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

Ed wrote in
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.


Ed,

I am not sure exactly what you mean by a "a wire antenna, 15 feet above a
flat metal roofed building, (it a NVIS antenna), and resonant at 6.2
MHz".

I assume you mean a centre fed dipole that is an electrical half wave at
6.2MHz.

The efficiency of the radiator component can be estimated from an NEC
model, it is probably very high and not the real issue.

Components of an antenna system interact with each other in a complex
way, and it is important to analyse the entire antenna system (radiator,
earth, transmission line, balun, ATU etc) to obtain a correct
understanding of how the antenna system works overall.

In your case, the feedline and ATU are the likely main contributors to
antenna system loss. Antenna system loss, transmitter behaviour, and
antenna pattern are the main contibutors to station performance.

To illustrate with an example. One Saturday a few months ago, I had 7MHz
three QSOs in a row with our new six hour hams who were each using an 80m
half wave dipole fed with a substantial run of RG58C/U coax and an ATU. I
explained to them that their antenna would work ten times better on 7MHz
if their antenna was half the length. A difficult concept for people with
six hours investment in ham radio to understand. A fourth person who had
heard one of the QSOs (and had apparently called me, but I couldn't dig
him out of the noise) emailed me saying he was using the same setup and
now knew the problem, it had to be the ATU. I replied to him that the ATU
probably wasn't losing all that much power, the high coax losses
protected the ATU from an extreme load and extreme loss. Again a hard
concept to swallow when the ATU is closer to the transmitter but
something downstream "protects" it from higher loss.


Owen

I am not sure exactly what you mean by a "a wire antenna, 15 feet
above a flat metal roofed building, (it a NVIS antenna), and resonant
at 6.2 MHz".

I assume you mean a centre fed dipole that is an electrical half wave
at 6.2MHz.


Yes. My appologies for leaving out a critical description. Its a
wire centerfed halfwave antenna, 15 feet above a flat metal roof, and
resonant at 6.2 MHz.

The people who installed it expected the tuner in the FT900 radio to
handle the matching. I explained that the tuner was never intended to
match an antenna that would be so far off resonance. To compound
matters, they are using at least 100 feet of 50 ohm coax... mostly RG-
8, I think.



The efficiency of the radiator component can be estimated from an NEC
model, it is probably very high and not the real issue.


Well, in this case, it IS a necessary issue. I wish to present the
probably actual feedpoint SWR to these people so they will realize how
far beyond tolerance they are trying to use that built in tuner. That is
why I was asking for SWR estimates for 3.95MHz, 7.2MHz, and 5.4MHz.

I intend to propose going to a resonant antenna, and use traps or
multiple dipoles to present a near 50ohm feedpoint to the coax and tuner
for those three frequencies of operation.


... so.... if anyone could run those figures, I'd sure appreciate it.


Ed

PS: Thanks for the info, though, Owen.

Ed wrote in
. 192.196:




I am not sure exactly what you mean by a "a wire antenna, 15 feet
above a flat metal roofed building, (it a NVIS antenna), and resonant
at 6.2 MHz".

I assume you mean a centre fed dipole that is an electrical half wave
at 6.2MHz.


Yes. My appologies for leaving out a critical description. Its a
wire centerfed halfwave antenna, 15 feet above a flat metal roof, and
resonant at 6.2 MHz.

The people who installed it expected the tuner in the FT900 radio
to
handle the matching. I explained that the tuner was never intended
to match an antenna that would be so far off resonance. To compound
matters, they are using at least 100 feet of 50 ohm coax... mostly
RG- 8, I think.



The efficiency of the radiator component can be estimated from an NEC
model, it is probably very high and not the real issue.


Well, in this case, it IS a necessary issue. I wish to present

Whilst neither of us KNOW whether it is an issue, I suggest to you that
based on experience radiator loss in this type of configurationis
probably less than 2%.

the
probably actual feedpoint SWR to these people so they will realize how
far beyond tolerance they are trying to use that built in tuner. That
is why I was asking for SWR estimates for 3.95MHz, 7.2MHz, and
5.4MHz.

You seem to be equating feed point VSWR and that seen by the built in
tuner. That might be the case if the transmission line was lossless. It
is unlikely to be, and we don't know again due to lack of information.

You might find my article at http://www.vk1od.net/LOLL/index.htm which
looks at multi-frequency use of a coax fed centre fed dipole of interest.
Though it models a 66' dipole at 30' height, some (most) of the messages
are relevant to your situation.

Perhaps on reviewing the article, you will see my point that radiator
loss is less important, and that understanding transmission line loss is
very important to your problem.

The article might even suggest some other options that you haven't
nominated.

Owen

You seem to be equating feed point VSWR and that seen by the built in
tuner. That might be the case if the transmission line was lossless.
It is unlikely to be, and we don't know again due to lack of
information.

Owen,

I appreciate your responses, 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.

What parameter is missing to provide a good model of this?


Ed

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.


Ed K7AAT

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

"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..."