"Ed_G" wrote in
. 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

Ed,

I would consider the following:

Mount the ATU at an accessible place on the masting near the lowest end.
Bond the ATT's ground terminal to the mast.

If there is a lighting protection conductor, steel rain gutter, steel
roof in proximity of the base of the mast, bond the base of the mast to
them also using substantial conductors (16mm^2).

At the top of the mast, bond one side of the dipole to the mast.

Run an RG213 or better coax from the ATU to the top of the mast, bond the
shield at both ends to the mast. Connect the inner conductor at the top
to the other side of the dipole, and at the bottom to the output terminal
on the ATU. You must treat the ends of the coax to prevent water ingress.
LDF4-50 would be an even better choice because its closed cell bonded
foam dielectric is better protection against water ingress (it also has
lower loss and higher breakdown voltage). I would not use a foil shielded
coax.

Connect the DC / control wires and input coax to the base of the tuner,
but route them through a common mode choke. You may well be able to use
RG58C/U for the input feed line (depending on length). Wind several turns
of the coax and DC / control wires together through a large ferrite
toroid (#43 should be fine). Two or three of these chokes should probably
be adequate. You may also want one or two chokes where you enter the
equipment room.

Treat all connections to prevent corrosion, especially considering
dissimilar metals.

There is likely to be common mode current on the feedline / mast above
and below the tuner. The chokes reduce the extent of it to minimise the
contibution of the feedline to the radiation system / RFI and conversely
feed line noise pickup, and act to reduce RF "flowing into the shack".


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.

You can take measures as above to minimise the downsides of the lack of
system symmetry.

SGC have recommended attaching symmetric loads to their assymetric tuners
for a long time (they sell tuners, and didn't at the time have a balanced
tuner), and hams have proved it "works" whatever that means.


Also, what recomendations do you guys have for use of a balun?

See the discussion above about a common mode choke.

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.

See above.

Owen