"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 (who used 1 mm diameter copper wire)

"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

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..."

"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..."

The answer to the question would seem to be that it would not work very well
just due to experiance. The swr would be off the charts and a rig that had
an internal tuner would not tune that kind of antenna over that frequency
range if the swr had to be under 3:1. Also the loss in the coax due to the
high swr would be very large..

.."

The answer to the question would seem to be that it would not work
very well just due to experiance. The swr would be off the charts and
a rig that had an internal tuner would not tune that kind of antenna
over that frequency range if the swr had to be under 3:1. Also the
loss in the coax due to the high swr would be very large..



Well, that was my original surmise of the situation. I was just
trying to have someone with expertize on EZNEC run the antenna at the
three frequencies listed in order to have the feedpoint SWR calculated
for each of those three frequencies. So far, no one, except Owen, has
come even close. I can work with the figures Owen provided for the one
frequency ( 3975) and do the rest of the calculations manually to
determine the SWR on that frequency, but had hoped for actual feedpoint
SWR data for all three actual transmit frequencies listed.

Thanks to all.

Ed

Ed wrote in
.92:

....
Well, that was my original surmise of the situation. I was just
trying to have someone with expertize on EZNEC run the antenna at the
three frequencies listed in order to have the feedpoint SWR calculated
for each of those three frequencies. So far, no one, except Owen, has
come even close. I can work with the figures Owen provided for the one
frequency ( 3975) and do the rest of the calculations manually to
determine the SWR on that frequency, but had hoped for actual feedpoint
SWR data for all three actual transmit frequencies listed.

Ed, I am giving you information that is not sufficient to solve the
problem.

Nevertheless, I defer to your opinion.

Read my earlier caveats about my reading of your limited information and
interpretation in a model.

Freq VSWR at feedpoint (RG8/U)
3.98 959
5.4 115
7.2 34

Owen

Ed, I am giving you information that is not sufficient to solve the
problem.

Nevertheless, I defer to your opinion.

Read my earlier caveats about my reading of your limited information
and
interpretation in a model.

It is apparent to me that we have not been communicating effectively.
I did leave out some info, such as the wire type, ( #18 stranded and
insulated) and the size of the flat metal roof ( can probably be treated
as infinite given its size), but only because I knew the basic losses
due to antenna size itself ( non-resonant ) would be so huge as to mask
any much smaller effects of these other items.


Freq VSWR at feedpoint (RG8/U)
3.98 959
5.4 115
7.2 34


My thanks on the above. That is something basic I can give to the
operators of this station to convince them we need to change the antenna
system. ( Giving them complex impedance figures, etc, would not fly with
that group!)

It is also obvious I need to get EZNEC. Soon as I have time to play
with it, I will download the freebie version for starters. I've seen too
much discussion here on this group not to realize it is worthwhile to
have, even if only occasionally used.


Ed K7AAT

"Ed" wrote in message
.92...
."

The answer to the question would seem to be that it would not work
very well just due to experiance. The swr would be off the charts and
a rig that had an internal tuner would not tune that kind of antenna
over that frequency range if the swr had to be under 3:1. Also the
loss in the coax due to the high swr would be very large..



Well, that was my original surmise of the situation. I was just
trying to have someone with expertize on EZNEC run the antenna at the
three frequencies listed in order to have the feedpoint SWR calculated
for each of those three frequencies. So far, no one, except Owen, has
come even close.

Hi Ed
Did you miss my post??? Or, did misunderstand your question. This
antenna is so easy to model with Roy's EZNEC that I suspect it would be an
excellant "first try" for you. Send Roy a few $$ and you will soon realize
that you have spent some money well.
I'd be willing to show you how to model it, and I am a novice user of
EZNEC. Your question certainly doesnt require an expert on EZNEC to be
answered very clearly.

Jerry KD6JDJ






I can work with the figures Owen provided for the one
frequency ( 3975) and do the rest of the calculations manually to
determine the SWR on that frequency, but had hoped for actual feedpoint
SWR data for all three actual transmit frequencies listed.

Thanks to all.

Ed

Hi Ed
Did you miss my post??? Or, did misunderstand your question.
This
antenna is so easy to model with Roy's EZNEC that I suspect it would
be an excellant "first try" for you. Send Roy a few $$ and you will
soon realize that you have spent some money well.
I'd be willing to show you how to model it, and I am a novice user
of
EZNEC. Your question certainly doesnt require an expert on EZNEC to
be answered very clearly.



No, Jerry, I did not miss your post.... but it did not answer my
question. You modeled the antenna I described, apparently, but not at
the frequencies of intended operation. Yes, it is a 75foot halfwave
center fed, etc, etc, but I was looking for either feedpoint impedence
or a calculated feedpoint SWR into 50 ohm transmission line on three
frequencies, 3993KHz, 5400 KHz, and 7240KHz. Owen has subsequently
provided the figures I was looking forward, although he wasn't happy
with it. He wanted more info to provide a more accurate model, and I
was only looking for a "close" figure.


Thanks for your input, Jerry.

Ed

..
Jerry Martes wrote:
"Ed" wrote in message
.92...
."

The answer to the question would seem to be that it would not work
very well just due to experiance. The swr would be off the charts and
a rig that had an internal tuner would not tune that kind of antenna
over that frequency range if the swr had to be under 3:1. Also the
loss in the coax due to the high swr would be very large..



Well, that was my original surmise of the situation. I was just
trying to have someone with expertize on EZNEC run the antenna at the
three frequencies listed in order to have the feedpoint SWR calculated
for each of those three frequencies. So far, no one, except Owen, has
come even close.


Ed, several years ago I had to find out why an almost identical
antenna system as you have discribed operating on similar frequencies
didnt work very well. System also used an ATU. Analysis showed data
very similar to the EZNEC values. I t was not practical to increase
the hiegth of the antenna so I went vertical using a 30ft fiberglass
whip, gov surplus. Also used inductors and capacitors switched in to
bring the antenna to resonance at the desired freq. Although we only
really used one the others had to be available. Although the ATU was
still used it was completely unneeded as only 3 frequencies were being
used seies inductance and capacitance selected by remotely controled
relays provide a match with the VSWR less than 1:.5 on any frequency .
Communication from NC to GA, FL, and DC were routinely possible with
100watts PEP SSB.


Jimmie