Suggestion ..

VK3XL wrote:
Hello all, I am planning to erect an 80 meter full wave loop antenna in
the back yard. The articals I have read so far all seem to feed the
antenna at a corner with either 450 ohm open wire line or feed direct
with 50 ohm coax. Due to the layout of my yard it would be preferable
that I feed the antenna at the mid point of one side of the antenna
with 450 ohm open wire line to a tuner (4:1 balun included in the line
and coax to the tuner). Is there any disadvantage in feeding the loop
mid way along one leg rather than at a corner?

any assistance greatly appreciated
Mike
VK3XL

I used a three element 4-square 'loop' for years here. If your loop can or
will be a vertical loop, even though the lower side of the square will be
parallel to the earth and low to it, I can offer you suggestions.

Do *NOT attempt to feed it in horizontal polarization. You may go up midway on
either side of the vertical wire elements. On one of them. all you have to do
is attach the braid of the coax to the center point of the wire loop. Then,
feed the loop with a gamma match segment made from, say six inch balanced line
insulators which bind to the wire of the loop at the far end to the gamma line
and through a variable capacitor to the center of the coax.

You tune the gamma match for proper SWR and so on and away you go with a really
decent feed line operation to your rig. You can swing the coax to either the
center support pole for it all, or move it perpendicular to the loop away from
it in any direction off the the rig site. I believe you will be VERY pleased
with the performance of the loop for DX purposes as the angle will favor
vertical polarization, but you do not need ground support wires like in a
vertical.

There are a number of references on how to more or less figure out the length
of the gamma section.

Then, after you figure out the general value of the capacitor you need, you can
pull off another stunt! You can use another chunk of your coax cable as a
substitute for that capacitor! It has so much capacity for so many inches or
feet. OK, you use the outer braid for one lead of you 'capacitor' and the
inner wire for the other. You start with a little longer than needed or too
much capacitance, then start snipping off the coax at the free end of this
contraption to lower the capacitance until it is just right.

At the free end, be sure to leave an inch or so of complete coax insulation at
the tip away out from the remaining braid. Then wrap it if you like with tape
to keep out the water and so on. Zippee! Fixed very durable capacitor for
this gamma feed device.

I've used this coax into capacitor technique for decades now on gamma matched
verticals and so on. Never a problem ever with it.

The trick for getting low angle performance from large wire devices like loops
on 40 and 80 is to get them to really be in vertical mode. And not dependent
on the ground below for RF current return like in verticals that are ground
plane devices.

But to do that you'll have to make the loop in a vertical mode, which, unless
you choose to use loading coils to fudge the vertical height at the tips of the
wire so you can keep the feed point balanced and a little lower, will require
some kind of high support for the 66 foot or so vertical side of the loop at
two ends of it.

I can assure you the fixed wire quads on 40 meters built this way have a VERY
good signal and beam operation from personal experience. And their DX
performance goes WAY up hotter if you feed them in vertical mode rather than in
horizontal more the simple way down close to the ground.




--


-- Sleep well; OS2's still awake!

Mike Luther