On Sun, 4 Oct 2015 15:24:39 -0400, rickman wrote:
On 10/4/2015 1:36 PM, 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.
Why not?
Because a free space model is defined as the absence of a "real
ground", "earth ground", or something sufficiently away from the rock
that you're standing on so that its influence is very small on the
model. That's usually measured in wavelengths. Offhand, anything at
least 10 wavelengths above the nearest ground structure (ground,
trees, buildings, etc) can be ignored. For VHF/UHF, that's a fairly
small distance. For HF, much longer.
Is not the ground just the other terminal on the antenna
connected to the radials?
Nope. Which radials? The radials in a common "ground plane" antenna
are certainly not considered an "earth ground". However, the buried
counterpoise that forms the other half of a monopole antenna is
certainly an earth ground. Note that I would need an NEC4 runtime to
model a below ground radial counterpoise system.
Ground doesn't have to be earth ground or
anything else. It is just a defined reference point.
I think the problem is too many definitions of ground here. In my
world, "earth ground" means just that. It's the rock you're standing
on. A "grounded" antenna, is one that uses the earth as the
counterpoise. A "safety or lightning ground" is a path for
atmospheric electricity and does not usually enter in the
calculations.
Maybe some examples might help.
1. I want to model a UHF (440 MHz) vertical "ground plane" antenna
mounted on a pole on my roof. The roof is 20ft high and the antenna
is mounted on top of a 10ft pole. How high above "ground" do I make
my antenna model?
2. Same antenna, but with a #12 solid ground wire running to a ground
rod pounded into the ground. How high above "ground" do I make my
antenna model?
3. Assuming the pole is made from fiberglass, and RF power amp is
mounted at the antenna (common for cellular TMA installs), will a free
space model work?
4. If the 10ft pole it transplanted to the ISS, is there an "earth
ground" and how high?
Scroll down for my answers.
Looks like I'll be busy for a few days, so I might as well answer my
own questions now:
1. 10 ft. At UHF, the house is considered part of the earth ground
especially if the roof is corrugated steel or full of chicken wire.
Same with any LARGE mounting structure (in terms of wavelengths).
2. 10 ft. The ground wire is presumably not part of the radiating
parts of the antenna and can be ignored if shielded by the ground
plane wires. If long enough, it looks more like an RF choke
(inductor) than an antenna element.
3. Yes, free space will work, although nearby metal structures and
wires need to be considered if they can be "seen" by the radiating
elements.
4. Toss a coin and 10 ft. The ISS is large enough (in terms of
wavelengths at UHF) to be considered a perfect ground, as in my
monopole examples. However, if you need an accurate model, the ISS
has enough complex shapes that can re-radiate RF, and should probably
be modeled as part of the antenna system, in free space and without
any influence from the planetary rock.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558