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

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?)

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?)


I don't think you are missing anything; your comments sound
reasonable to me. The building is the nearly new local firehouse HQ.
The Chief and staff have outlined very strict parameters for our antenna
location and setup. They fabricated the aluminum mast and will install
it on the side of the building per their own design. Even at that, they
gritted their teeth at the visual impact it has. No stand-off
supported open ladder line for us! We must work with this and I don't
see any feed alternative than coax. The only thing we 'may' have some
flexibility with is the length of the antenna. ( Sorry Cecil, I guess
I should have mentioned that up front ). We are limited to the length
of the side of the building, which I estimated would give us up to 40
feet of element from the top of the mast. However, in hindsignt, I
believe we could at that end point drop some additional length down off
the end for a longer antenna.... would probably give us better operation
on 75 & 80.


Ed

Ed_G wrote:
They fabricated the aluminum mast and will install
it on the side of the building per their own design.

Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.
--
73, Cecil http://www.w5dxp.com

Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.



I fully agree, but we have absolutely no input in regards to the mast
they have provided, and to attempt to persuade them otherwise would only
jeopardize the situaion, so we must accept what they have provided and work
with it.


Ed

Ed_G wrote:
Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.

I fully agree, but we have absolutely no input in regards to the mast
they have provided, and to attempt to persuade them otherwise would only
jeopardize the situaion, so we must accept what they have provided and work
with it.

There is someone, somewhere, who has the authority to
change the mast from aluminum to fiberglass for the
purpose of multiplying your radiation efficiency by
a factor of 7. I would seek that person out and bend
his/her ear.
--
73, Cecil http://www.w5dxp.com