Dave Platt wrote:

Hi. Can anyone recommend a source of 36:50 ohm ununs?

I'm adding an inverted L for 160 meters and would like to add an unun of
36 ohm to 50 ohms to reduce my SWR. EZNEC shows it would lower it
substantially and when you're running barefoot on 160 you need all the
help you can get.

One gotcha on this: before you add such an unun, make sure that your
feedpoint impedance is actually close to 36 ohms. It may not be,
depending on your actual soil and ground-plane impedance and losses,
and an unun of this sort might actually make matters worse.

Yes, I'm considering that, and figure if it occurs I'll add more radials.

One of the local ham clubs has a regular Field Day practice of sending
up a quarter-wave vertical wire, attached to a helium balloon, to
operate on 160 meters at night. They had planned to use an unun of
around that ratio to match it to the feedline... but when they tried,
it made the SWR at the transmitter rather worse.

I suggested that since they'd only run four radials (along the ground
surface), they had a rather poor ground plane and probably had quite a
few ohms of ground-loss resistance at the feedpoint. The simple
assumption of "quarter-wave vertical over a perfect ground plane" just
didn't apply very well. I think they just chose to drive the antenna
directly and live with the SWR on the line. It's possible that they
chose to hook up the unun "backwards".

Depending on the soil, and radials installed below your Inverted L,
you might encounter the same issue. And, depending on your soil type
and weather, it's possible that your ground losses might vary quite
a bit over the course of a year.

Adding an adjustable transmatch right at the base of the antenna would
have been a more flexible solution for them - not as broadband as an
unun, but more adaptable to different ground-loss conditions. Might
work for you, too (although I grant it's likely to need a rather large
inductor!)

Large Inductors aren't a problem - keeping a tuner weathertight on BC's wet
coast is, however.

Also... I'd suggest calculating the actual "excess loss" in your coax,
if you accept the mismatch at the feedpoint and match to 50 ohms at
the rig. At 2 MHz I'd guess that the excess loss from even a 2:1 SWR
would be quite low.

How do I calculate "excess loss?


(My copy of "Reflections" is at home, and I can't recall whether
Walt Maxwell W2DU provides a winding formula for a 36:50, if
that's actually what you do turn out to need. I'll try to remember
to check. My guess is that you'd probably end up having to wind your
own.)

Thanks - I'll see if I can find a copy.
--
bobrsmits.ca