On 25 feb, 00:10, Richard Clark wrote:
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.

That advantage of high impedance is also the disadvantage, 1500 Ohm
end-fed impedance, or higher, is not uncommon. With 400W input power,
this leads to 1100Vp voltage (at 1500 Ohms). Without careful
construction, E-field at sharp edges will exceed 3000V/mm easily. This
will not result in full air breakdown (due to strong nun-uniformity of
E-field, but will result in undesired corona discharge.

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.

End fed impedance for 3cm thick radiator is about 1500 Ohms, hence
radiator current (middle) is about 5 times higher then feed current to
the quarter wave radial. Therefore current*length product for radiator
is 10 times as high as for the radial. When the radial is vertically
oriented (worst case situation) influence on field from radiator is
+/- 10%. So very worst case you are talking of 1dB. When the radial
runs horizontally, the effect on the vertical component under low
elevation angle is negligible. As the original question relates to
amateur service, mentioning: "hence do not have large influence on
radiation pattern" is justified, in my opinion.


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.

Mostly CB antennas are mounted on a metal mast, so part of the return
current goes through the mast. You are right, in some cases this may
lead to significant common mode current, but looking to my experience,
this seldom resulted in untunable systems when lambda/dradiator is
high. I did experience problems in antennas for VHF where thickness
of radiator is no longer thin to wavelength. Fortunately, at such
frequencies a simple ground is easy to make.

73's
Richard Clark, KB7QHC

Wim
PA3DJS