SHORTENED AND MUlTIBAND ANTENNAS
It would be just terrific if we could all have a full-sized antenna on every band, far above the ground, but this is rarely possible. Life is full of compromises and a typical antenna system is a good example! Two of the most common antenna aspects requiring compromise, especially on the HF bands, are the number of separate antennas that can be erected and their length. To address the length issue, amateurs use a number of techniques to shorten the antennas while still maintaining acceptable electrical performance. There are also a number of ways to make a single physical structure work well on a number of different frequencies. The next two sections give a few examples of common techniques and the tradeoffs they require.
Loaded Whips
The most difficult place to achieve effective HF antenna performance is a mobile station. Not only is the antenna system exposed to the mobile environment's vibration, temperature extremes and corrosion, but the antenna's size is quite limited by the constraints on vehicle size and maneuverability. Ten and 12 meters are the only bands on which "full-sized" 1/4 wavelength ground-plane antennas can realistically be deployed.
Practically, mobile antennas for HF are almost all some variation on the whip a flexible, vertical conductor attached to the vehicle with a threaded or magnetic mount. The vehicle acts as a ground-plane for the antenna. Whips are usually 8 feet or less in length. At 21 MHz and lower frequencies, the antenna is "electrically short," meaning less than 1/4 wavelength long. As the operating frequency is lowered, the feed point impedance of such an antenna is a decreasing radiation resistance in series with an increasing capacitive reactance as shown by the equivalent circuit in Figure 9.7. A full-size 1/4 wavelength whip's radiation resistance is approxi-mately 36 Ohm.
Figure 9.8 The capacitive reactance of a whip antenna can be cancelled by adding an equivalent amount of inductive reactance as a loading coil in series with the antenna.
To tune out the capacitive reactance and resonate the antenna, a series inductive reactance, or loading coil is used. (Remember that resonance occurs when the feed point impedance is entirely resistive.) The amount of inductance required is determined by the desired operating frequency and where the coil is placed along the antenna. Figure 9.8 shows the loading coil as an inductance in series with the whip.
Base loading (placing the loading coil at the feed point, assumed to be at the base of the antenna) requires the lowest value of inductance for a given antenna length. As the coil is moved along the whip farther from the feed point, the required amount of inductance increases. This is because the amount of capacitive reactance increases as the feed point moves closer to the end of the whip. The addition of the resonating inductance has one drawback in that SWR bandwidth of the antenna system is reduced from that of a full-size, Yt wavelength antenna. The reduction in bandwidth occurs because reactance of the tuned system increases more rapidly away from the resonant frequency than does the feed point reactance of a non-tuned antenna.
One advantage of placing the coil at least part way up the whip, however, is that the current distribution along the antenna is improved, and that increases the radiation resistance. The major disadvantage is that the requirement for a larger loading coil means that the coil losses will be greater, although this is offset somewhat by lower current through the larger coil. Center loading has been generally accepted as a good compromise in mobile antenna design.
Figure 9-9 A drawing showing a typical bumper-mounted HF-mobile antenna. Optional guy lines help stabilize the antenna while the vehicle is in motion.
Figure 9-9 shows a typical bumper-mounted, center-loaded whip antenna suitable for opera-tion in the HF range. The antenna could also be mounted on the car body proper (such as a fender or trunk lid). The base spring acts as a shock absorber for the base of the whip, since continual flexing would weaken the antenna. A short, heavy, mast section is mounted between the base spring and loading coil. Some models have a mechanism that allows the antenna to be tipped over for adjustment or for fastening to the roof of the car when not in use. Optional guy lines can be used to stabilize the antenna while in motion.
A word of caution - the bumper mount technique assumes that the bumper (or its mounting brackets) are metal and bonded to the rest of the car's body, With modem cars increasingly made from nonconductive materials, it is important to be sure that whatever mounting technique is used makes good connection to as much of the vehicle's metal frame and surfaces as possible!
