Topband: shunt feed help needed

[i4jmy@iol.it]

> Yes, it does.  The "secret" in shunt feeding a tower is to find the
> 50-ohm point where the shunt wire attaches to the tower.  There will 
be
> some inductive reactance in additon to the 50 ohms of R, so a series
> capacitor is need at the feedpoint to cancel that reactance, ending up
> with a pure 50-ohm resistive load.


The point where to attach the gamma rod, the distance from tower and 
the ratio between the tower and the rod itself play all a significant 
role.
What finally should be a convenient 50 +JX is the impedance at the feed 
end of the rod. Incidentally, although scarcely used, there the 
impedance could be also a 50 -JX and the reactive series device could 
be perfectly an inductor to transform into a 50 + j0.
Last but not least, when the tower is exactly resonant, or you move the 
resonance doing something mechanical or you do it electrically 
otherwise there won't be a match with a gamma, just as it happens with 
yagis.
Practically speaking to avoid climbing too many times I'd try a rough 
PC modeling to get an idea of what happens varying spacing, rod 
thickness and tap height. 

73,
Mauri I4JMY