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2016 Extra Class Study Guide: E9D - Directional antennas, antenna efficiency

Posted: 29 Feb 2016 12:26 PM PST
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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 stations 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.

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