Topband: Feed Line

[George & Marijke Guerin]

Hi Tom, Bill & Topbanders,

I think one of the original questioners on this 450 ohm feedline was wanting
to feed the high impedance point of a 1/2 wave antenna where the current max
is 1/4 wave off ground, and the tee top extends out from that point.
Essentially an inverted 1/4 wave vertical with two elevated radials fed
against ground with some radials under the whole thing.

With a feed point impedance of maybe 2000 ohms the 1/4 wave transmission
line impedance becomes about 100 ohms, which is easy to handle.  However,
Tom is correct, that the twin lead or ladder line is balanced, and the
antenna is being fed against ground or unbalanced.

There appear to be at least two other solutions:
1. a transmission line stub, similar to the write ups for half squares and
bobtail curtains.
2. A tank coil tuned to 160 meters at the feed point with a tap or link for
50 ohm coax to the radio and the top or a high tap for the high impedance
antenna feed point.  The tank coil approach is in figure 12-19 of ON4UN's
1994 edition of Low-Band DXing.  Note the inverted ground plane is simply
the middle part of a bobtail curtain, omitting the inverted L portions on
either end.

Comments?

73        George        K8GG




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