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?

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

wrote in message
oups.com...
Is that a joke?

No, in most implementations, ground radials are very much attempting to
simulate a solid, perfectly conducting plane under the antenna. Given
enough of them, they do a reasonably good job.

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

As you go to few and fewer radials, the 'pull' of the 'simulated' ground
becomes 'weaker' in a sense and the radiation pattern of an antenna with
horizontal radials tends to have its maximum at an angle significantly above
the horizontal plane. By angling the radials downward, the radiation
pattern is pulled back downward and the maximum radiation is again more or
less horizontal.

(If you don't like this 'maybe intuitive to me and not at all to you'
explanation, you can simulate an antenna with radials in, e.g., ezNEC and
see what the actual results are...)

---Joel Kolstad

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 in message
...
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.

Doesn't it somewhat depend on frequency? I.e., how electrically large those
radials appear to the antenna?

I ask due to having seen how commercial AM radio station antennas are
built -- usually something pushing a dozen radials, often over a wire mesh
as well.

I'm thinking that in the case of a commercial station, they often multiple
phased antennas to try to precisely control their radiation pattern, in
which case have each antenna be 'as ideal as possible' probably helps.

---Joel

Joel Kolstad wrote:
wrote in message
...
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.

Doesn't it somewhat depend on frequency? I.e., how electrically large those
radials appear to the antenna?

I ask due to having seen how commercial AM radio station antennas are
built -- usually something pushing a dozen radials, often over a wire mesh
as well.

I'm thinking that in the case of a commercial station, they often multiple
phased antennas to try to precisely control their radiation pattern, in
which case have each antenna be 'as ideal as possible' probably helps.

---Joel

Notice the words "For an elevated antenna" which presumes you are working
at a frequency where there is no problem with 1/4 wave radials.

For low frequencies, as in AM broadcast and the lower HAM bands, elevated
antennas become impractical and must be ground mounted, which means the
radials are usually buried as well as there may not be enough room
for 1/4 wave radials.

For radials on or in the ground, usually 4 to 8 1/4 wave radials is
good enough.

If space is limited so 1/4 wave radials aren't possible, the number
required goes up.

The ARRL Antenna Handbook has a good discussion on this.

You might also look at http://www.cebik.com/radio.html which has a couple
of articles about radials, buried and otherwise.

--
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...