On 8/29/2024 6:57 AM, Stanley Zawrotny wrote:
I have used two 80-6M off center fed (OCF) dipoles for years. They
cover all bands, including the WARC. You can make or buy one for 160 M,
but its performance may change.
Antennas that don't match the line at frequencies where we want to use
them cannot be effectively choked to kill receive noise. That wasn't a
problem 20-30 years ago when their use was pushed in the Handbook and by
other authors. It IS a problem today, when we're surrounded by dozens of
noise sources in our own home and more in the homes of our neighbors. A
choke that does anything useful will fry with even as little as 100W.
Off-center fed antennas are even worse, because they are inherently
imbalanced, thus more noisy, and much more likely to fry the choke. Yes,
chokes are sold that don't fail in these antennas, but they also don't
do anything useful. :)
I've had very good success with fan dipoles. My "JA Killer" is an 80/40
fan, fed with big coax (RG8/RG11). Mine is high, so it's fed with RG11.
Before I had aluminum, I had a low 20/15/10 fan in Chicago that guys
told me was loud on the east coast, and before I had aluminum here in
NorCal, I had a pair of 20/15/10 fans at right angles that played very
well.
Inexpensive, simple construction. Bare copper for the longest element,
THHN for the others, 1/2-in PVC conduit for spacers with holes drilled
for the wires; spacers held in place by soldering around them on the
bare copper. I use spacing of about 10-in for the 80-40, about 8-in
between elements of the 20-15-10.
The longest element has full SWR bandwidth of a single dipole, the
shorter elements have about half the SWR bandwidth. I model in NEC,
using a lot of segments, and carefully varying the number of elements to
to keep them as nearly as possible the same length. The total length
of wire in each element determines the resonance.
For a while, I also had an 80/40 fan with loading coils near the ends of
the long elements and short extensions beyond the coils to resonate it
on 160M. That worked well too, BUT -- almost any horizontal dipole on
160 is a low dipole, which makes for both very high ground losses AND
poor field strength at low angles. That antenna was at 120 ft, and my
100 ft vertical nearly always beat it, often by a lot.
That loading coil idea is a great way to add a second harmonically
related band -- I've designed 40/20 dipoles for use on county
expeditions for CQP and 7QP that worked well. The idea there is that if
you're activating a rare county, those two bands will get you to most
serious contester who need the county. For our club, as sponsors of CQP,
the goal of county expeditions is simply to make sure that there
reasonably good stations to work in all of our 58 counties, many of
which have few if any active hams.
73, Jim K9YC
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