"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

"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

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

"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

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

Owen Duffy wrote:
Ed wrote in
. 192.196:

but I am at a loss as to what
additional
information one would need to model a center fed 75 foot long half
wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat
metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on
any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz.



Small correction.... the feedline is also not important since all I
am
asking for is essentially the feedpoint impedance of this antenna at
those three frequencies of operation.

If the metal roof was infinite (or very large compared to the dipole),
the feedpoint Z looks to be around 3-j711 at 3.95MHz. (This is probably
not a good model of your scenario though.)

Is that all you really need to assess the outcome?

100' of RG8 with such a load will lose 20.6dB.

The ATU will see around 4-j55, so you may well be losing 3dB

Radiator loss (with with you seemed concerned) depends on the wire (which
you haven't told us about), and it might be a dB with such a low
feedpoint R.

The coax loss dominates the problem.

There is a lot more to quantifying the problem, and designing a solution
than knowing feedpoint Z at three spot frequencies for a dipole over an
inadequately specified metal roof.

However, we do know that a coax fed centre fed dipole is not an efficient
multi-frequency antenna if the coax is of significant length (meaning
basically more than nearly zero).


Owen

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


Here's your answer:
Freq SWR
3.98 3,240 (three thousand !!)
6.2 2.08
7.2 39.3

Details:
Used 4NEC2 (free)
5 meters above ground
Length of antenna = 11.62 m
Impedance at 3.98 MHz = 3-j710
Impedance at 6.2 MHz = 24 +j3
Impedance at 7.2 MHz = 54+j318
Limitation - real earth used vs flat metal roof


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73,

Rick K2XT