On Sat, 23 Feb 2008 22:54:37 +0100, "Francesco L."
wrote:

I was wondering if it's possible to shorten the radials from the standard
1/4 wavelength to another value, a smaller one. For example, in a 20 meters
antenna, which is the best value of length for shortened radials or, in
alternative, until which value is possible to go to without superlosses?
Thanks in advance

Hi Francesco,

On the ground, or elevated?

73's
Richard Clark, KB7QHC

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

On 24 feb, 15:15, "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

Hello Francesco,

It depends heavily on the antenna. When your antenna is an
electrically half wave radiator, the input impedance is in the kOhm
range and "ground" becomes less important. Many CB 27 MHz antennas are
half wave antennas (without any radials). For other lengths, "ground"
is important.

The smaller your antenna the lower the radiation resistance (=higher
feed current), and the more important the "ground" system.

For fertile wet ground and buried radials, the 0.25lambda is no longer
a magic number as the attenuation is very high. For dry Rock/sand,
dielectric properties dictate and some standing waves appear in the
ground conductor. As long as the real part of the ground impedance
is less then the real part of the input impedance of the radiator, it
is OK.

If possible, I prefer elevated radials. In many cases, they can be
shorter than 0.25 lambda (add more of them). The disadvantage is that
your floating ground becomes capacitive and you get a common mode
voltage on the cable screen, so you should add a common mode choke.
When you make them very short, the electrical situation is upside
down: your radiator functions as ground and your floating radial
network is the antenna.

An advantage of floating radials and the high end of HF is that your
radiation center is higher and ground properties become less
important, so you can have less loss (also in nearby structures) and
less noise from electrical equipment.

To give a more precise answer, one need to know your local conditions,
structural limitations, frequency and antenna type to be used, etc.
For low frequency, "Ground systems as a factor in antenna
systems" (Brown, Lewis Epstein, 1937) maybe interesting for you in
case of buried radials.

I know this doesn't answer your question, but I hope it will help you
a bit.

Best regards,


Wim
PA3DJS
www.tetech.nl (Dutch).

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

Hi Francesco,

In the air:
You are going to need a tuner for a standard height antenna with
shorter radials. Instead, you can try adding loading coils to each
radial (about midpoint). This will take experimentation to achieve
resonance. So plan on putting it up and taking it down many times.
You will also need a good choke at the feed point (aka 1:1 W2DU
BalUn); and another one a quarter wave down the line. Some who report
here say you will need more radials the lower the antenna is, and the
closer to ground.

On the ground:
As many small radials as you can make, as long as the radiator. If
that is too long (will not fit in garden), then simply fill the area
you can. "Fill?" For argument's sake (a starting point) a dozen or
more 0.1 wavelength radials. If this is too long, increase the count
and make them the maximum length you can.

Read Wim's advice for halfwave radiators. They have the reputation of
being ground free, but it will ease tuning if you build some radials -
whatever length, whatever count (not critical). If you don't build
these; then matching becomes a function of line placement. If the
line changes, so does the match.

73's
Richard Clark, KB7QHC

I believe other folks have mentioned that when the radials are buried,
no specific length is required for the antenna to be resonant. The more
radials you use and the longer they are, the lower the loss. There's a
point of diminishing returns for both length and number, and it turns
out that if you have only a few radials, making them very long doesn't
help much.

Elevated radials normally have to be close to a quarter wavelength long
for resonance unless they're very close to the ground in which case they
need to be somewhat shorter. You can use the same techniques to shorten
elevated radials, though, as you do a vertical radiating element. You
can add a loading inductance at the feedpoint or farther along each
radial, you can use a capacitive "hat" at the end, or some combination
of the two. And just as happens with loaded verticals, the result will
be narrower bandwidth, lower feedpoint resistance at resonance, and
potentially higher loss.

Roy Lewallen, W7EL

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

"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

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

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