I no longer do this at my current station but used to have an 80M delta loop
(single) and did exactly what Tony suggests. I used open wire - 450 Ohm line
as the stub and I just had a shorting clip to short circuit the stub at the
base of the delta loop for SSB and unclipped it for CW. Vertically polarized
exactly as Tony describes. For 2:1 Bandwidth, I have never seen a way to use a
delta loop from 3850 - 3500 vertically. It might be possible horizontally but
if so its only because the low height above ground is making it so ineffective
for DX the impedance is broad. Not a good way to achieve the desired results.
73
Ed N1UR
-----Original Message-----
From: CQ-Contest [mailto:cq-contest-bounces@contesting.com] On Behalf Of Tony
Brock-Fisher via CQ-Contest
Sent: Monday, July 29, 2019 8:47 AM
To: reflector cq-contest
Subject: Re: [CQ-Contest] delta loop question 80m
Darrel-
I have a pair of delta loops in a phased configuration. They are very
effective. Here are my thoughts:
-There are two ways of feeding them which result in either horizontal or
vertical polarization. For low-angle DX work you want the vertical polarization
(unless you can put?? the loops up higher than a half wavelength above ground).
-For the vertical polarization, feed the equilateral triangle full wavelength
of wire at a point 1/4 wavelength down from the apex, near a corner.
-For vertical polarization, the loops need not be very high off the ground. The
base leg of mine are only about 6-10 feet off the ground.
-An advantage is that this gives you?? a vertically polarized antenna that
needs no ground radials! The base leg acts as a radial.
-The natural resonant impedance of a single full-wave loop is around 100 ohms.
Use a 1/4 wavelength (electrical) of 75 ohm coax to match to 50 ohms.
-You asked about bandwidth. In any given configuration, 2:1 SWR bandwidth is
NOT large enough to cover both phone and CW. Pick one.
-Seeing as you posted this to the CQ-Contest forum, I'll give you the
contesting answer to tuning:
?????? Tune the loops for phone. Then when a CW contest comes along, walk out
to your back yard and add about a 13' stub to the middle of the base leg. This
is easily accessible from the ground. This stub is run at 90 degrees to the
base leg and parallel to the ground. It will tune the loop down to the CW band.
If you want to get clever it could be done with a vacuum relay.
Enjoy!
-Tony, K1KP
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