Topband: 160 Meter Tuning Question
w8ji at w8ji.com
Mon Aug 6 05:37:23 PDT 2012
> How much series C will it take to resonate a set of three guy wires 120
> degrees apart that are 145 feet long each.
> They are fed at the 60 foot level of a Rohn 25 tower, and slope down to 10
> feet at the ends. They are guy wires that cannot be changed.
> I want them to resonate on 1.830 khz. They are to be the elevated radials
> for my vertical.
Why do you want to resonate them?
The shield voltage is what drives common mode onto the coax. With the
radials slightly off-tune, the only problem is the shield connection voltage
is a bit higher. Voltage driving the shield is never near zero anyway unless
the ground system is very large.
If it were my system, I'd be using a reasonable isolation on the coax shield
anyway. Maybe something like 500 ohms or more, with the shield grounded on
the house side of the isolation. Then I'd just tie the radials together and
series or shunt tune the radiating element with a single capacitor to
compensate any reactance.
Series tuning would not modify resistive part of feed impedance, shunt
tuning would change it to a higher value. Say it was (just guessing) 35 +j30
at the feedpoint. A series 30 ohm reactance capacitor would make it 35 j0.
The parallel equivalent impedance would 60.7 +70.8. So if we parallel tune
the reactance out with a -j70 shunt capacitor it would be 60.7 j0 ohms.
With two caps, one series and one shunt, we could make it 50 j0.
I'm not sure what the impedance would be. This is just to illustrate if
shield voltage is made a non-issue with an isolator on the shield, the
radials can connect to the shield without any care in tuning and it won't
change a thing except reactance of feed impedance. That error can be
corrected equally on either side of the coax, because we all know a
feedpoint always has the same current on each side if the coax is not
Of course some people write entire articles about Kirchhoff not applying to
RF, but we know that isn't true if we understand displacement current (stray
More information about the Topband