Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT

"Ed_G" wrote in message
. 192.196...

Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The

I think you are going to have a difficult time operating on 80 meters if you
can not get around 120 feet of overall length. Then it may be difficult on
40 meters. If limiated to one single wire , coax fed, look at the off
center fed antenna.

"Ed_G" wrote in message
. 192.196...

Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT

If the SGC-237 coupler is on the roof and properly grounded, try it the way
the SGC rep suggested. Any feedline radiation should occur from the antenna
side of the coupler and might help with getting a bit more signal out. Go
with the simplest solution first and run a single coax to the coupler from
the transceiver. Maybe it will work just fine.

If there are problems with RF on the coax outer, add some ferrite beads over
the coax to form an RF choke.

Look at more complex solutions when you have actually identified a problem
rather than worrying in advance over something that probably won't happen.

Mike G0ULI

On 29 Feb 2008 00:51:16 GMT, "Ed_G"
wrote:


Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT
My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.

The antenna is in an environment that will not model well. The
radiation from the coax will have an effect on the aggregate
performance but nothing you can really measure.

Although I doubt anyone can explain just how it works, the SGC-237 and
the wire you have described will work fine.

Modern antenna tuners perform a lot like Magic in my estimation.

My own feeble experiments have led me to believe that it is worthwhile
to put an antenna analyzer on the configuration and make sure that the
array is NOT resonant on any frequency of interest. The tuner seems to
like that best.

Power supply: I have a very old telephone power supply tweaked down to
12 volts. I leave it on 24/7.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.

The antenna is in an environment that will not model well. The
radiation from the coax will have an effect on the aggregate
performance but nothing you can really measure.

Although I doubt anyone can explain just how it works, the SGC-237 and
the wire you have described will work fine.

Modern antenna tuners perform a lot like Magic in my estimation.

My own feeble experiments have led me to believe that it is worthwhile
to put an antenna analyzer on the configuration and make sure that the
array is NOT resonant on any frequency of interest. The tuner seems to
like that best.

Power supply: I have a very old telephone power supply tweaked down to
12 volts. I leave it on 24/7.


Thanks for the feedback, John. I imagine that we will end up doing
as you suggested. We WILL test the antenna on the ground with a
temporary mast to see if there are any issues, but we also realize that
things can, and probably will, change when it is permanently mounted on
the building. One of the reasons I am posing these questions here now
is that once the antenna is up, it will be difficult to get the
building personal ( its a firehouse ) to lower the mast for changes.
Its a rigid one piece aluminum mast that will be bolted at its base to
the building.

73

Ed K7AAT

Ed_G wrote:
The above setup is a given, with no room for compromise.

Your antenna is already quite compromised with a
~490:1 SWR on 75m at the antenna feedpoint. The line
loss with RG-213 is about 4.5 dB and the tuner is
required to match 0.55-j48 ohms, another lossy
situation. Your antenna system efficiency may be ~10%.

How about turning your 13 foot pole into a radiator
and using the dipole wires for a top hat?
--
73, Cecil http://www.w5dxp.com

Your antenna is already quite compromised with a
~490:1 SWR on 75m at the antenna feedpoint. The line
loss with RG-213 is about 4.5 dB and the tuner is
required to match 0.55-j48 ohms, another lossy
situation. Your antenna system efficiency may be ~10%.

How about turning your 13 foot pole into a radiator
and using the dipole wires for a top hat?


The aluminum 2" mast will be side mounted on the wood building...
about 5 feet below the edge of the roof. The remaining 14 or so feet
will be above the roof. With two 35 foot wires off the top, do you
really think that would work to our needs better? There is one
problem with this setup, though.... I do not think there is a suitable
ground on the building roof. I suppose it is possible to run a simple
wire straight down from the area near the bottom of the mast to the
ground..... the SGC-237 would still have to sit on the roof and be
connected to this now lowered feedpoint.....

I think it would be easier to extend the length of our inverted V
legs. I had posted their length between 35 and 40 feet, but if we run
out of building to extend the ends to, I suppose we could go on
straight down from the end mounts, toward the ground a bit. Even an
additional 10 feet could be had on each leg with the new ends being
still 10 feet above the ground. It IS a wooden structure, for the most
part.

Ed

John Ferrell wrote in
:

My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.


John,

Any conductors or transmission lines carrying a net current (or common
mode current) will contribute to radiation. If your meaning of "antenna"
is something that contributes to radiation, then the feedline on both
sides of the ATU might be part of the "antenna" no matter what you might
declare.

If Ed connects parallel line from the centre of the dipole to the hot and
common terminals of the ATU, there is likely to be common mode current on
the parallel line adjacent to the ATU. If the only connection on the tx
side of the ATU is the coax, then it will also have a common mode current
adjacent to the ATU and near enough to equal to the common mode current
on the other side of the ATU.

That is not to say it won't work. People regularly build antennas with
radiating feedlines, some (mainly commercial interests) even call that
out as a significant advantage.

Nevertheless, there are downsides and some measures to minimise the
common mode current on the feedline may be warranted... or even
necessitated down the track.

Ed has been deliberating over a solution to this problem for a long time.
It has been discussed to some depth!

Owen

Ed_G wrote:
There is one
problem with this setup, though.... I do not think there is a suitable
ground on the building roof.

I was thinking you could lay some radials on the roof
to obtain your ground plane. You could also insulate
the pole from the mounts using sections of PVC pipe.

I think it would be easier to extend the length of our inverted V
legs. I had posted their length between 35 and 40 feet, but if we run
out of building to extend the ends to, I suppose we could go on
straight down from the end mounts, toward the ground a bit.

I had assumed that your "no compromise" statement included
antenna length. A full-sized dipole would certainly solve
your 75m problem but then the full-wave dipole would be
mostly non-functional on 40m when fed with coax.

Here's another idea. *Change the pole support to fiberglass*,
use heavy duty 300 ohm balanced line, and run the 300 ohm
feedline inside the fiberglass pole to the dipole. You could
use the G5RV length of 51 feet per dipole element. With such
a configuration, the feedpoint impedance on 3.8 MHz would be
about 20-j130 ohms and on 7.2 MHz would be about 100-j450 ohms
which the SGC would have no trouble matching. This could
multiply your system efficiency by maybe 6-8.
--
73, Cecil http://www.w5dxp.com

Antenna tuners 'like' antennas that are 'too long' better than
antennas that are 'too short'. That doesn't say anything about how
well that 'too long'/'too short' antenna will perform, just that the
average tuner will find it easier to deal with one that's 'too long'.
(Easier to 'cram' more capacitance into a tuner than inductance.) If
this antenna is a doublet, 'balanced', why would you need a
'groundplane'? And while I'd guess that it'll never be 'ideal', the
building is acting as a 'groundplane' anyway, sort of.
- 'Doc

(What am I not understanding about the situation?)