On Sun, 9 Jul 2006 23:53:40 +0100, "David" nospam@nospam wrote:
Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.

Hi David,

Well, given your repetition of "ground plane," be cautioned that is
not one-and-the-same meaning for radials (even if they are called part
of a ground plane antenna).

73's
Richard Clark, KB7QHC

Cecil Moore wrote:
David wrote:

Can someone provide a full description of how a quarterwave vertical
antenna
with radials works?


Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.

I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.

Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.

Learning this stuff can be a lot of fun. It can be frustrating, too.

Good Luck,

John
AB8WH

"David" nospam@nospam wrote in message
...
A normal ground plane is a large sheet of metal that reflects the radio
wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

Effectively, yes.

A metallic surface (your "sheet of metal") can be replaced by a partially
metallic surface -- within limits. If you keep the size of any gap under
1/10 wavelength, the surface will appear solid. This I know from satellite
reflector work.

The use of four radials appears to be a compromise for using a solid
surface, but it obviously works. The RF sees these radial wires and behaves
like we want. I think adding more radials will always make a better
counterpoise, but I also think you reach the point of diminishing returns
pretty quickly. (We aren't the first ones to speculate about this, after
all :-)

"Cecil Moore" wrote
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
==========================================

What everybody forgets about is that the velocity factor along
ground-mounted radial wires is about half of the free-space value.

Consequently, the 1/4-wave resonant length is crudely only half of the
elevated value.

On the other hand, the resonant length is very non-critical because Q
is very small - Q is only 2 or 3 and is even smaller at the high end
of the HF band.
----
Reg, G4FGQ

Reg Edwards wrote:

What everybody forgets about is that the velocity factor along
ground-mounted radial wires is about half of the free-space value.

Consequently, the 1/4-wave resonant length is crudely only half of the
elevated value.

On the other hand, the resonant length is very non-critical because Q
is very small - Q is only 2 or 3 and is even smaller at the high end
of the HF band.

Actually, at HF and average ground, the velocity factor below the ground
is about 1/4 to 1/5 the free space value. And no resonance at all is
usually apparent because of the high loss.

EZNEC isn't among the "everybody" who's forgotten it. Choose any real
ground, open the Utilities menu and click Ground Info, and you'll see
the velocity factor along with other information. But it's seldom of any
practical use.

Roy Lewallen, W7EL

Cecil Moore wrote:
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
jawod wrote:
I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.
Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.
Learning this stuff can be a lot of fun. It can be frustrating, too.

Just last month, with four elevated 40 meter radials 6 feet high, the
antenna was about 5 dB weaker than the very same antenna with 16
radials laid directly against soil.

This basic result repeated at three different soil locations on three
different bands, 160, 80, and 40, so it is not a fluke.

In my last quick measurement on 7MHz:

16 long radials directly on the earth (no attempt to make resonant
since they have very low Q) 0dB reference

8 long radials on the ground -1.3dB reference

4 long radials on the ground -3dB reference

4 resonant elevated radials at six feet -5.6dB reference

73 Tom

Sometimes the more I learn the less I know.

I have dabbled a lot along these lines in the last few months. I have
determined my methods are faulty.

When I compare two or more antennas for gain I have no means to
measure the actual gain because I really don't know what the radiation
pattern is in real life.

However, comparing measured gains with calculated gains has given me
more confidence in the EZNEC calculations.

I have limited my test antenna to a 30 foot vertical with radials
consisting of electrical extent ion cords connected in parallel
stretched out on the ground. I seem to be manipulating the take off
angle and the impedance of the feed by adding and subtracting these
radials. The vertical seems to be more quiet (fewer signals) than a
dipole but pretty much the same strength on those it hears. The
reference dipole is the 40M section of my CushCraft A3S Beam at about
40 feet.

The only certain conclusions I have made are that getting high
confidence numbers about radials is a lot of work and probably beyond
my resources. The ARRL Antenna Handbook and EZNEC are usually right.

Usually right...
If you lie to EZNEC it will lie right back to you with an even bigger
lie. Be very careful with assumptions!

The Antenna Handbook...
There is still the unresolved issue of conjugate matching. I noted
last week or so that a copy of Walter Maxwell's book that retailed for
$19.95 went for about $75 on EBAY.

John W8CCW

On 10 Jul 2006 04:01:01 -0700, wrote:



Cecil Moore wrote:
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
jawod wrote:
I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.
Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.
Learning this stuff can be a lot of fun. It can be frustrating, too.

