On 1/8/2022 8:27 AM, CUTTER DAVID via Topband wrote:
Rob
I recall a discussion on here some years ago which proposed that, whilst being
an amazing antenna for top band, if you could achieve it, the broadcast model
was not necessarily the best use of resources for amateur purposes, on the
basis that broadcasters are mainly interested in ground wave to cover a defined
relatively short range service area, whereas amateurs a more interested in dx.
Without arguing the "best use of resources" issue, work on the ground screen
that improves ground wave will also improve sky wave (DX)
I don't recall how that discussion ended, but for purposes of saving wire, at
least, the K2AV folded counterpoise (FCP) must be about as good as anyone could
attain. How it performs against that broadcast model would be of interest.
David G3UNA
Maybe it saves wire, but IMHO, it's a terrible ground screen. To see what I
mean by "ground screen" I quote a post to this reflector by my friend Eric,
N7CL, from a long time ago.
Wes N7WS
Quote:
To: <topband@contesting.com>
Subject: TopBand: Elevated GP vs. Vertical Antennas
From: n7cl@mmsi.com (Eric Gustafson Courtesy Account)
Date: Fri, 13 Mar 1998 17:14:28 -0700
I apologize for this but I'm not directly responding to the post
of yours referred to in the header of this message. I Just
grabbed it to respond to because it was a convenient way to get
the subject references correct. I don't believe that the quoted
insert from Tom is taken from that message.
>In a message dated 98-03-08 19:05:39 EST, W8JI writes:
>
>> The 160 meter skin depth (distance where current drops to 37%
>> of the value traveling in the conductor) is about 30 meters in
>> poor soil, ten meters in good soil, and about a foot in salt
>> water. Current effectively goes to zero at about 10 skin
>> depths.
>
>
The above is a very interesting statement. Let me see if I can
make use of it to illuminate _why_ the full ground screen (either
on or in the ground surface) provides a system with less loss
than a system of six or fewer elevated radials.
First, I'd like to separate the discussion to a strict comparison
of one ground system versus the other when used to provide the
ground system for (nearly at least) identical vertical
radiators. Please note that I'm NOT saying that elevated radials
don't work, or that they don't work well enough to make a useful
antenna. Only that when used near the earth's surface, they
cannot provide the same high level of efficiency as the canonical
full size sufficiently dense ground screen under the same
radiator. We are talking about ground mounted (less than 1/8
wave above grade) vertical radiators which are less than or equal
to 1/4 wave long.
The quantity of DX that can be worked with tuned rain gutters is
irrelevant to this discussion. The only thing we are talking
about is how many dB stronger (or weaker) is the same radiator
with ground system (A) versus the same antenna with ground system
(B).
Frequently it is possible to work DX effectively even on topband
with an antenna that is 25 dB weaker than a full size, full
effeciency vertical. This is because the fade margin for the big
antenna is 50 or 60 dB and -25 dB doesn't take the link SNR below
zero. Here we are talking about two systems which are within 4
to 6 dB of each other.
OK Now, from Tom's statement above it is pretty clear that unless
some means is taken to prevent the antenna's near field zone from
"seeing" the earth under the antenna, a large volume of earth
will be involved in the near field interaction. The poorer the
earth conductivity is, the larger the volume will be that is
involved. As the apparent surface conductivity of the earth's
surface increases (or is artificially increased with radials),
the volume of lossy earth exposed to the near fields decreases.
Eventually a point is reached where further surface conductivity
increases do not produce lower losses because the dominant loss
mechanism for the antenna system is no longer the near field
interaction loss with the earth material.
The effect of increasing the surface conductivity has been to
prevent the fields from penetrating into the earth material under
the antenna. Hence the name "ground screen". The ground system
when sufficiently dense has "screened" or shielded the earth
material from the antenna's near fields.
Sufficiently dense means that the greatest distance between
conductors in the screen is 0.015 wavelengths or less. For 1/4
wavelength radials, this requirement is met with 104 radials.