Other common types of mobile HF antennas include a magnetic-mounted, two-section tubular fiberglass base helically wound with the antenna conductor and topped with a short metal whip, The entire base becomes the loading coiL These inexpensive antennas work on a single band, requiring multiple antennas to be carried for operation on multiple bands, They give good performance for their modest price, At the other end of the price scale are the tunable "screwdriver" antennas similar to that in Figure 9-9 but for which the loading coil' inductance is adjusted from inside the vehicle. The name derives from the small de motor used to tune the coil, similar to fhose found in electric screwdrivers.
Losses in the loading coil can be reduced if the required loading coil inductance is reduced, allowing a smaller coiL To use a smaller coil, the capacitive reactance that must be tuned out must also be reduced. One method of decreasing capacitive reactance is to increase capacitance. The top loading method is one such technique.
Top loading adds a "capacitive hat" above the loading coil, either just above the coil or near the top of the whip. The "hat" usually consists of short wires perpendicular to the whip, often with the ends of the wires connected by a metal ring for additional strength. The added capacitance reduces the resonating value of inductance and the size of the loading coiL This reduces the loading coil's resistive loss and improves the antenna radiation efficiency.
ข้อสอบที่เกี่ยวข้อง
For a shortened vertical antenna, where should a loading coil be placed to minimize losses and produce the most effective performance?
A. Near the center of the vertical radiator
B. As low as possible on the vertical radiator
C. As close to the transmitter as possible
D. At a voltage node
Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?
A. To swamp out harmonics
B. To maximize losses
C. To minimize losses
D. To minimize the Q
What is a disadvantage of using a multiband trapped antenna?
A. It might radiate harmonics
B. It can only be used for single-band operation
C. It is too sharply directional at lower frequencies
D. It must be neutralized
What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?
A. It is increased
B. It is decreased
C. No change occurs
D. It becomes flat
What is an advantage of using top loading in a shortened HF vertical antenna?
A. Lower Q
B. Greater structural strength
C. Higher losses
D. Improved radiation efficiency
Why is a loading coil often used with an HF mobile antenna?
A. To improve reception
B. To lower the losses
C. To lower the Q
D. To cancel capacitive reactan
What is an advantage of using a trapped antenna?
A. It has high directivity in the higher-frequency bands
B. It has high gain
C. It minimizes harmonic radiation
D. It may be used for multi-band operation
What happens at the base feed-point of a fixed-length HF mobile antenna as the frequency of operation is lowered?
A. The resistance decreases and the capacitive reactance decreases
B. The resistance decreases and the capacitive reactance increases
C. The resistance increases and the capacitive reactance decreases
D. The resistance increases and the capacitive reactance increases
ERROR DETECTION AND CORRECTION
Even the best transmitter and receiver systems cannot guarantee 100% accuracy of data transmitted across an air link. There are just too many ways that Mother Nature can disrupt the signal; noise, multi path and fading are just a few causes of errors. To get an idea of what can happen to a data signal, try to copy RTTY signals that are weak or fluttery from ionospheric variations!
In recognition of the realities of radio propagation, data communications engineers have devised a number of strategies. The first challenge is to find out when an error has occurred! This is called error detection. Without some clue about what the data should have been, . however, there is no way to detect errors. To be able to discern transmission errors, information describing the data is sent along with the original data.
Error detection data can be as simple as the parity bit of ASCII data discussed earlier. The addition of the single parity bit is a great improvement over no error detection at all, at the cost of a small decrease in data rate. Parity cannot be used to detect any type of error, however. Parity can only detect errors that cause the number of even or odd bits to differ from that which the system has been configured to expect. You can probably imagine how changing one bit would cause the parity check to fail, but what if two bits are changed so that the net number of even or odd bits is unchanged? Parity checking can only detect single-bit errors.
Another popular technique of error detection, used by packet radio's AX.25 protocol, TCP/IP networking systems, and Ethernet (among many others) is checksums. Originally, this was just the sum of all the data values in a packet, sent as a single byte at the end of the packet. It was compared to the sum computed by the receiver. If the sum matched, the entire packet was judged to be good. The simple checksum has evolved into a more sophisticated technique called the cyclical redundancy check or (CRC). The two-byte CRC is a lot like a checksum and is evaluated by the receiving system in the same way as a checksum. Using a CRC detects most errors.
