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
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