"Jerry Martes" wrote in
news:_XUqi.1027$FO1.53@trnddc05:


"Owen Duffy" wrote in message
...
Owen Duffy wrote in
:

"Jerry Martes" wrote in
news:ScTqi.3733$8u1.2359@trnddc07:


"Ed" wrote in message
92.196...

This is the first time I've done this. If the need arises again,
I'll
have to download and try EZNEC, I guess.


Meanwhile, I am concerned about the efficiency of our ARES
Command
Center HF antenna system.


Could someone please advise me what feedpoint SWRs would be
seen on a
wire antenna, 15 feet above a flat metal roofed building, (it a
NVIS antenna), and resonant at 6.2 MHz.


It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz


The 100 watt radio has one of those built in tuners that can
only
handle 3:1 SWR. I suspect we are terribly inefficient.



Jerry, are you saying that your model suggests the Z~=72+j0 at
resonance, and the half wave dipole is ~0.1m above a perfectly
conducting plane. It doesn't seem right.

That should read "0.1 wl above..."

Hi Owen

I was kinda surprised too. But, I have alot of faith in EZNEC.
The
problem of determining the input impedance of such a simple antenna
over a perfectly conducting ground seemed too easy to model, so I did
that.
EZNEC tells me that the 75 foot copper wire thats 0.040 diameter and
15
feet above ground looks like 72.33 +j0.5972 ohms.at 6.4 MHz.

Ok, I get around 20 ohms at resonance.

We have obviously made some different interpretations somewhere... but I
expect a number smaller than 72 due to the proximity of the conducting
plane.


My experience with Roy's program has always indicated that it is
more
accurate than my actual measurements. I would appreciate hearing
where EZNEC cannot be trusted.

I didn't say that, nor did I imply so, and to infer so is a YKW trick.

It is not the tool, it is the difference in the way we have each built
the model. As it happens, I am using EZNEC 3, but I expect I would get
exactly the same answer from any NEC-2 based tool, 4NEC2 for instance. (I
have mailed you my model, you can compare them and see where they are
different.)

Owen

Glossary: YKW=You Know Who