Once the system has detected an error, what it decides to do about it is another matter. This moves the process from error detection to error correction. The simplest form of error correction is ARQ (Automatic Repeat Request), introduced in the section on PACTOR earlier in this chapter. If the receiving system detects an error, it requests a retransmission of the corrupted packet or message by sending a NAK (Not Acknowledge) message to transmitting station. The information is retransmitted until the receiver responds with an ACK (Acknowledge) message. If the errors persist long enough, the system gives up and drops the connection.
Another popular error correction technique is to send some extra data about the information in the packet or message so that the receiving station can actually correct some types of errors. This technique is called Forward Error Correction or (FEC). The term "forward" stems from sending the error correction data "ahead" with the original information. The combination of the FEC data and the algorithm by which errors are detected and corrected is called an FEC code.
There are many types of FEC codes. Reed-Solomon, Hamming, BCH and Golay codes are all used in consumer electronics. FEC data is sometimes spread out over several data packets to account for fading. FEC is used with digitized voice to help preserve the quality of the received speech. This is why digital voice systems (such as mobile phones) tend to have good quality up to a certain error threshold and then become completely garbled - their FEC algorithm fails at that point.
One type of FEC builds the error correction into the structure of the transmitted data itself. Instead of allowing any possible sequence of symbols to be sent over the air link, the system called Viterbi encoding restricts the sequence to a smaller number. That way,
the receiving system has fewer possibilities to choose from when deciding what symbol was received. The receiver then reports the most likely sequence of symbols to have created the signal received the Viterbi path.
ข้อสอบที่เกี่ยวข้อง
What do the letters FEC mean as they relate to digital operation?
A. Forward Error Correction
B. First Error Correction
C. Fatal Error Correction
D. Final Error Correction
How is Forward Error Correction implemented?
A. By the receiving station repeating each block of three data characters
B. By transmitting a special algorithm to the receiving station along with the data characters
C. By transmitting extra data that may be used to detect and correct transmission errors
D. By varying the frequency shift of the transmitted signal according to a predefined algorithm
How does ARQ accomplish error correction?
A. Special binary codes provide automatic correction
B. Special polynomial codes provide automatic correction
C. If errors are detected, redundant data is substituted
D. If errors are detected, a retransmission is requested
What is the advantage of including a parity bit with an ASCII character stream?
A. Faster transmission rate
B. The signal can overpower interfering signals
C. Foreign language characters can be sent
D. Some types of errors can be detected
L-Networks
The simplest LC impedance matching network is the L-network. Figure 6-40 shows its four variations that have both an inductor and capacitor. (There are four additional variations that either have two inductors or two capacitors, but they are much less common.) The choice of circuit to be used is determined by the ratio of the two impedances to be matched and the practicality of the component values that are required.
The L-network in Figure 6-41 will transform to 50 Ohm any higher impedance presented at the input to the feed line. (At least it will if you have an unlimited choice of values for Land C.) Most antennas and feed lines will present an im¬pedance that can be matched with an L-network.
To adjust this L-network for a proper match, the coil tap is moved one turn at a time, adjusting C for lowest SWR at each step. Eventually a combina¬tion should be found that will give an acceptable SWR value. If the impedance at the input to the feed line is lower than 50 Ohm, the circuit can be "turned around" to reverse the transformation ratio. Matching networks made entirely of inductors and capacitors work equally well in either direction!
The major limitation of an L-network is that a combination of inductor and capacitor is normally chosen to operate on only one frequency band because a given LC combination has a relatively small impedance-matching range. If the operating frequency varies too greatly, a different set of components will be needed.
Pi and Pi-L Networks
Most tube-type amplifiers use pi-network output-coupling circuits as shown in Figure 6-42. The most common form of this network consists of one capacitor in parallel with the input and another capacitor in parallel with the output. An inductor is in series between the two capacitors. The circuit is called a pi-network because it resembles the Greek letter pi - if you use your imagination a bit¬with the two capacitors drawn down from the ends of the horizontally drawn inductor.
To adjust the pi network in a power amplifier for proper operation. the tuning capacitor (CL) is adjusted for minimum plate current. and the loading capacitor (C2) is adjusted for maximum permissible plate current. The adjustments are interactive. so this procedure is usually performed several times to reach the optimum settings.
