียวกับวงจรวัดกระแส RF

What we Have

The object here was not to get an exact measure of the antenna current. What I wanted
was a relative measure, so I could see if anything was going wrong, or use it as a way to
adjust my antenna tuner for the maximum signal to the antenna. Commercial stations use a current meter, mounted at the connection of the feed line to the antenna, to monitor output. Since they know the antenna impedance and their meters are calibrated, they can determine precise power into the antenna.

If you enjoy low power (QRP, typically 5 W or less) or very low power (QRPP, less than 1 W) operation, more turns may be needed and can easily be added. Similarly, if your meter does not have enough sensitivity, more turns may be called for. If after assembly the core is not firmly in place, held by the friction of the secondary on the primary wire, a drop of glue can be used to secure it all together.

Putting it to Use

The current meter consumes a miniscule fraction of the output power, so, can be left in the line, or removed when not in use. You may even find a meter case and a surplus meter at a hamfest that will work with it to give continuous indication without tying up your bench meter. I suggest writing the relative current indication for each band in your log. Later, if something seems amiss, it is then an easy job to compare your readings to the recorded ones to find out if the problem is in your antenna system.



Figure 2 — Schematic diagram of the relative RF current meter. Nothing about the circuit is critical. See text for parts information.

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