On Sun, 24 Feb 2008 15:15:34 +0100, "Francesco L."
wrote:

Hi Francesco,

On the ground, or elevated?

Both cases please. I did a reasearch on arrl antenna book, low band dxing
and newsgroups and got a bit confused, so I need more tips. There are too
many options, I mean: at a certain height above the ground, atop the roof,
on the ground and so on, so I'm trying to collect as much info as possible
in order to get the overall picture.
Thanks


If you are calling 'low band' those frequencies below the AM broadcast
band, any antenna you use will be of compromise size (less than 1/4
wave tall, less than 1/4 wave ground radials and less than 1/2 wave
high.) A full length dipole is probably a challenge.

That being said, I operate mobile HF mostly. The car body is
generally a poor, but raised ground plane. On 40 and up,
communications are competitive enough to make contacts and communicate
during normal open band hours (give or take SW Broadcast stations.)
Because of power restrictions, 60 Meters works well mobile as
competition is restricted to ERP 50 watts as radiated from a dipole
antenna. 75/80 Meters is tough due to band conditions and antenna
size in wavelengths and the number of high power stations on the air.

Your ground plane antenna whether on the ground or in the air will
likely be very short. Even though its height and the length of the
ground radials (raised or on the ground) will be longer, think of the
effect of the mobile. Even though it has a shortened ground radial
system, it does work. If you can match the antenna, you can probably
work with it.

My experience is that the better the ground and the longer the
vertical element (within limitations), the better the antenna works. I
doubt there is much data reflecting various lengths of shortened
ground radials as there could be too many variations. However, there
is a manufacturer who has a vertical trap-multiband antenna that uses
one tuned ground radial for each band made up of helical wire and/or a
loading coil.

If you can't make a full 1/4 wave vertical, then do your best to make
it as tall as possible, as many, and as long radials as you can, or
make the antenna as high as you can with four radials as long as you
can, maybe loaded by helical or loading coil windings.

I hope this helps
Buck
N4PGW

--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

If you can't make a full 1/4 wave vertical, then do your best to make
it as tall as possible, as many, and as long radials as you can, or
make the antenna as high as you can with four radials as long as you
can, maybe loaded by helical or loading coil windings.

My referral to "low band dxing" was just incidental. That was because
info about ground radials are scattered here and there. My target is
to realize a 2 element phased array (which I'm already assembling)
completely homemade and was wondering about the impact that a few or a
lot of radials can have on the system. Obviously I'm talking about a
fixed station, not mobile. Anyway thanks for the contribution, much
appreciated, of you all.
Francesco, ik8vwa

Buck, N4PGW wrote:
"---or make the antenna as high as you can with four rafials as long as
you can, maybe loaded by helical or loading coil windings."

Yes, a ground-plane antenna needs resonant radials or at least with its
vertical radiator it needs to make a resonant system.

According to J.D. Kraus, radial conductor ground-plane antennas were
originated by G.H. Brown. Story is that Brown said only two radials were
needed (for equilibrium?). RCA`s marketing department insisted on four
and that`s how it`s been ever since.

Best regards, Richard Harrison, KB5WZI

"Francesco L." wrote in news:f225cfd7-6bad-45a4-
:



If you can't make a full 1/4 wave vertical, then do your best to make
it as tall as possible, as many, and as long radials as you can, or
make the antenna as high as you can with four radials as long as you
can, maybe loaded by helical or loading coil windings.

My referral to "low band dxing" was just incidental. That was because
info about ground radials are scattered here and there. My target is
to realize a 2 element phased array (which I'm already assembling)
completely homemade and was wondering about the impact that a few or a
lot of radials can have on the system. Obviously I'm talking about a
fixed station, not mobile. Anyway thanks for the contribution, much
appreciated, of you all.
Francesco, ik8vwa


In Low band Dxing and also QST there was information on the number of
radials verses their length. If I remember correctly the basic information
was that distances between the far ends (end that is the farthest from the
feed point) of the radial needed to be some set fraction of a wavelength at
the highest frequency to maximize the performance of a vertical with
radials of any length. So as the radials become shorter fewer of them are
needed to meet this requirement. This not to say that few short radial
works as well as longer radials but for any length of radials there is an
optimal number required and putting down more gains very little. For ¼ wave
radial this number is 120 but far fewer are needed for say 1/8 wave
radials.
I don’t remember the wave length that the article suggested so I will not
make a wild guess.


John, W3JXP

Richard Harrison wrote:
. . .
According to J.D. Kraus, radial conductor ground-plane antennas were
originated by G.H. Brown. Story is that Brown said only two radials were
needed (for equilibrium?). RCA`s marketing department insisted on four
and that`s how it`s been ever since.

That's a true story, also related in Brown's autobiography _And Part of
Which I Was_. Two radials are necessary for near cancellation of their
fields, to allow a circular pattern. The marketeers felt that it didn't
look symmetrical enough to be marketed as an omnidirectional antenna so
insisted on adding two more.

Two radials produce a very circular pattern in a directly horizontal
direction. The pattern becomes non-circular above and below horizontal,
but only very slightly for moderate angles above and below. I took
advantage of knowing that story when I was asked not long ago to design
an omnidirectional antenna to go in a small essentially flat volume. It
ended up as a variation of a ground plane with two radials, implemented
as flat traces on a PC board substrate. I built a prototype with copper
tape and had it measured in an anechoic chamber at a test lab. It had
the most circular pattern the lab technicians had ever seen, better than
their very expensive reference antenna. The extreme quality of the
circularity was actually a lucky coincidence because there are factors
such as feedline coupling which cause some variation even in a lab
environment. But it left no doubt that the pattern circularity was in
fact very good in spite of the apparently asymmetrical construction.
Brown was right.

George Brown, one of the pioneers of television, was also the inventor
of the turnstile antenna, widely used for TV broadcasting. And he's the
Brown of Brown, Lewis, and Epstein's seminal paper on radial ground
systems. He also had a legendary sense of humor. One of his most famous
stunts was substituting a blue-dyed banana for the yellow one in a bowl
of fruit used to test the first color TV broadcast, causing a great deal
of head-scratching among the engineers at the other end trying to figure
out what was wrong with the color transmission. Walt Maxwell, W2DU,
worked with Brown at RCA.

Roy Lewallen, W7EL