RadioBanter - View Single Post - 2 baluns on doublet feedline to overcome impedance changes?

Roy Lewallen · July 6th 09, 10:43 PM posted to rec.radio.amateur.antenna

wrote:
hi, everybody;

i am interested in constructing a choke balun for balancing a doublet
75 metres long, centre-fed with 30 metres of ladder line, an l-match
between the rig and the feeder;

i have read around a bit, including roy lewallen's splendid article
below; it seems that a balun maintains balance best when its inductive
reactance outweighs the line impedance by a factor of at least 4;

at about 2 mhz, the impedance at the bottom of my feeder will be high,
resulting in considerable imbalance on the line (more so that the
frequency is low, where the balun's inductive reactance is lower); i
have two questions;

1) would a balun at the antenna end of the line (where the impedance
is much lower) maintain balance in both antenna and line?

The impedance seen between the two line conductors isn't what determines
how well a balun works. What's important is that the common mode
impedance of the balun is at least several times (ideally ten or more
times) the common mode impedance seen at the balun insertion point. The
latter isn't easy to determine. If the balun has adequate impedance
relative to the common mode impedance, it will maintain balance at that
point. Basically, if you put the balun at a point where the common mode
current is high (which is a point of low common mode impedance), it'll
reduce the common mode current at that point to a low value. If there's
no substantial current due to mutual coupling (due, for example, to a
non-symmetrically placed feedline), this will balance the entire system.
If you put it at a point where the common mode current is low, it won't
change things much.

If there's common mode current due to coupling, putting a balun at one
point will change the current distribution but not necessarily balance
the whole system; you can still have common mode current on the feedline
but with a different distribution. When that happens, two baluns placed
about a quarter wavelength apart will usually disrupt all possible
distributions and result in good balance.

But the points of maximum and minimum common mode current will be
different for each band, and will depend on the length of the feedline
and the total path to the Earth. So there's no single solution that's
best for all circumstances. You're usually stuck with trying various
combinations until a satisfactory solution is found.

2) and then how about another balun at the l-match end of the line,
for frequencies where the antenna-end impedance is high?

What this will do is assure that you don't have a significant amount of
common mode current at the rig, which is a good thing. Putting one at
the antenna assures that the antenna legs have equal and opposite
currents, which is another good thing. What happens between depends, as
I described above, on a number of factors.

Roy Lewallen, W7EL