[TowerTalk] Simpler Version of Shunt Fed Tower - 80M?
RadioIR at charter.net
Sat Feb 4 21:02:23 EST 2006
A 55 ft tower with SteppIR on the top is a good size for an 80 meter
vertical. This arrangement should be resonant somewhere around 2.9 MHz.
In order to do a match with a single capacitor for the CW end of the
band, the shunt wire needs to go about 45 feet up. For 1500 watts, the
voltage on the cap will be around 1200 volts. Bandwidth will be very
With this tap point, it's hard to get a good match for the high end of
the band, and a tap point of 45 feet on a crank-up, is also not too
convenient. So, if you move the tap point to the top of the first
section, about 22 ft up, you can also do a match but you will have to
use a more complicated matching network. There are several solutions
for this network. If you use a T network, same as in many tuners, this
will give you wider bandwidth than the single cap solution. Choosing a
minimum value for the inductor will give lowest voltage on the caps, but
this voltage will still be high, 2000 to 3000 volts for 1500 watts. You
can probably also find a two-component solution for this network. I
hesitate to give you exact values for components because they will
change depending on exactly how you construct the shunt wire, but
components will be in the range of 1000pf, 1.8 uH, 150pf.
You can use air variables provided you keep them dry and add some design
margin for high humidity. I hesitate to use fixed value HV mica
transmitting caps, because when I lived in Florida they never lasted
more than about 6 months. Close lightning strikes would always take
them out. I had to use something that would stand arc-over and recover.
Now a few words of caution are in order. I also have a 55 crankup and a
4 element SteppIR, so I have already looked into doing this, but decided
against it. SteppIR elements are insulated from the boom, so if you use
this as a vertical when the SteppIR elements are extended, there may be
large voltages across the balun in the SteppIR. I was concerned that
this might create a voltage breakdown across primary to secondary. I'm
not sure if that fear is well founded or not, I just didn't want to take
the chance. I made some assumptions about the stray capacitance across
this transformer and did some simulations, and the results were not too
comforting. (It would have been really neat if the center tap of the
secondary of this transformer were grounded. You would have a remotely
tuned top hat.)
Also as you noted, the coax standoff arms can possibly create a problem.
With these cables 2 to 3 ft from the tower and not connected at both top
and bottom of the tower, the amount of current you have flowing in them
is pretty much random, depending on cable lengths. It seems like a good
way to possibly blow the rotator indicator pot. The stepper motors are
also connected to one of these cables, although I doubt that this is a
problem. If these cables are not connected at the bottom of the tower,
you can also expect that RF currents will flow down them back to the
shack. RF back into the shack on the SteppIR control cable is a good way
to blow the SteppIR controller box.
Now, having said all that, I know that some have tried this. Maybe
those guinea pigs will offer their results. I would like to know anyone
experienced any of the problems listed above.
Brian Maynard wrote:
>This is a question for the group. Thanks for reading and responding
>as you can. Brian, K1NW
>I have a USTowers 55' crank up with a 4-L SteppIR. I would like to
>put up a shunt fed wire for 80M only. I have read up on this and
>despite it being called a "poor-man's vertical" I find the ~$400+
>price tag for vacuum variable capacitors to be very prohibitive.
>My question is:
>If I am only looking for resonance on 80M can I just use high power
>capacitors (recommendations welcome) and skip the vacuum variables?
>Also I am using the standoffs for my feed and control lines, and
>could not possibly tie the cables for these to the tower legs (I
>raise and lower the tower regularly) - how will this affect
>performance of the 80M wire?
>TowerTalk mailing list
>TowerTalk at contesting.com
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