You can convert the L-network of Figure 6-41 into a pi-network by adding a variable capacitor to the transmitter side of the inductor. This effectively creates two L-networks back-to-back with each L-network sharing half of the series inductance. Using this circuit, any value of load impedance (greater or less than 50 Ohm) can be matched using some values of inductance and capacitance, so it provides a greater impedance-transformation range.
Because of the series coil and parallel capacitors, this circuit acts as a low-pass filter to reduce harmonics, as well as acting as an impedance-matching device. (A pi-network with two coils shunted to ground and a series capacitor would make a high¬pass filter, and is virtually never used as an amateur output-coupling circuit.) Harmonic suppression with a pi-network depends on the impedance-transformation ratio and the circuit Q. While the L-network's Q is fixed as a consequence of the frequency and impedance transformation ratio, the pi-network's Q can be adjusted by selecting different combinations of component values. Circuit design information for pi-networks appears in The ARRL Handbook.
If you need more attenuation of the harmonics from your transmitter, you can add an L-network in series with a pi-network, to build a pi-L-network. Figure 6-43 shows a pi-network and an L-network connected in series. It is common to combine the value of C2 and C3 in single variable capacitor, shown in Figure 6-43B as C4. The pi-L-network thus consists of two series inductors and two shunt capacitors. The pi-L-network provides the greatest harmonic attenuation of the three most-used matching networks - the L, pi and pi-L-networks.
ข้อสอบที่เกี่ยวข้อง
What is a Pi-L network, as used when matching a vacuum-tube final amplifier to a 50-ohm unbalanced output?
A. A Phase Inverter Load network
B. A network consisting of two series inductors and two shunt capacitors
C. A network with only three discrete parts
D. A matching network in which all components are isolated from ground
What is one advantage of a Pi matching network over an L matching network?
A. Q of Pi networks can be varied depending on the component values chosen
B. L networks can not perform impedance transformation
C. Pi networks have fewer components
D. Pi networks are designed for balanced input and output
What advantage does a Pi-L-network have over a Pi-network for impedance matching between the final amplifier of a vacuum-tube type transmitter and an antenna?
A. Greater harmonic suppression
B. Higher efficiency
C. Lower losses
D. Greater transformation range
Which of the following describes how the loading and tuning capacitors are to be adjusted when tuning a vacuum tube RF power amplifier that employs a pi-network output circuit?
A. The loading capacitor is set to maximum capacitance and the tuning capacitor is adjusted for minimum allowable plate current
B. The tuning capacitor is set to maximum capacitance and the loading capacitor is adjusted for minimum plate permissible current
C. The loading capacitor is adjusted to minimum plate current while alternately adjusting the tuning capacitor for maximum allowable plate current
D. The tuning capacitor is adjusted for minimum plate current, while the loading capacitor is adjusted for maximum permissible plate current
How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network’s
input and output?
A. Two inductors are in series between the input and output and a capacitor is connected between the two inductors and ground
B. Two capacitors are in series between the input and output and an inductor is connected between the two capacitors and ground
C. An inductor is in parallel with the input, another inductor is in parallel with the output, and a capacitor is in series between the two
D. A capacitor is in parallel with the input, another capacitor is in parallel with the output, and an
inductor is in series between the two
A T-network with series capacitors and a parallel (shunt) inductor has which of the following properties?
A. It transforms impedance and is a low-pass filter
B. It transforms reactance and is a low-pass filter
C. It transforms impedance and is a high-pass filter
D. It transforms reactance and is a narrow bandwidth notch filter
Which of the following is the common name for a filter network which is equivalent to two L networks back-to-back?
A. Pi-L
B. Cascode
C. Omega
D. Pi
MAGNETIC CORES
As you've seen, inductors store magnetic energy, creating reactance. Inductors are usually visualized as the classic winding of wire from one end to the other of a round form this winding shape is called solenoidal giving rise to the common term "coil." An inductor's core is whatever material the wire is wound around, even air. (An inductor whose core consists of air is called air-wound.)