Just last month, with four elevated 40 meter radials 6 feet high, the
antenna was about 5 dB weaker than the very same antenna with 16
radials laid directly against soil.

This basic result repeated at three different soil locations on three
different bands, 160, 80, and 40, so it is not a fluke.

In my last quick measurement on 7MHz:

16 long radials directly on the earth (no attempt to make resonant
since they have very low Q) 0dB reference

8 long radials on the ground -1.3dB reference

4 long radials on the ground -3dB reference

4 resonant elevated radials at six feet -5.6dB reference

73 Tom
John Ferrell W8CCW

David wrote:
Image theory is for a perfect groundplane e.g. large area metal sheet. The
wave emitted by the vertical radiating element is reflected by the ground
plane.

Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.
Currents flowing in skin depth of ground plane emit a wave of opposite
polarity to cancel out the wave at the boundary of the ground plane, thus
making the electric field in the ground plane zero. The wave of opposite
polarity is the reflected wave. The reflected wave appears to be coming from
an image antenna. Image theory is a mathematical model for solving antenna
simulations where there is a monopole over a ground plane.

How do the radials reflect the wave? If they are not a good enough ground
plane because of the gap, how do they reflect? I cannot see the transition
from ground plane to radials, when looking at image theory.

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. Then imagine thin radial slots spread
around the vertical. Since these slots do not cross any current path
that is needed to produce the image, they have little effect on the
image. Widen those slots, and decrease the number of them, and
eventually you get to a ground radial system with only a few radials.
There has to be a transition point, where the radials are only a
poor approximation of the original disk. The question is, how well
must you approximate the disk to get a reasonable approximation of the
far field radiation pattern it would have helped produce?

Sal M. Onella wrote:
"David" nospam@nospam wrote in message
...

A normal ground plane is a large sheet of metal that reflects the radio

wave

emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?


Effectively, yes.

A metallic surface (your "sheet of metal") can be replaced by a partially
metallic surface -- within limits. If you keep the size of any gap under
1/10 wavelength, the surface will appear solid. This I know from satellite
reflector work.

The use of four radials appears to be a compromise for using a solid
surface, but it obviously works. The RF sees these radial wires and behaves
like we want. I think adding more radials will always make a better
counterpoise, but I also think you reach the point of diminishing returns
pretty quickly. (We aren't the first ones to speculate about this, after
all :-)



Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from 4 or
more radials. EZNEC shows very little change in terminal impedance and
pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

Cheers,
John

John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from 4 or
more radials.

Well, in theory yes, but in the real world , usually no. The reason
being the decoupling. Four or more radials will decouple the line
quite a bit better than two. I did tests adding radials to a VHF
ground plane, and I saw improvement with each addition of radials
I tried. Eight radials was a noticable improvement over four.
But I always put it down to the improved decoupling of the feedline,
rather than any big decrease in ground losses.
I imagine if you used separate decoupling sections to avoid feeder
radiation, the number of radials would matter little if any.
As far as elevated ground planes vs ground mount...Cecil does
have a point. It's common knowledge that a real low ground plane
generally sucks. You need many, many, more radials to equal
the ground loss of one at 1/2 wave up. While I don't doubt that
the low ground plane was beaten by the ground mount in Tom's test,
very few people actually run ground planes that low. If they do, they
can count on me to berate them for it.. IE: I often jumped on Cecil
for using one at appx 1/8 wave, and wondering why it didn't work too
well.
Thats too low, unless you have a lot of radials. In my observations
comparing ground planes, you really need to be at least 1/4 wave in
the air if you are going to use only four radials. Even then, thats not
optimum. At 1/4 wave up, 8-12 radials is closer to optimum. Four
radials at 1/4 wave is appx equal to about 60 on the ground.
By "optimum", I mean equals 120 radials on the ground...
Myself, I had a full length monopole on 40 m, with 32 ground radials.
It was rarely much better than my dipole on medium long paths of say
1500 miles. When I elevated the antenna to 1/4 wave, and used only
four radials, the performance was much better. Like day and night
really. So I agree, if you run an elevated GP, it needs to be up in the

air, or else you will need many radials. At 1/8 wave up, you need appx
60 radials to equal the 4 radials of the same antenna at 1/2 wave up.
I've heard many a tale of people running low band ground planes, real
low
to the ground, and having bad results. But you won't hear those bad
stories from the ones that run them at 1/4, 1/2 WL up.
MK