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Richard Clark · February 24th 09, 11:10 PM posted to rec.radio.amateur.antenna

On Tue, 24 Feb 2009 10:19:14 -0800 (PST), wrote:

Hello Ken,

I would not use the 5/8 wave antenna, unless you can make a dipole of
1.25lambda. The 5/8 wave vertical only gives the published gain over a
large good conducting ground plane. 3 or 4 quarter wave radials may
provide a reasonable floating ground for feeding the antenna, but it
is not a large ground plane.

This and other points are deceptive. First, the performance you site
is indeed due to the plane of ground (not to be confused with our
usage of the term ground plane) to the extent of the conductivity of
ground out about 5 to 10 wavelengths away from the antenna. No
practical ground system is going to impact that.

The ground system placed below the antenna WILL impact gain, only
insofar as it shields the ground's loss contribution. Hence the large
number of radials.

Using a half wave has the disadvantage of the more complicated feeding
network. You may expect impedances up to kOhm range (depending in
thickness of the radiating element), so you need some high voltage
evaluation of your structure in case of 100W input power.

The advantage of the half wave is exactly for its high impedance in
relation to the loss of ground. The far ground still dominates low
angle launch characteristics, but if (like the large number of radials
offers) you lose less to ground, you have more in the air in all
directions.

The advantage is the low requirement for the (floating) ground at the
feed point. Just 1 or 2 quarter wave radials are sufficient. These
radial wires may also slope down, as they carry low current, hence do
not have large influence on radiation pattern.

If there is just 1, or if the 2 are not symmetrical, then the DO
contribute to the radiation pattern lobe shape. As to the degree or
notice, that is variable to the user/listener.

When you have some
metal structure around you, you can use that as ground, eliminating
the need for radials. When you look to half wave CB antennas, most
ones do not have radials at all.

They probably rely on the coax shield as a return path, which makes it
notoriously unreliable in its state of tune.

73's
Richard Clark, KB7QHC

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