TopBand: Shunt matching tower for 160M

Earl W Cunningham
Wed, 03 Dec 1997 00:57:45 EST

Steve Zettel <> wrote:
>Just finalizing details prior to placing some parts orders for my
>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
>populated by insulated-element F12 antennas, top-loading contributed by
>other antennas will be nil.
There WILL be some top loading effect, Steve.  Not just from the booms of
the antennas, but from the driven elements also (due to the coupling to
the tower from their coax feeds which run down the tower).

>I plan to run a 90' heavy gauge wire from the top of the tower over (and
>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
>hardline from matching cap to shack.
The structure may already be close to an electrical 1/4-wave on 160m.  If
it is, read below regarding your concern about the gamma capacitor.

>I was looking at the prices in the Surplus Sales of NE catalog, and the
price of 
>vacuum variables took my breath away. But I am concerned with
>/condensation problems attendant with using an air-varible out at the
>Also arcing (not running HP yet, but that is in the future plans). 
If the structure is close to an electrical 1/4-wave, the RF voltage
across the capacitor will be on the order of only about 700 to 800 volts
with 1 KW applied, so a vacuum variable shouldn't be necessary.  The
value of gamma capacity required for shunt-feeding an electrical 1/4-wave
structure will lie in the range of 400 to 500 pf.  The 50-ohm tap point
for the shunt feed wire on the same structure will be about 30 to 35 feet
above ground, with about 24" spacing.
If you plan to use 75-ohm hardline for the feed, the 75-ohm tap point
will be a few feet higher above ground.

Structures with a 1/4-wave resonant frequency of less than or greater
than 1.825 mHz require a smaller value of gamma capacity.  The RF voltage
across the capacitor will be greater and the 50-ohm tap point will be
higher above ground.

BTW, a good rule-of thumb for calculating the RF voltage across the gamma
capacitor in a shunt feed system with 1 KW applied and fed with 50-ohm
coax and the tuned value of capacity is known is:

                     V = 388,000/capacity (in pF)

I've only modeled one shunt-fed structure where the feedline was 75-ohm
hardline.  That one was 1/4-wave resonant in the AM BC band, so the
capacitor value was relatively small and the RF voltage across it was
large.  So, I can't say for sure what the capacity and voltage would be
if you use 75-ohm line, but I imagine that the values wouldn't be too
much different.

>Also, ballpark figures for a range of capacitance needed for this
configuration?  (I am "antenna-modeling and mathmatically-challenged",
shall we say).
See above

>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
>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
>the antenna boom/side-arm assembly, not so good from a shunt rod
>from what I understand).
If 30' up isn't high enough to match 50- (or 75-) ohm coax, then widen
the shunt-wire spacing until the resistive part of the impedance is 50
(or 75) ohms.
Other alternatives would be to use a slant-wire shunt feed (at, say 20'
or 25' up), or to use an omega match rather than a gamma match.

BTW, my two 72' top-loaded towers are 1/4-wave resonant at about 2050 to
2100 kHz.  With 500 W applied to each tower (1 KW total), I have about
1900 volts across the 1/16" plate spacing 250 pf air variable gamma caps
(set at about 150 pF to tune out the reactance), and have never had any
arcing problems.

I also shunt-feed them on 80m, where the RF voltage across the 1/10"
plate spacing air-variable gamma caps is quite high.  Running 1 KW out
(500 W to each tower), arcing is also non-existent.  

Hope this helps.

73 es GL, de Earl, K6SE

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