On 10/4/2015 5:08 PM, Jeff Liebermann wrote:
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.

I don't know why you are talking about "a real ground" when the context
was a ground plane antenna.

"It is a ground plane with 4 radials (typical). Free space."

Clearly that can exist. You said you can't have a "ground plane". The
antenna has a ground plane no matter where it is.


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".

No one but you is talking about an "earth ground". The comment was
simply about a ground plane antenna in free space. I don't think the
name "ground plane antenna" requires the antenna to have any relation to
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.

But no one said anything about an "earth" ground except you. A "ground
plane" antenna is the topic. No one else said anything about a
"grounded" antenna.

Have you had too much coffee today?

--

Rick

On Sun, 4 Oct 2015 19:44:24 -0400, rickman wrote:

I don't know why you are talking about "a real ground" when the context
was a ground plane antenna.

"It is a ground plane with 4 radials (typical). Free space."

Clearly that can exist. You said you can't have a "ground plane". The
antenna has a ground plane no matter where it is.

Argh. You're right. I misread the original question.

Have you had too much coffee today?

No. I've been stacking firewood, dragging junk around, and doing
other odd jobs around the house today. When I get tired, I sit down
at the computah and post wrong information and bad advice at about 1
hr intervals. Maybe I shouldn't do that.

Sorry(tm).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sunday, October 4, 2015 at 9:05:23 AM UTC-5, John S wrote:
How less efficient is a short vertical than a 1/4 vertical?

I would like to see some numbers. It is a ground plane with 4 radials
(typical). Free space.

Assume a source at the base. The type of source is your choice. EZNEC
defaults to one amp, but can be changed to a constant power of your choice.

I'm sure I've left out additional requirements, but maybe this will be
a healthy discussion even so. Suggestions are welcome.

One could spit out numbers for days given all the possibilities.
Too vague are the specs.. IE: you state four radials, but model in
free space. So would ground losses be an issue or not? Also the height
above ground in wavelength makes a large difference. Four radials at 1/2
wave up provide low ground losses, but four radials at 1/10 wave up are
not so hot. Much higher ground losses.

Being as all short radiators radiate nearly all power fed to them, barring
any small resistive losses, the only thing left are the matching losses.

And for playing "what if", a program like "vertload" could be used for
getting an idea of the efficiency of the various length radiators and
spit out the number of turns needed, etc.. Will give the efficiency using
whatever ground number you punch in as I recall.
That's what I used to use when building mobile whips. I think it also
lets you adjust the whip both below and above the coil. So you can vary
the location of the coil.

The only problem with vertload is it's old and DOS I think, so newer OS's
won't run it without a DOS BOX or whatever.. XP will run em as is..
So I can still use them on my old laptop. This box is Win7 64, and it won't
run em without the DOS program, which I haven't bothered with yet.
But they say it will work.. I forgot the exact name of the DOS emulator,
would I'm sure google knows what and where it is.