Topband: shunt feed help needed

[Dave Heil]

i4jmy@iol.it wrote:
> 
> > 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.

There seems to be much reinvention of the wheel going on in this
discussion :)

Pete--If you want to use the gamma match, you can change the point at
which the tower is tapped and/or change the spacing of the gamma wire to
the tower.  If you want to get the job done in less time, go to an omega
match and use the shunt capacitor.  The best article I've ever found on
the topic is John True's May 1975 piece in "Ham Radio".  It provides and
electrical height for your tower for several types of top loading and
offers graphs which point to the proper tap point for the electrical
heights.  He uses a spacing of ten plus or minus two inches and other
graphs are provided to help find the necessary series and shunt
capacitance.  There are graphs for shunt feeding towers on 40, 80 and
160m.  I've never had a problem using his figures and I've done the
shunt feed for top band at quite a number of loations.

That said, I'm currently using one crankup tower and one fixed tower
with a Hazer (with all coaxial cables and control cables external to the
towers) so I'm going with an inverted "L" and will use a vacuum relay to
switch capacitance in and out in order to cover most of the band.

If you don't have access to the article I've referenced, e-mail me with
particulars on your tower and I'll send you information on the tap point
and capacitances needed.

73,

Dave Heil K8MN