Solenoidal coils make great figures in books, but are actually fairly uncommon in electronics. A winding of wire around a hollow form filled with nothing but air is a relatively inefficient way to store magnetic energy. A form made of magnetic material increases the storage of energy because it focuses the magnetic field created by the current in the surrounding winding. The stronger magnetic field increases the inductance of the inductor.
Inductance is determined by the number of turns of wire on the core and on the core material's permeability. Permeability refers to the strength of a magnetic field in the core as compared to the strength of the field with a core of air. Cores with higher permeability have more inductance for the same number of turns on the core. In other words, if you make two inductors with 10 turns around different core materials, the core with a higher permeability will have more inductance.
Manufacturers offer a wide variety of materials, or mixes, to provide cores that will perform well over a desired frequency range. Powdered-iron cores combine fine iron particles with magnetically-inert binding materials. Combining materials such as nickel-zinc and manganese-zinc compounds with the iron produces ceramic ferrite cores. The chemical names for iron compounds are based on the Latin word for iron,ferrum, so this is how these materials get the name ferrite. By careful selection of core material, it is possible to produce inductors that can be used from the audio range to UHF. Inductors with magnetic material cores are also called ferromagnetic inductors.
The choice of core materials for a particular inductor presents a compromise of features. Powdered-iron cores generally have better temperature stability. Ferrite cores generally have higher permeability values. however. so inductors made with ferrite cores require fewer turns to produce a given inductance value.
Core Shape
The shape of an inductor's core also affects how its magnetic field is contained. For a solenoidal core, the magnetic field exists not only in the core, but in the space around the inductor. This allows the magnetic field to interact with, or couple to, other nearby conductors. This coupling often creates unwanted signal paths and interactions between components, so external shields or other isolation methods must be used.
To reduce unwanted coupling, the donut-shaped toroid core is used. When wire is wound on such a core, a toroidal inductor is produced. Nearly all of a toroidal inductor's magnetic field is contained within the toroid core. Toroidal inductors are one of the most popular inductor types in RF circuits because they can be located close to each other on a circuit board with almost no interaction. See Figure 4-29 for a photo of a variety of toroidal inductors Toroidal inductors are used in circuits that involve frequencies from below 20 Hz around 300 MHz.
ข้อสอบที่เกี่ยวข้อง
What material property determines the inductance of a toroidal inductor with a 10-turn winding?
A. Core load current
B. Core resistance
C. Core reactivity
D. Core permeability
What is the usable frequency range of inductors that use toroidal cores, assuming a correct selection of core material for the frequency being used?
A. From a few kHz to no more than 30 MHz
B. From less than 20 Hz to approximately 300 MHz
C. From approximately 1000 Hz to no more than 3000 kHz
D. From about 100 kHz to at least 1000 GHz
What is one important reason for using powdered-iron toroids rather than ferrite toroids in an inductor?
A. Powdered-iron toroids generally have greater initial permeabilities
B. Powdered-iron toroids generally have better temperature stability
C. Powdered-iron toroids generally require fewer turns to produce a given inductance value
D. Powdered-iron toroids have the highest power handling capacity
What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor?
A. Toroidal cores contain most of the magnetic field within the core material
B. Toroidal cores make it easier to couple the magnetic energy into other components
C. Toroidal cores exhibit greater hysteresis
D. Toroidal cores have lower Q characteristics
What is one reason for using ferrite toroids rather than powdered-iron toroids in an inductor?
A. Ferrite toroids generally have lower initial permeabilities
B. Ferrite toroids generally have better temperature stability
C. Ferrite toroids generally require fewer turns to produce a given inductance value
D. Ferrite toroids are easier to use with surface mount technology
รางวัล CQ WW DX Contest - CW 2009 , Award ของ HS8JYX ประเภท QRP 20 มิเตอร์
CQ WW VHF Contest ปีนี้ HS8JYX เลือกแข่งขันแบบ Single Operator ออกอากาศที่บ้าน เขตพื้นที่ NJ98kc อ.เมือง จ.กระบี่
สายอากาศที่ใช้เป็นสายอากาศ V2 แบบ 4 ชั้น ความสูง ประมาณ 10 เมตร ผลการติดต่อ สามารถติดต่อได้ 92 QSO จาก 5 Grid Locator
QSL Card จาก JH4UYB Masaki Okano
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