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Wimpie[_2_] · April 4th 14, 05:58 PM posted to rec.radio.amateur.antenna

El 04-04-14 18:25, escribió:
On Friday, April 4, 2014 8:39:52 AM UTC-5, Wimpie wrote:

For zero elevation (relevant for line of sight comms in VHF and up),

the gain will virtually not improve when using horizontal 3/4 WL

radials. The horizontal radials don't contribute to the vertically

polarized wave.

Horizontal no.. Try them sloping. As far as I know, the only
case where horizontal radials are preferred would be with the
5/8 radiator with 1/4 radials. In that case, they are better
straight out than sloping. But if I remember right, all other
cases will be better with sloping radials.

You are right. The feed current for a 5/8 has almost opposite phase
over the current in the upper half of the 5/8 radiator. So when you
slope the quarter wave radials, the radiator current has almost
opposite phase w.r.t. the phase of the radiator, so the field of the
radials is out of phase with the field from the radiator.

When you make the radials also 5/8 lambda long, you have both phase
reversal in the radiator and the radials, and then the radial current
is almost in phase with the radiator current (as in a 1.25 lambda
center fed dipole). You have more gain (as you already mentioned), but
the radial network is no longer a good floating ground, so you need
some CM suppression in the feed line or mast.

With tall masts (especially without metal guys), you can have really
weird results.

And for an even better pattern use sloping 5/8 WL radials, which

will start approaching the gain of a dual 5/8 WL collinear.

Agree, that really helps! You have to take care of mast/feeder

radiation that may distort the pattern (hence gain) significantly.

Sure. But one should do that with any elevated vertical for the
best performance.

I've compared 1/4 wave GP's, 1/2 wave's with decoupling, and

5/8 GP's with 3/4 and 5/8 radials on 10m to distant local

stations, which is a good test of low angle performance.

The 5/8 was always the best antenna in the real world using

low angle space wave paths 30-40 miles across town.

Probably this was because of the added heigth for the 5/8 lambda

radiator over a half or quarter wave, or you had (somewhat) sloping

5/8 WL radials.

They were sloping. But... I remember that I started out with
a 5/8 with 1/4 radials. It was also better than the 1/2 wave even
with the theoretical problems.. It could have been due to the higher
radiator, but not sure.. I'm not sure if an extra 5 feet in height
would make that big a difference on a 30 mile local path when the
base was a fixed 36 feet high in all cases. Could be, with the current
distribution of the usual 5/8 wave element.

You are right, 5 feet over 36 feet doesn't make a big difference (I
think 1 dB), provided that the 36 feet is in the clear. Maybe you had
possitive contribution from mast current (I am in guessing mode now...).

Other thing with the half wave end-fed can be absorption of the
plastic insulation. To make UV-resistant black they frequently use
carbon black. When the concentration is too high the dissipation
factor of the plastic increases signficantly. Recently I had really
weird results with a half wave monopole design. Everything went wrong.
It was the black UV-stabilized HMPE plastic containing around 2.5%
carbon black. When changing to green HMPE everything was according to
my calculations.

--
Wim
PA3DJS
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