Jeff Liebermann wrote:
On Sun, 4 Oct 2015 18:56:18 -0000, wrote:
Jeff Liebermann wrote:
On Sun, 4 Oct 2015 09:05:57 -0500, John S wrote:
I would like to see some numbers. It is a ground plane with 4 radials
(typical). Free space.
I just noticed the contradiction. You can't have a grounded antenna,
or a ground plane in free space, where there is no ground. Also, as
Jim mentioned, ideal antennas in free space have no dissipative
losses. Try again please.
What do you mean you can't have a ground plane in free space?
There are no ground losses in free space, but ground plane antennas, i.e.
a radiator with radials, work just fine.
I'll stand my ground and defend my assertions on good grounds. Permit
me to explain from the ground up.
If you mount a monopole on the ISS, the NEC model would probably use
the space station surface as a ground. That's not exactly what I
would call "free space" because it costs so much to get into space,
but that doesn't enter into the calculations. The space station would
form a suitable ground plane where its presence in outer space is an
incidental coincidence. However, that's not the same as an "earth
ground", which is what I mean by a "real ground". Also, If I fire up
4NEC2, and setup the antenna in free space, the various grounding
options become grayed out. That would suggest that there ain't no
such thing as a ground or "earth ground" in expensive, errr... free
outer space.
Of course, I could design what is commonly called a "ground plane
antenna", which would rhetorically have a "ground plane". However,
that's not the same as an "earth ground". For example, if you
actually model a "ground plane antenna" over an earth ground, you
might end up with two grounds, which make little sense. Also,
adjusting the height above ground in a "ground plane antenna" make
equally little sense. The radials that form a "ground plane" should
be renamed to something more definitive, such as a conical
counterpoise or conical grounded sleeve antenna.
Don't know about 4NEC2, but EZNEC has no problem with a ground plane
antenna in free space.
The ground plane in a ground plane antenna usually refers to the radial
elements attached to the bottom end of the radiator, so I don't see
any problem with such a configuration.
Now whether or not a ground plane antenna is a usefull design to use in
space is a separate issue.
But what is interesting is to model a ground plane over real ground
and step the height above real ground and observe what happens to the
pattern as the height goes from zero to a couple of wave lengths.
While getting a 40M ground plane up a half wave length would be a
challenge, at 12M and above it is not, and at 6M, most ARE mounted
at about 1 wavelength.
Ummm... how do I model coffee grounds or has this discussion ground to
a halt?
Depends on whether or not I have intrigued you enough to grind the
numbers for a ground plane at various heights.
--
Jim Pennino