Now, as a practical matter, relaxing the criteria to 0.03
wavelengths for a radial system results in only about a 0.5 dB
reduction in measured field strength. So 60 radials this long
would produce a system which is very nearly as good as one
meeting the full density criteria.
For radials only 1/8 wavelength long, the requirement is met with
only 52 radials (26 if you are willing to give up 0.5 dB). Note
that these numbers are very consistent with the rule of thumb
(often repeated) that if the radials must be this short, there
isn't any point in having more than 30 or so. This does NOT mean
that 30 or 80 or 300 1/8 wavelength radials will produce an
antenna system as effecient as one that has 104 or more 1/4
wavelength radials. If the radiator is near 1/4 wavelength long,
the near field zone will extend beyond the 1/8 wavelength radius
of the screen and some efficiency will be lost.
Also note that it does not really matter very much wether these
radials are on the ground surface or elevated above the ground
surface. The effect is the same.
When we reduce the number of radials to a small number (four for
this discussion), the story is quite different. Just to be
clear, for the rest of this discussion I'm talking about four
resonant (or nearly resonant) radials spaced at 90 degree
intervals and elevated 16 feet or less above ground surface.
I'd like to start by stating that an antenna with four elevated
1/4 wave radials can be just as efficient as one with 120
elevated 1/4 wave radials. But this is true ONLY if the radial
system (and the base of the antenna) is elevated 3/8 wavelength
or more above the ground surface. On topband this would mean
elevating the radial system 197 feet! To be fair, 3/8 wave is
the distance where the effect stops being measurable. Not much
efficiency is really lost until the elevation is reduced to
somewhat less than 1/4 wavelength. But even 1/8 wavelength
elevation (where significant efficiency IS lost) would require
the feedpoint and radials to be 65 feet in the air.
So why is it that the four radials don't work as well near the
ground as they do when high up in the air? The answer is that
they DO. They do work as well at providing the radiator with a
low impedance something to be driven against, that is.
However, they don't help maintain system losses at the same low
level that a sufficiently dense screen would. There are two
reasons for this. (Well, actually, there are two ways to look at
one reason for it.) First, the four radials do not provide an
effective screen to keep the near field zone from extending below
the plane of the radials and therefore interacting with the lossy
earth below. Second, although the fields generated by the
current in the radials do (very nearly) cancel in the far field or
radiation zone, in the near field they do not even come close to
complete cancellation. So the interaction of these fields due to
RF current in the radials with the lossy earth induces some loss
into the system.
This does not mean that four elevated radials won't work. It
just means that over lossy ground, they wouldn't work _as well
as_ the full density ground screen. The amount of the additional
loss will vary depending on the exact amount of elevation and the
nature of the earth under the antenna. But a good useable
average number to use for planning purposes is about 4 dB. It
can be as high as 6 dB.
Ground level (far field) field strength measurements are
perfectly adequate to verify and compare the performance of these
systems. The screen density affects only the radiation
efficiency of the system. It does not significantly modify the
radiation pattern of the antenna in the elevation plane. So a
change in the "ground wave" field strength is accompanied by a
proportional change in the intensity of the radiation in the
antenna's main lobe which is launching the sky wave.
To significantly modify the elevation pattern of the antenna, the
ground screen must be dense and extended out to beyond 1/4
wavelength from the base of the antenna. The region beginning at
1/2 wavelength and extending out to 3 wavelengths or so from the
base exerts the most influence on the antenna elevation pattern
of interest to amateurs.
I don't personally know of any amateur installations where the
ground screen was extended out far enough to modify the elevation
pattern of a 1/4 wave vertical antenna operated on topband. But
erecting a vertical about 1/2 to 3/4 wavelengths up the beach
from a large body of salt water would be a really good way to
achieve a very low takeoff angle (over some range of azimuths).
This would be true if either ground system were used. However,
the full density screen would still put about 4 dB more energy
into the low angle main lobe.
73 Eric N7CL
End quote.
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|