TopBand: 160m shunt feeds

[Bill Tippett]

        Earl K6SE mentioned the use of EZNEC for modeling verticals of
different heights.  For those of you with antenna modeling software,
this is an excellent way to estimate the resonant frequency of your
system.  Model your setup to determine the resonant frequency of
your system, and then refer to figure 9-61 in ON4UN's book to determine
approximate tap height and series capacitance.  I used this technique
when I shunt fed my 178 foot tower in December and the SWR was 1.5 on
my first attempt.  By dropping the tap point from 70 feet to 60 feet,
I got about 10 watts reflected with 1500 out.  This took just a couple
of hours once I had the Gamma wire (#12 wire at 48 inch spacing) and
series air variable 250 pF capacitor in place.

        Incidentally, EZNEC will also allow you to model Gamma feed
systems if you really want to get fancy and are familiar with using
the software.  

                                                73,  Bill  W4ZV

P.S.  To answer N4KG's question about determining resonance empirically,
see figure 9-60 in ON4UN's book which describes how to do it with a GDO.


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