My planned inverted L has morphed a bit ... I modeled it – it is planned as a
95 – 100’ vertical section (depends on the amount of droop I can remove from a
~400’ line from the top guy ring at 122’ to a tree that is about 45’ high –
spaced about 100’ from the tower) ... I found that the angle involved doesn’t
impact going from an L to a T config. I would still get nearly complete
cancelation of the horizontal radiation that the L would have ... so now it’s a
planned T ...
and when modeled the Rr is about 33 ohms. So I was thinking of doing what I did
in Arizona on a similar antenna (there it was an L) which is to use an
autotransformer. The feedline connects to one end of a ... I believe it was 11
turn coil around a 2.4” core with the other end going to ground and the antenna
tapping in a few turns down from the top. This gave me a nearly 1.1:1 SWR and
covered from 1.8 well past 1.94 or so with less than a 2:1. It worked extremely
well (worked DXCC from Arizona – all 100 countries from scratch since I’d never
been on this band before, in 89 days) ...
but after reading FMI’s transformer book there’s a few notes/comments that
autotransformers are less efficient .... is this indeed the case? Should I
instead be using an L network (with I believe lower bandwidth)?
thoughts anyone?
Gary
K9RX
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I agree with the comments so far, and also that a 2:1 transmission line
transformer is a good method. But if you are in a hurry, here is an very easy
approach.
Just use a hairpin match. Mine was just some 1/8" copper tubing (but #8, 10 or
#12 wire would be fine too) wound around a wine bottle for about 6 to 8 turns.
Remove the bottle and reserve it to drink upon success! The coil goes from the
feedpoint to ground. (Yes, just like you are shorting the antenna. But you
really aren't.) Then just stretch it open or closed to get best SWR from an
analyzer.
In my case it brought my antenna from about 25 ohms up to about 45 ohms to give
a plenty good enough SWR. Please note that this method will also shift the
resonant frequency, so you may have to shift top wire length a bit as you
adjust the coil. It took me half an hour start to finish to get a good match,
no parts required. Many 40M shortened yagis use this method.
73,Drew K3PA ------------------------------
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