"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.

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.

Jerry