The common-mode voltage at the feedpoint of an OCFD can be many times
the differential voltage across the feedpoint.
Take the common example of a 132ft wire fed 44ft from one end at a
height of 35ft. On 80m the offset feedpoint impedance might be, say,
70+j0 Ohms; so at the 100W power level the feedpoint differential
voltage would be around 84v rms and the current 1.2A.
But if we now consider the two legs of the dipole separately we find
that the impedance wrt ground looking into the short 44ft leg is
dominated by a reactance of about -j300 Ohms, and the impedance wrt
ground looking into the long 88ft leg is dominated by a reactance of
about +j300 Ohms. Taken together, those two reactances cancel and we are
left with a moderate, resistive, differential feedpoint impedance;
however the feedpoint current of 1.2A flowing through those large
individual reactances elevates the feedpoint voltages to something well
over 300v with respect to ground. The effect is analogous to the very
high voltages experienced when feeding highly reactive vertical antennas.
Now scale those numbers for a power level of 1500W and the CM voltage
rises to 1160v. That could result in 135W dissipation in a balun, even
if it had a CM impedance of 10,000 Ohms - that's why designing a balun
for an OCFD is a significant challenge, particularly if you want to run QRO.
Choosing a "flux-coupled" transformer balun over a "transmission-line
choke" balun can alleviate some of the potential overheating problems,
but it doesn't necessarily reduce the CM current. Its CM impedance is
almost totally determined by the primary/secondary stray capacitance, so
the CM impedance falls at high frequencies and is often inferior to a
choke balun.
Take a look at the comparative CM impedance figures for various balun
topologies in the table on page 9 here:
http://www.karinya.net/g3txq/baluns/baluns.pdf
Steve G3TXQ
On 26/02/2015 00:30, Hans Hammarquist via TowerTalk wrote:
if you try to put a common mode choke at the antenna I can imagine difficulties
with overheating etc as the voltage is high. I allow the unbalance current
through the feeder and I stop it after the tuner.
A well design (unlike mine) Windom, or Off-Center-Fed antenna has a resistive 200 -
400 ohm impedance at the feeding point allowing for a balun at the feeding point and
a reasonable SWR in a coax feeder. I will not recommend what type of balun to use
but imagine that the "right' balun reduces any common mode current outside the
coax.
I have not problem with overheating maybe my 100 W is one reason.
Hans - N2JFS
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