"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.


Thanks for any response.


Ed K7AAT


Hi Ed

I probably dont quite understand your situation. But, if your
question
is referenced to a 75 foot long center fed dipole 15 feet above a
metal ground, EZNEC shows the terminal impedance will be 72 ohms at
6.4 MHz. The VSWR is under 3:1 from about 6.05 MHz to 6.5 MHz.


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.

Owen