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.

wrote:

M. J. Powell wrote:
In message .com,
writes
I have seen some with 4 elements, some with 5. Also is the spacing
between them important?

A ground plane is an attempt to simulate the earth, with better
conduction, so the more radials the better.

Mike
--
M.J.Powell


Is that a joke?

I'd call it poorly put, but not a joke.

On another note, why are some of the radials bent to a 45 degree angle
and some are not?

To get a better match to 50 Ohms.

--
Jim Pennino

Remove -spam-sux to reply.

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

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...

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

Sorry, fellas, I intended to mention in my previous post that I worked
with Dr. George H. Brown in his antenna lab at the RCA Laboratories in
Princeton, and he took great delight in telling me about the 'ruse'
that Dan, K6MHE,related concerning the ground plane with only two
radials. 'Ya got that right, Dan Boy!

Walt, W2DU

wrote:

In a ground plane, what dictates the number and spacing of
radials?

The height of the antenna above ground in wavelengths. Yes, at a 1/2
wave
or more up, even just two radials are pretty decent. But at 1/8 wave or

lower, 2 radials are just barely above the "waste of time" level as far
as
reducing ground losses... Take two ground planes. Both are at 20 ft at
the base. Each has two radials. But one is for 145 mhz, and the other
is
for 1.85 mhz. Do they have equal ground losses? Not hardly... MK