To: <topband@contesting.com>
>Date: Fri, 11 Oct 1996 10:21:01 -0600
>X-Sender: laurao@vcn.com
>From: Laura Ostrem <laurao@vcn.com>
>Sender: owner-topband@akorn.net
>
>I have been following a few threads on elevated radials, and I'm getting a
>little worried that I may have to switch to littering my south pasture with
>the standard 120 quarter wave wires for 160 mtrs.
>
>I saw a posting stating that 4 elevated radials (10 ft high I'll assume),
>would be 5-6 db down from the 120 radials on the ground.
>
>What are your thoughts? My modeling program won't handle wires that close
>to the ground, it just treats them as perfect ground.
>
Snip...
>
> 73
>
> Jay WC7M in Wyoming
>
Jay,
There seems to be a lot of confusion over the issue of elevated radials and
vertical antenna ground system losses in general. To compound the problem,
none of the most popular antenna modelling programs handle any ground
system other than an infinite perfectly conducting plane very well.
My experience during testing of antennas for military use suggests the
following:
1. There are a number of components to what we routinely group together as
"ground losses". I think the understanding of what is going on may be
enhanced when we start talking about these loss components separately.
I would like to apologize in advance for a somewhat oversimplified
but nonetheless long winded treatment of these topics.
First, there is what I like to call the "connection" loss. This is the
effective resistance of the earth terminal connection of the antenna
system to the flow of whatever RF current the antenna system is
attempting to pump into and out of the earth on the wire that connects
it to earth. This is most easily visualized when the ground system
consists of a single ground rod at the base of a 1/4 wave (or shorter)
antenna.
Second, there is the loss due to the interaction of the near-field
energy storage fields of the antenna with nearby lossy ground,
vegetation and structures. This loss component behaves slightly
differently depending on wheter the antenna in question does most of
its near field storage in the magnetic field or in the electric field.
For our usual amateur discussion of short linear verticals and
horizontal dipoles, the near field storage is predominantly electric.
Lets refer to this as "near-field" loss.
Third, there is the RF radiation far-field interaction with the
(somewhat less nearby) ground around the antenna. This is the beam
forming or elevation pattern affecting interaction of the RF field with
the surrounding ground. This interaction is very difficult to
completely describe since the nature of it changes with the distance
from (and therefore the angle to) the antenna's phase center and the
plane of the "RF earth". The frequency involved is another variable
factor here. As someone earlier pointed out, the "skin depth" of the
earth is significant at 1.8 MHz. But as the grazing angle is decreased
to approach and exceed the critical angle, very little penetration
occurs (this is not to say that loss is completely eliminated).
2. Short (1/4 wave or less) base fed vertical antennas require significant
RF current to flow in the ground return terminal of the feedpoint.
If the base is at ground level and no metallic ground "screen" (to mean
either radial system or actual mesh screen covering a large enough
area) is provided, then the losses are dominated by the first two kinds
("connection" and "near-field") for obvious reasons. The third kind
(radiation field losses) are also present but are swamped in magnitude
by the first two. I am assuming a single ground rod attachment for
ground return currents here.
If we add just two slightly elevated resonant 1/4 wave radials (one at
0 and the other at 180 degrees), we can reduce the "connection" loss
component to a very small value. The RF ground return current can be
made to flow almost exclusively in the resonant radials with very
little loss. Radiation fields from these radial wires are very small
due to nearly complete cancellation in the far field. However, we are
left with a very significant amount of "near-field" interaction loss,
and the radiation field interaction loss.
How significant is the "near-field" component? Usually between 4 and 6
dB depending on the exact nature of the local earth and surroundings.
The farther we raise the base (and the two radials) from the earth, the
more we can reduce the effect of the "near-field" losses. How far must
we elevate the antenna to eliminate the "near-field" losses? Our work
(mostly between 9 and 18 MHz) showed diminishing returns setting in
around 3/8 wavelength above earth surface and loss of measurability
somewhere just beyond 1/2 wavelength. On top band, even the 3/8
wavelength number translates into a very diffcult support structure.
Imagine a 1/4 wave vertical with its BASE at almost 200 feet!
If instead of raising the structure, we begin adding radials to
"screen" the "near-fields" from "seeing" the underlying lossy earth, we
can also reduce the effect of the "near-field" losses.
Did someone ask how many radials does it take to eliminate the loss? I
thought so. Well, in a nearly ideal flat, large enough, open field
without any vegetation we found that in terms of length (assuming ideal
screen density), returns again diminished in the 3/8 to 1/2 wavelength
range (for a full size 1/4 wavelength radiator). In terms of screen
density (with various lengths of radial), diminishing returns began
when the distance between the open ends of the radials was less than
0.03 wavelength. Loss of measurability occurred at around half of that
or about 0.015 wavelength. Note that there is nothing resonant in this
ground screen. It can be replaced (or large areas of its central zone
can be) with square welded intersection mesh. Using the mesh has no
measurable effect so long as the comparison is between mesh with 0.015
wavelength or less openings and an identical (size and shape) radial
screen with no more than 0.015 wavelength spacing between the radial
ends.
So, does this answer the question "Do elevated radials work?" Yes. The
answer is that they are effective at reducing the "connection" loss. And
if they are "elevated" far enough (along with the base of the antenna),
they work as well as a full density on-the-ground screen in terms of
radiation effeciency (ignoring changes in the shape of the resulting
elevation pattern).
So it is probably the case that both the guy who says "I added 4 elevated
radials to my vertical antenna and the performance improved greatly" and
the guy who says "I and others have evaluated 4 elevated radials against a
full ground screen and the elevated radials loose by 4 - 6 dB" are correct.
Interestingly, our work also showed that physically short (less than 1/8
wavelength) top loaded (hat + inductor) verticals required _less_ ground
screen in terms of radial length to get to diminishing returns. Required
screen density was the same. Unfortunately, as the radiator gets shorter,
the losses from the ratio of radiation resistance to ohmic losses in the
radiator and loading and matching system component losses overcome the
benefit of needing a smaller ground screen area.
Anyone notice that I didn't talk about radiation field interaction losses
very much? There are a number of reasons for that. First, I'm not
confident that I can quantify it very well. Second, there is very little
that the average ham (or on top band, even the rich, obsessed, landed gonzo
contester) can really do about it. This is because the distances from the
base of the antenna which are important for DX (low angle beam forming)
range from about 0.75 to 3 wavelengths. The exact boundaries are somewhat
dependent on the height of the antenna's effective phase center above the
plane of the surrounding "RF earth". Those numbers are approximately
correct for a ground mounted 1/4 wave vertical. So the "perfect" RF
radiation field ground plane requires enough 1580 foot long radials to keep
the tip to tip distance to 0.015 wavelength. That is 630 1580 foot long
radials! Even Uncle Sugar declined to do that.
The bottom line is that any individual amateur has to be limited to what he
_can_ do at his QTH. To that end, my recommendation for anyone driving
a ground mounted short vertical antenna is to use as many on the surface
radials as he can reasonably achieve. But no more than are required to
limit the tip to tip distance to 0.03 wavelengths or a bit less. Note that
the less space you have for radial length, the smaller the number of
radials you need to get into diminishing returns. And, yes, DX can be
worked with numbers of radials ranging from zero through "enough".
However, with "enough" you will be louder.
73, Eric N7CL
--
------------------------------------------------------------------------------
Eric Gustafson N7CL | The mountains are high and the Emperor
6730 S. Old Spanish Trail | is far away.
Tucson, AZ 85747 |
| You can't work 'em
INTERNET: n7cl@mmsi.com | if you can't hear 'em.
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