There are a couple of things I see wrong with the models and
information given...
First the guy said he had a 2 element shorty 40 on top of this
tower.. and that the reflector was insulated from the boom.. I
also assume that the driven element is insulated from the
boom.. and tower had a 4 foot mast on it...
you forget to include the BOOM in the model... its probably
about 22 feet or so.
Next problem the insulators between the reflector and boom and
between the driven element and boom? .. just how much
spacing do you have here?.. this is going to be the max voltage
point of this shunt fed tower?... good chance it will arc those
insulators unless your running 100 watts or less...
If you pull a wire off the top of the tower ... then you turn the
gounded tower into an inverted-L antenna and the high voltage
point is at the end of the wire and not where it will kill the guys
40 meter yagi... you can also make up a balanced to
unbalanced balun to feed the 40 that WILL make it DC
grounded at 160 meter frequency.. then the driven element will
be part of the top loading.. also don't see why the reflector can't
be grounded to the boom directly..
bet you still have trouble getting 50 ohms on a 48 foot tower....
even with gamma wire at the top of tower...
It might be a compromise but why not make up the gamma to
be 25 ohms and series out the J then feed the Gamma with a
simple 2/1 Un-Un ??
John k9uwa
John Goller, K9UWA & Jean Goller, N9PXF
Antique Radio Restorations
k9uwa@arrl.net
Visit our Web Site at:
http://www.JohnJeanAntiqueRadio.com
4836 Ranch Road
Leo, IN 46765
USA
1-260-637-6426
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