There are THREE issues that are improved with ground radials, NOT just
1. Reduce ground resistance as a series resistance loss at the
feedpoint. Since a shortened radiator has a lower radiation resistance,
having a really low series resistance loss at the feedpoint is even more
important. This is what is addressed by the two elevated radials. In
that configuration you have a controlled counterpoise with a low series
resistance (as opposed to a radial-less groundrod).
2. Reduce ground resistance loss due to penetration by E-plane radiation
in the area immediately under the antenna. Most of this would be reduced
by a *dense* ground screen directly under the antenna, whether or not
used as a ground current sink as in a traditional vertical. The elevated
radials do not address this improvement from a dense ground screen
3. Reduce reflection loss in "first bounce" radiation. You may think of
the vertical as a stack of "point" radiators. Each point radiates in a
nearly 360 degree sphere. In the angles above the horizon this either
radiates or cancels with other "point" sources to produce a pattern.
>From the point source to a point *below* the horizon, there is a
"bounce" path. If the bounce path hits a spot that is part of a dense
radial system, the bounce is efficient. If the bounce path hits salt
water or salt marsh or a dense ground screen, the bounce is efficient.
If the bounce hits plain dirt or worse, a lot is lost. The larger the
area of efficient bounces, the lower the angles where the below the
horizon radiation is redirected to reinforce the radiation above the
horizon, and the lower the loss due to inefficient "bounces". Two
elevated radials do nothing to improve the area of "efficient" bounces.
The subject of how many radials is "dense enough" has been touted as
anywhere between 16 and 60 and up, depending on the author. (Look up
W8JI's posts on this, he's done actual testing and comparisons.)
The biggest advantage of a buried dense radial system over an elevated
one, is that it's buried and you can grow grass on top of it. Up in the
air gets ice, falls down, and you can see it, etc, etc.
Oddly enough, with a really dense ground screen, IF a shortened radiator
is kept efficient, and IF the reduced bandwidth of the shortened antenna
is tolerable to the operator, lower angle radiation is improved, because
the ground screen does not have to extend as far to reinforce a given
angle. HOWEVER, the IF's are huge, and there is more high angle
radiation from a shortened radiator, which may or not be bad, depending.
In the ground screen you suggest, you need to pay attention to "dense".
Depending on who you talk to, dense starts somewhere between 60 and 120
radials. Quite a few will say if you put down 60 radials with most
directions out 130', your antenna will play very well. EVERYONE will say
if you put down 120 radials with most directions out 130', your antenna
will play very well. There will be some lessening of low angle
performance in the direction of the 60 footers.
Some people do not want a hot antenna at ground level or anywhere low
enough for someone to touch.
In that case, do the ground screen AS IF you were going to use it as the
shield connection, AND use the two elevated radials, with the antenna
base and the hot connection point up the 8 feet or so for safety. In
this case, the two elevated radials take care of Issue 1) above, and the
ground screen takes care of issues 2) and 3). If you do this out a pier
on salt water and run the two radials out over and above the water, it's
the best of both worlds.
I have seen vertical installations where I was convinced maybe 10
percent at best was radiated. The rest was series resistance keeping the
sod warm, and the SWR low. Given how many absolutely abysmal radial
systems there are, it is understandable that F12 talks about two
elevated radials. That AT LEAST largely takes care of Issue 1), arguably
the worst of the three. (Just largely, because if no wire or salt water
underneath, there is some induced loss in the ground)
Guy Olinger, K2AV
Apex, NC, USA
----- Original Message -----
From: "Rick Williams" <email@example.com>
Sent: Tuesday, October 23, 2001 11:44 AM
Subject: [TowerTalk] vertical antennas
> I am in the process of erecting a Force 12 160VX (60ft tall 160 mtr
> vertical. The insturction manual addresses using 2 elevated
> instead of more traditional ground radials.
> After talking to a few other hams on the "pros and cons" along with
> experiences with each; I have come to the completely unscientific
> to use ground radails.
> Even in this area I have found some different ideas. Mainly, their is
> train of thought that ground radials need not be longer than the
> the vertical radiator (in my case, 60 ft.). The other train, always
> quarter wave radials (130 ft) regardless.
> I wonder what the "thread" is on this. In my situation, I can lay 130
> radials for 270 degrees. In the other 90, radials are going to
> length to about 60 ft for the shortest (perpendicular to the property
> Is this going to noticable affect the performance.
> Thanks for any help/ideas.
> Dick Williams K8ZTT
> Franktown CO
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