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[TowerTalk] Phased Loops ?

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Subject: [TowerTalk] Phased Loops ?
From: (Tony Brock-Fisher)
Date: Thu, 6 Mar 1997 15:38:43 -0500
I'd highly recommend a pair of phased loops on 80 meters are nearly
as effective as a 4-square, with the exception of the directional
flexibility (only 2 directions switchable). Major advantages over
the 4-square are no expensive phasing box and no major landscaping
job installing radials.

I compared my 80 meter totals to two years ago in ARRL CW, and find
that after adding the second loop, the line score about doubled
in Q's. Of course, other factors could be present.

Here's my recipe:
Here's my simplified version of the ON4UN feed for 80 meter delta loops.

My delta loops are triangles; apex up at 70 feet; base leg about 8 feet
off the ground. The apices are spaced 20 feet apart with a 2 inch PVC
crossarm mounted on the tower. They are fed near a lower corner; actual
feedpoint wants to be 1/4 lambda from apex.

I have had both single loop and now phased pair. Feedpoint impedance
seems close to 100 ohms in both cases.

For phased pair, each feedpoint is run to a central switch box. The runs
are matched length RG-11/U FOAM coax, 36 feet long. FOAM is required
to get the right velocity factor.

The switch box is a relay that selects one feed to go to the shack (driven
element). The relay also shorts out the end of the other feed (reflector).
The shorted 36 foot section gives the required loading inductance.
Both feedpoints have ferrite bead baluns.

Here's the trick:
After the relay box, I use another 16 feet of RG-11/U FOAM coax. This
adds to the 36 feet on the driven element to form a quarter wave
matching section. Voila! 50 ohm impedance at the end of the 16 foot
section. Just run 50 ohm coax back to the shack.

SWR is very reasonable (1.5:1 or better at all times). F/B is at least
15 db. I retune the loops for CW versus SSB by adding/removing about
12-15 feet in the base leg - which can be done from the ground in
about 20 minutes.

The antenna has great performance, probably 1-2 db gain over a single
loop, and doesn't need radials!

-Tony, K1KP,

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