Hello 160M fanatics. The following, very lengthy, was copied from another
reflector. I found it very interesting.
73 Carl KM1H
>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 modeling 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 difficult 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 efficiency (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
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