Dear Topbanders,
Dennis has a good solution written below with about 116.5 feet of tower
plus the "X" for top loading. Some items to consider:
1. The X is copper, the Rohn tower sections are zinc galvanized, so a
transition of an alloy like brass or bronze is needed between them to
prevent a wet cell battery effect that will eat up the connection. Graphite
filled conductive grease may also help.
2. Since the tower is slightly top loaded with the X, the impedance over
a lossless ground is less than 36 ohms. If one has 15 - 20 ohms of ground
losses, then the match is fine. If one can handle the ~1.5:1 SWR from a
nearly perfect ground at the radio end of the coax, all the better. The
TenTec L-net tuners do a fine job if a better match is required or one wants
to transmit way up the band.
3. If one needs to be closer to 50 ohms at the feed point, consider
another section of tower, or a longer stinger to get 50 ohms resistive load
with little or no loss. The inductive reactance can be tuned out with a
series capacitor at the feed point. For example, if antenna impedance, Zant
= 50 + 50j then a series capacitor of about 1750 pfd will tune out the
inductive reactance.
My personal, theoretical votes are to put up 13 or 14 sections of Rohn tower
with insulated base and tune out the reactance at the radio end of the coax,
or to put a yagi antenna on top of 100 - 120 feet of grounded base tower and
shunt feed the tower with an omega or gamma match.
73 George K8GG
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