Extra Class question of the day: miscellaneous antenna questions

This section consists of a miscellaneous selection of antenna questions. We’ll start with some questions about grounding, then talk a little bit about vertical antennas, then mobile antennas, and finally directional antennas.

Much has been written about station grounding. One thing’s for sure. A station’s safety ground is not adequate as an RF ground. The reason for this is that conductors present different impedances at different frequencies.

A wide flat copper strap is the type of conductor that would be best for minimizing losses in a station’s RF ground system. (E9D14) The main reason for this is that at RF tends to be conducted near the surface of a conductor. The more surface area there is, the lower the impedance to ground.

To  minimize inductance, it’s best to keep the RF ground connection as short as possible.  An electrically-short connection to 3 or 4 interconnected ground rods driven into the Earth would provide the best RF ground for your station. (E9D15)

For many amateurs, there first antenna is a trapped vertical antenna. Mine was a Hy-Gain 14AVQ, which was resonant on 40m, 20m, 15m, and 10m.  One advantage of using a trapped antenna is that it may be used for multiband operation. (E9D12) Another big advantage is that it doesn’t require a lot of space when compared to a dipole antenna.

A disadvantage of using a multiband trapped antenna is that it might radiate harmonics. (E9D07) For example, if your 40m transmissions have high harmonic content on 20m, and the multiband vertical is also resonant on 20m, it will radiate those harmonics.

Another disadvantage is that they are generally shorter than 1/4 wavelength. The bandwidth of an antenna is decreased as it is shortened through the use of loading coils. (E9D08) Not only do they have a smaller bandwidth, but loaded verticals are also less efficient than full, quarter-wavelength verticals. One way to lessen this disadvantage is to use top loading. An advantage of using top loading in a shortened HF vertical antenna is improved radiation efficiency. (E9D09)

Mobile antennas are almost always shorter than a quarter wavelength. What happens to the feed point impedance at the base of a fixed-length HF mobile antenna as the frequency of operation is lowered is that the radiation resistance decreases and the capacitive reactance increases. (E9D13) To transform this impedance to 50 ohms, they use a loading coil. The function of a loading coil as used with an HF mobile antenna is to cancel capacitive reactance. (E9D11)

Because short verticals, such as those used in mobile installations are inherently inefficient, you should do whatever you can to make them as efficient as possible. An HF mobile antenna loading coil should have a high ratio of reactance to resistance to minimize losses. (E9D06)

The ratio of reactance to resistance is called Q. A high-Q loading coil should be placed near the center of the vertical radiator to minimize losses in a shortened vertical antenna. (E9D05)

An antenna that used to be very popular when TV antennas used 300-ohm feedline is the folded dipole. The reason for this is that that approximate feed point impedance at the center of a two-wire folded dipole antenna is 300 ohms. (E9D10) Amateurs would use the 300-ohm feedline for both the antenna elements and the feedline, and then use a balun or some other matching device to match the 300 ohm impedance to the transmitter output impedance.

Finally, there are four miscellaneous questions about directional antennas. The first is about how beamwidth and antenna gain are related. The beamwidth of an antenna decreases as the gain is increased. (E9D03) This is intuitively obvious. An antenna does not amplify a signal but instead focuses the power. So, when we say that the gain is increased, what we’re really saying is that we’re focusing the power into a smaller beam.

On the VHF and UHF bands, Yagi antennas are operated either horizontally for weak-signal work and vertically for FM operations. In some cases, however circular polarization is desirable. You can use linearly polarized Yagi antennas to produce circular polarization if you arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom and feed them 90 degrees out of phase. (E9D02) The disadvantage to this approach is, obviously, that you need two antennas, instead of just one to achieve circular polarization.

For satellite operation, some hams have antenna systems that can tilt up and down as well as rotate. This is so the antenna can point directly at the satellite as it passes overhead. It is desirable for a ground-mounted satellite communications antenna system to be able to move in both azimuth and elevation in order to track the satellite as it orbits the Earth. (E9D04)

Parabolic antennas are often used at microwave frequencies to direct a signal in a particular direction. One thing to keep in mind is that gain increases by 6 dB if you are using an ideal parabolic dish antenna when the operating frequency is doubled. (E9D01) Also keep in mind that, as pointed out earlier, the beamwidth is narrower as well.

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