Yagi antennas

On July 1, 2020, the Extra Class question pool is being updated, making this post obsolete. See the corresponding post from my 2020 version of No Nonsense Extra Class License Study Guide.

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E9D – Directional antennas: gain; Yagi antennas; losses; SWR bandwidth; antenna efficiency; shortened and mobile antennas; RF grounding

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

Directional antennas

When designing a Yagi antenna, you might think that the most important parameter is forward gain. What usually occurs if a Yagi antenna is designed solely for maximum forward gain, though, is that the front-to-back ratio decreases. (E9D13) In other words, the antenna becomes more bi-directional than simply directional.

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.

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.

Antenna efficiency, shortened and mobile antennas

Designing an efficient mobile HF antenna is perhaps the toughest job for a radio amateur. Mobile antennas, almost by definition, must be shorter than a quarter wave and present a capacitive load. 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. (E9D10)

The function of a loading coil as used with an HF mobile antenna is, therefore, to cancel capacitive reactance. (E9D09) In effect, loading coils to make the radiator of a short vertical antenna look electrically longer.

Because short verticals have a low radiation resistance, they are inherently inefficient, and you need to 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. (E9D04) A high-Q loading coil (one with a high ratio of reactance to resistance) should be placed near the center of the vertical radiator to minimize losses in a shortened vertical antenna. (E9D03)

Unfortunately, what happens to the bandwidth of an antenna as it is shortened through the use of loading coils is that it is decreased. (E9D06) In other words, what happens as the Q of an antenna increases is that the SWR bandwidth decreases. (E9D08)

One way to improve the efficiency of a short vertical is to use a technique called top loading. An advantage of using top loading in a shortened HF vertical antenna is improved radiation efficiency. (E9D07) This is most often accomplished by using a “capacitance hat” on the top of the vertical element.

Often, antennas use traps to yield multi-band operation. A disadvantage of using a multiband trapped antenna is that it might radiate harmonics. (E9D05) 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.

RF grounding

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. (E9D11) The main reason for this is that 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. (E9D12)

For many amateurs, their first antenna is a trapped vertical antenna. Mine was a Hy-Gain 14AVQ. 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.