On Thu, 13 Jan 2005 18:56:36 -0600, Cecil Moore
wrote:

W9DMK (Robert Lay) wrote:

(ZZZPK)
wrote:
capacitance is prop to gap between plates.

I think you meant that it's inversely proportional to the gap between
the plates.

I've made the same mistake - said "capacitance" when I really
meant "capacitive reactance".

No way, Cecil - you haven't made any mistakes this whole year!

Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

W9DMK (Robert Lay) wrote:
No way, Cecil - you haven't made any mistakes this whole year!

Actually, it was probably a mistake to quote the "IEEE
Spectrum" magazine article regarding cellphones and
brain tumors. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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Walter Maxwell wrote:
But a ground plane antenna suffers no
ground losses, so using as many as possible only applies to radials
on
or in the ground.

Walt, W2DU

Or low to the ground in terms of wavelength. But you wouldn't have to
use
"as many as possible". With most VHF/UHF ground planes, they are
usually
several waves up off the ground, and ground loss is very low. But if
you
take a low band ground plane, say 1/8 wave off the ground, you will
need
quite a few more radials than 4, to equal the benchmark of 120 radials
on
the ground. According to charts I've seen, and also backed up with real

world results, a ground plane 1/8 wave off the ground will need appx
50-60 radials to equal the 120 on the ground. At 1/4 wave up, about
8-10
or so. At 1/2 wave up, 4, 3, or just 2 radials will all work fairly
well.
Of course, I would always prefer four, over two. Even if ground loss
was
not a factor due to being high up, the decoupling of the feedline is
better with four, than two. In real world tests on VHF, I've noticed a
noticable difference going from 4 radials, to say 8 , or even 10. And
that was several waves up...So adding more radials does continue to
improve the antenna. Probably more due to the improved decoupling of
the feedline, rather than lower ground loss I would suspect...I've
heard
many a tale of disappointed hams having poor results with "low" ground
planes, and not enough radials. But thats usually on 160,80, or 40
meters.
Actually, I don't know of any ground plane users on 160, but I have
heard of quite a few on 80m..Four radials at 10 ft up on 80m, is better

than four radials on the ground, I think, but not by a large amount...
Ground loss will be fairly substantial if the ground quality is
mediocre.
In my HF experience with them, the lowest I would use four radials, and

expect *good* performance, would be at 1/4 wave up. When I would lower
the mast to 1/8 wave up, you could see quite a difference. That was on
40m, where I ran a full size GP at 36 ft at the base, on a pushup mast.

BTW, that was a great DX and late night antenna on 40m...I also had a
24 volt relay to switch a base loading coil in for 17m use as a 5/8 GP.
I changed bands here in the shack, by unplugging the transformer, for
40m use..That bypassed the 17m coil.
MK

Joel Kolstadt wrote:
"Hmm...how about...three! ground radials?"

Three radials should be fine where they are elevated to such height that
they capture all the electric lines of force from the vertical
radiator.Radials are balanced and their currents travel in offsetting
directions. The radial system does not radiate itself because of its
offsetting balances. Elevated radials must shield the earth from induced
current. This requires few radials when the radials are far above the
earth, but where the radials are near to the earth, many radials are
needed to capture all the electric lines and shield the earth from lossy
currents.

The number of radials and their effect on pattern and efficiency of
radiation from a vertical antenna is well addressed by ON4UN in
"Low-Band DXing". This is found in Chapter 9 of my 2nd edition. Choose
the efficiency and elevation angle you are willing to accept ftom the
graphs and tables presented.

Best regards, Richard Harrison, KB5WZI

On 14 Jan 2005 11:09:33 -0800, wrote:


Walter Maxwell wrote:
But a ground plane antenna suffers no
ground losses, so using as many as possible only applies to radials
on
or in the ground.

Walt, W2DU

Or low to the ground in terms of wavelength. But you wouldn't have to
use
"as many as possible". With most VHF/UHF ground planes, they are
usually
several waves up off the ground, and ground loss is very low. But if
you
take a low band ground plane, say 1/8 wave off the ground, you will
need
quite a few more radials than 4, to equal the benchmark of 120 radials
on
the ground. According to charts I've seen, and also backed up with real

world results, a ground plane 1/8 wave off the ground will need appx
50-60 radials to equal the 120 on the ground. At 1/4 wave up, about
8-10
or so. At 1/2 wave up, 4, 3, or just 2 radials will all work fairly
well.
Of course, I would always prefer four, over two. Even if ground loss
was
not a factor due to being high up, the decoupling of the feedline is
better with four, than two. In real world tests on VHF, I've noticed a
noticable difference going from 4 radials, to say 8 , or even 10. And
that was several waves up...So adding more radials does continue to
improve the antenna. Probably more due to the improved decoupling of
the feedline, rather than lower ground loss I would suspect...I've
heard
many a tale of disappointed hams having poor results with "low" ground
planes, and not enough radials. But thats usually on 160,80, or 40
meters.
Actually, I don't know of any ground plane users on 160, but I have
heard of quite a few on 80m..Four radials at 10 ft up on 80m, is better

than four radials on the ground, I think, but not by a large amount...
Ground loss will be fairly substantial if the ground quality is
mediocre.
In my HF experience with them, the lowest I would use four radials, and

expect *good* performance, would be at 1/4 wave up. When I would lower
the mast to 1/8 wave up, you could see quite a difference. That was on
40m, where I ran a full size GP at 36 ft at the base, on a pushup mast.

BTW, that was a great DX and late night antenna on 40m...I also had a
24 volt relay to switch a base loading coil in for 17m use as a 5/8 GP.
I changed bands here in the shack, by unplugging the transformer, for
40m use..That bypassed the 17m coil.
MK

You're quite correct, Mark, but when I mentioned 'ground plane' I'm
thinking in terms of VHF/UHF, with the antenna many wavelengths above
ground.

Thanks for clarifying my omission of terms.

Walt, W2DU

(Robert Lay) wrote:

: : If not, what reason does he give for the technique?
:
: capacitance is prop to gap between plates.
:
:
: I think you meant that it's inversely proportional to the gap between
: the plates.
:

its still proportional....

Cecil Moore wrote:

: W9DMK (Robert Lay) wrote:
:
: (ZZZPK)
: wrote:
: capacitance is prop to gap between plates.
:
: I think you meant that it's inversely proportional to the gap between
: the plates.
:
: I've made the same mistake - said "capacitance" when I really
: meant "capacitive reactance".
: --

capacitance and capacitive reactance are two separate things.

one is always there and the other needs something else

ZZZPK wrote:
"Capacoitance and capacitive reactance are two separate things."

Yes. But, they are inextricably related by:

Capacitive reactance = 1 / 2 pi f C
where pi = approx. 3.1416,
f = frequency in Hertz,
Capacitance is in Farads, and
capacitive reactance is in Ohms.

Best regards, Richard Harrison, KB5WZI

On Sat, 15 Jan 2005 15:15:31 GMT,
(ZZZPK)
wrote:

(Robert Lay) wrote:

: : If not, what reason does he give for the technique?
:
: capacitance is prop to gap between plates.
:
:
: I think you meant that it's inversely proportional to the gap between
: the plates.
:

its still proportional....


That doesn't qualify as worthy of a response.
Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

(Robert Lay) wrote:

: : I think you meant that it's inversely proportional to the gap between
: : the plates.
: :
:
: its still proportional....
:
:
: That doesn't qualify as worthy of a response.


a relative bearing is a bearing.

inverse proportional is a proportion.



a is a subset of b

get the idea ?