Topband: shunt feed help needed

[k6se@juno.com]

Mauri, I4JMY implied that if the vertical was 1/4-wave resonant that it
could not be shunt-fed.

There is no problem shunt-feeding a 1/4-wave vertical in the normal
fashion.  In fact, a 1/4-wave vertical has several advantages over
shunt-fed verticals that are not 1/4-wave resonant, as follows:

1) The 50-ohm tap point is closest to the ground as compared with a
non-1/4-wave shunt-fed vertical, on the order of only 15 feet high.  This
means less climbing.

2) The 2:1 SWR bandwidth is better than with a non-1/4-wave shunt-fed
vertical.

3) The RF voltage across the "gamma" capacitor will be less than with a
non-1/4-wave shunt-fed vertical, on the order of about only 600 volts
with 1000 watts applied.

However, it is not necessary to strive for 1/4-wave resonance.  Virtually
any size vertical can be successfully shunt-fed and perform well.
==========
Mauri also said that it is possible to have a negative (capacitive)
reactance when shunt feeding, which would require a series inductor to
cancel the reactance.

To encounter capacitive reactance when shunt feeding a tower is only
possible when that structure is greater than 1/2-wave electrically.  If
it is, there will be two 50-ohm tap points, one capcitively reactive and
the other inductively reactive.

For example, on a 300-foot tower (without top loading) shunt-fed at about
200 feet high would yield 50 - j580 ohms on 160m, requiring a series
inductor.  If the same tower is shunt-fed at about 63 feet, the impedance
would be 50 + j870 ohms, requiring a series capacitor.

I'm quite sure you would perfer the lower tap point to avoid climbing
higher and to avoid having to use a potentially lossy coil.  Besides,
probably 99% of amateur towers are less than 1/2-wave electrically on
Topband, so it is very unlikely that you will run into this "problem".

73, de Earl, K6SE