Walter Maxwell wrote:

The terminal resistance will be 50 ohms at some angle in between, and
is usually close to 45°.

Not to mention that's a damn handy angle when you need the
radials to double as guy wires... ;)

The terminal resistance will be 50 ohms at some angle in between, and
is usually close to 45°.


Not to mention that's a damn handy angle when you need the
radials to double as guy wires... ;)


Not to mention further, it helps keep birds off!!



Ed

You only need two to result in field cancellation
to prevent radiation. However, that is only the
case in free space. Any near objects or objects
in electrical contact may distort
the field from one of the elements differently
from the other, so resulting in less-than-perfect
cancellation, especially in the case of ground radials.

"Dan Richardson" wrote in message
...
On 10 Jan 2005 08:48:37 -0800, wrote:
I have seen some with 4 elements, some with 5. Also is the spacing
between them important?
The ¼-wave groundplane was developed by George Brown 1938. Here's a
partial quote from his book:
"... In our initial experiments we found that only two horizontal rods
(ground rods) functioned as well as four. Many people from the
Broadcast Sales organization came by to view our tests and they always
expressed doubts as to the ability to radiate uniformly when only two
ground rods were used. To quiet them, we used four ground rods for a
while, thus stilling the criticism. When the antenna became really
popular, we did not dare confess to our ruse."
There you have it from the inventor of the antenna.

Such an excellent and succint didactic exposition deserves
wider recognition.....

"Walter Maxwell" wrote in message
...
Assuming the terminal resistance of a resonant dipole is 72 ohms,
then a ground plane separating the halves of the dipole means the
terminal resistance of each half is 36 ohms. Thus the terminal
resistance of the half-dipole over the ground plane is also 36 ohms.
The terminal resistance of the half dipole operating against the
radials bent down can then be any value between 36 and 72 ohms,
depending on the angle of the bending. If the bending changes the
angle from 90° to 180° the resistance has changed from 36 to 72 ohms.
The terminal resistance will be 50 ohms at some angle in between, and
is usually close to 45°.
Hope this helps in understanding what occurs from bending the radials
downward.
Walt, W2DU

PS. The same effect occurs in the so-called "capacity hat".
The waves rushing out and returning in both directions result
in field cancellation and no radiation from the hat in the
ideal case. "Capacity Hat"? - Because the delayed returning
waves in recombination at the top of the radiator behave
as though they have picked up a capacitive phase change.

"Airy R.Bean" wrote in message
...
You only need two to result in field cancellation
to prevent radiation. However, that is only the
case in free space. Any near objects or objects
in electrical contact may distort
the field from one of the elements differently
from the other, so resulting in less-than-perfect
cancellation, especially in the case of ground radials.

"Dan Richardson" wrote in message
...
On 10 Jan 2005 08:48:37 -0800, wrote:
I have seen some with 4 elements, some with 5. Also is the spacing
between them important?
The ¼-wave groundplane was developed by George Brown 1938. Here's a
partial quote from his book:
"... In our initial experiments we found that only two horizontal rods
(ground rods) functioned as well as four. Many people from the
Broadcast Sales organization came by to view our tests and they always
expressed doubts as to the ability to radiate uniformly when only two
ground rods were used. To quiet them, we used four ground rods for a
while, thus stilling the criticism. When the antenna became really
popular, we did not dare confess to our ruse."
There you have it from the inventor of the antenna.

On Tue, 11 Jan 2005 08:50:27 -0000, "Airy R.Bean"
wrote:

Such an excellent and succint didactic exposition deserves
wider recognition.....

Whilst an interesting contribution, for which thanks are due to the
OP, many of us knew this already. Didn't you, Bean? If you already
knew this, why have you not explained it before? Can't you write in
excellent, succinct, and/or didactic fashion? Or have you just gone up
the learning curve?

"Walter Maxwell" wrote in message
.. .
Assuming the terminal resistance of a resonant dipole is 72 ohms,
then a ground plane separating the halves of the dipole means the
terminal resistance of each half is 36 ohms. Thus the terminal
resistance of the half-dipole over the ground plane is also 36 ohms.
The terminal resistance of the half dipole operating against the
radials bent down can then be any value between 36 and 72 ohms,
depending on the angle of the bending. If the bending changes the
angle from 90° to 180° the resistance has changed from 36 to 72 ohms.
The terminal resistance will be 50 ohms at some angle in between, and
is usually close to 45°.
Hope this helps in understanding what occurs from bending the radials
downward.
Walt, W2DU


--
from
Aero Spike

Ok, I think I am learning something here. If radials simulate earth,
would using a solid steel plate instead of radials be better?

wrote:
Ok, I think I am learning something here. If radials simulate earth,
would using a solid steel plate instead of radials be better?

OK till it rusted away. Some very fine antennas have used
metal roofs for their ground planes.
--
73, Cecil http://www.qsl.net/w5dxp


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wrote:
Ok, I think I am learning something here. If radials simulate earth,
would using a solid steel plate instead of radials be better?

Depends on how you define "better".

For an elevated antenna, once you get beyond about 3 or 4 radials, the
increamental difference in performance for added radials is such that
you would never notice it in a practical application.

A solid plane has the disadvantages of being heavy, has a much larger
wind loading, and is difficult to "droop" to get closer to 50 Ohms.

You can download the demo version of EZNEC from http://www.eznec.com/
and model a simple vertical with varying numbers of radials and see
for yourself.

--
Jim Pennino

Remove -spam-sux to reply.

On Tue, 11 Jan 2005 09:41:57 +0000, Spike
wrote:
many of us knew this already. Didn't you, Bean? If you already
knew this, why have you not explained it before?

Hi OM,

Much the same faint complaint could be lain against you, which is to
say, seeing as you "knew this" why didn't you explain it as well?

This is simply stealing Walt's thunder. It takes only once, and that
moment passed.

73's
Richard Clark, KB7QHC