QSL Card จาก JA6WFM Kumamoto Japan
3EL Yagi
IC-7800 100watts out and N1MM
วันจันทร์ที่ 12 กรกฎาคม พ.ศ. 2553
QSL Card จาก JA6WFM Kumamoto Japan
QSL Card จาก JA6WFM Kumamoto Japan
3EL Yagi
IC-7800 100watts out and N1MM
วันพุธที่ 23 มิถุนายน พ.ศ. 2553
วันจันทร์ที่ 21 มิถุนายน พ.ศ. 2553
วันศุกร์ที่ 18 มิถุนายน พ.ศ. 2553
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
Your calculations are right on. That’s all there is to it. But do keep in mind just a few potential pitfalls:
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
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.
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.
วันพฤหัสบดีที่ 17 มิถุนายน พ.ศ. 2553
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.
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.
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