"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 :-)

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

Ok I am getting confused. You are saying that a groundplane will not
work as good a a ground mounted vertical ? At what angle are you
talking about? Are you more interested in working 500 miles or 6,000
miles?

Ron

Ron wrote:
Ok I am getting confused. You are saying that a groundplane will not
work as good a a ground mounted vertical ? At what angle are you
talking about? Are you more interested in working 500 miles or 6,000
miles?

Ron

Nope. I think the elevated ground plane is superior to the
ground mount as long as it's high enough in the air to avoid
excess ground loss. As far as long haul, there may not be
too large a difference if each system is equal as far as ground
loss. IE: a ground mount with 120 radials, and a GP at 1/2 wave
high with 4 radials should show about the same efficiency. So
for long haul dx, they should be fairly close in theory. But...
You have a better ground/space wave with the elevated antenna.
This can come in handy when talking 50-100 miles away when
the band doesn't support NVIS with a dipole, etc..
When you run the elevated antenna, you must always think of
height in terms of wavelength, not feet or meters. A 2 meter GP
can be fairly low, and still very efficient. But not a low band GP.
A half wave is a different height on each band. Being I recommend
a minimum of 1/4 wave height when using only 4 radials, that
can be pretty high on a lower frequency. On 40m, I ran one at
36 ft at the base of the radiator. Thats just over 1/4 wave up.
If I ran the same antenna on 80m, I would have to mount it at
72 ft to have the same efficiency. About 145 ft on 160m.
Soooo...If you can't go that high, you must increase the number
of radials to lower the ground losses to a equal number.
If you have a ground mount with 120 radials, you need about 60 radials
if the antenna is at 1/8 wave. About 8-12 radials if the antenna is at
1/4 wave. About 3-4 radials if the antenna is at 1/2 wave. All these
have the same appx ground losses. So you can see, if you run a 80m
ground plane at 15 ft, the ground losses will be high unless you use
a whole lot of radials. So in that case, it's really more practical to
use the ground mount unless you don't mind all that wire in the air.
But equal loss ground mount vs ground plane? I'd take the ground
plane anyday... I ran one on 40m and it kicked serious butt on
long haul dx. And yes, I use the verticals on the low bands for
mostly long haul. I use dipoles, etc for NVIS.
MK

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. 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.

George Brown, the inventor of the ground plane antenna, found that only
two radials were necessary. But when his company went to sell it, the
marketing department decided that no one would buy a two-radial ground
plane antenna in the belief that it would be omnidirectional. So they
added two more to make it "look" more omnidirectional. The four-radial
ground plane persists to this day.

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Did this antenna include any provision to prevent current on the
outside of the feed line?

Which direction did the feed line exit the antenna?

John Popelish wrote:
Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper
tape on a Duroid dielectric material, a lot like the window antenna
John described. An omnidirectional pattern was a requirement, and I
was concerned that either the flatness of the tape or the presence of
the dielectric might have some impact on the circularity of the
pattern. So I had it tested at a local lab. It was the most circular
pattern they'd ever seen, having about 1 dB maximum difference between
any two directions.


Did this antenna include any provision to prevent current on the outside
of the feed line?

Which direction did the feed line exit the antenna?


I don't know about Roy's antenna, but this subject has come up before,
and at the time I made a two meter vertical ground plane with only two
radials. No matter how I oriented the antenna, radially, I got the same
signal strength on my field-strength meter. And yes, I took precautions
to make sure the feedline wasn't radiating. (Many ferrite beads at
strategic places on the feedline to the point that feedline radiation
was undetectable.) If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

It's pretty easy to understand. Any two radials,
180 degrees apart and high enough, should theoretically
cancel each other's radiation in the far field.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote

It's pretty easy to understand. Any two radials,
180 degrees apart and high enough, should theoretically
cancel each other's radiation in the far field.
--
73, Cecil
=====================================

If they don't cancel-out each other in the near field then they don't
cancel-out each other in the far field either.

A pair of radials behave as a continuous dipole fed at its center via
a single wire. And it radiates.

A circular disk, diameter = 1/2 wavelength, fed at its centre
radiates.

But don't ask me what its radiation resistance is. It must be very
low.
----
Reg.