TopBand: Shunt matching tower for 160M

[Eric Gustafson]

>Date: Tue, 2 Dec 1997 14:56:08 -0700
>From: Steve Zettel <zettel@homer.libby.org>
>
>Just finalizing details prior to placing some parts orders for
>my Topband, Contesting Tower. I'd like to tap the wisdom of the
>reflectors before I do though, and perhaps avoid an error or
>two.
>
>I plan to shunt feed my 90' Rohn 45G tower on 160M. Since it
>will be (soon!) populated by insulated-element F12 antennas,
>top-loading contributed by the other antennas will be nil.

Don't discount the top loading.  Check the tower resonant
frequency before beginning.



>I plan to run a 90' heavy gauge wire from the top of the tower
>over (and maybe slightly down) to a fir tree a ways from the
>tower. This should be long enough to ensure the tower/wire combo
>is resonant somewhere below 1.8-2.0 MHZ (inductive).  Matching
>will be by shunt feeding with a vacuum-varible in series,
>hardline from matching cap to shack.
>
>I was looking at the prices in the Surplus Sales of NE catalog,
>and the price of vacuum varibles took my breath away. But I am
>concerned with weatherizing/condensation problems attendant with
>using an air-varible out at the base. Also arcing (not running
>HP yet, but that is in the future plans).  Comments?
>
>Also, ballpark figures for a range of capacitance needed for
>this configuration?  (I am "antenna-modeling and
>mathmatically-challenged", shall we say).
>
>And finally (for now!), suggestions for the configuration of the
>shunt fed leg. I have plenty of 3/4" hardline to use as a
>conductor, but my lowest sidemounted and rotatable antenna on
>the tower is at 30', so I can't run up past that (unless I make
>the shunt leg REALLY close to the tower face to avoid
>interference with the antenna boom/side-arm assembly, not so
>good from a shunt rod situation, from what I understand).
>
>Thanks for you assistance,
>
>Steve Zettel  KJ7CH
>near Libby, MT  (no snow YET at 2100')
>

Steve,

Here is a way to drive that tower that may be more appropriate
than shunt feeding (given the antennas part way up).  If you can
use it, it will also save you from spending a lot of money and a
lot of bother with the vacuum caps.

First, determine the tower resonant frequency (with all the stuff
on it that is going to be there).  If it is already below 2 MHz,
stop.  This method won't work for you.

If the tower assembly is in fact short, then proceed.

Take an impedance bridge or one of the MFJ or Autek analyzers up
the tower along with the tower end of your loading wire.  Choose
a point more than half way up and establish a feedpoint for the
loading wire attachment point (shield to tower and center
conductor to wire - wire insulated from tower).  Of course, the
loading wire will need to be approximately the right length and
positioned approximately as it will end up in the final
instllation.  This can be _very_ approximate at this point.

Measure the RF resistance (ignore reactance for now) at this
feedpoint.  If it is above 50 ohms, choose a lower spot on the
tower.  If it is below 15 ohms, choose a higher point on the
tower.  I would shoot for a value between 25 and 35 ohms.

At this point you can proceed by cut and try or by calculation
and then much less cut and try.

Calculate the value of shunt inductive reactance required to
match the resistance up to 50 ohms (pretending that the system is
supplying the necessary capacitive reactance).  Install a
sufficiently robust coil of that value acros the feedpoint.  Only
you can determine how robust that coil must be but a 4 inch
diameter coil of 1/4 inch copper tubing airwound at 1/2 inch
spacing is more robust than anyone could need even if running
what is euphemistically referred to around here as "contest
power".  If you really want to go nuts, silver plate it.  I
haven't had any real troubles with #12 bare copper wire.

Now finalize the mechanical support and routing for the loading
wire.  Adjust the length of the loading wire to put the system
resonance where you want it.  By this time you should have run
the feedline you will be using down the inside of the tower and
off to your rig.  Ground the feedline to the tower at the
feedpoint and at the base of the tower.

you may need to cut and try the exact inductance of the shunt
coil and the loading wire length to get 50+j0 located on the
frequency where you desire it.  I usually find that the 2:1 SWR
bandwidth is wide enough for me.  Most amps can handle 2:1 well
enough.  Tuner losses will not be large for SWRs less than 3:1 or
so.

If you are really anal about losses, you can make the thing
easily adjustable by using a small (2 foot radius) end "hat" that
hose clamps to the loading wire.  Then set the wire length so
that when the hat is at the end, the system resonates just below
the band.  You can then simply loosen the clamps, slide the hat
up to where the resonance is in the desired spot and retighten
the clamps.  Of course you will have to be able to get to the end
easily.  Just arrange to support it from a rope/pulley
arrangement and let it down to do the adjustment.  In an
afternoon, you can make a chart of inches from the end versus
resonant frequency to make it easy to preset where desired.  Now
you can dispense with any matching devices at the exciter end of
the feedline.

Voila!  You now have an easily tuneable loaded tower TX antenna
that requires ZERO vacuum variables, can handle any amount of
power you care to feed it, and is at least as efficient as a
shunt loaded tower.  This system will operate as a vertical.
Only negligible amounts of power will go to horizontal radiation
from the wire.

73,  Eric  N7CL

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