[Towertalk] log feed systems

[Tom Rauch]

> W8JI says "never tape the feedline along the boom of an lpda,
> as some mfr's suggest..."   While I appreciate the coupling issues...
> what alternative would you suggest?  

Since such antennas generally have two booms functioning as a 
balanced line, you run into a problem if the common-mode impedance at 
the top of the tower happens to be low at the operating frequency if 
you tape the feedline directly to one boom.

This is similar to problems that CAN occur with dipoles or regular 
yagis, where the feedline becomes part of the radiating system. Of 
course any ill-effects depend greatly on tower and/or feedline 
length, and can range from nothing (with luck) to a loss in F/B or 
increase in RFI problems.

A friend of mine had a Tennedyne log and wiped out TV sets (all on 
CATV) on 20 meters until he moved the feedline off the "hot" lower 
boom. He also had arcing problems between the shield (through the 
coax jacket) and the tower!

If you look at these antennas carefully and think about how they 
actually work, the balun (assuming it is a 1:1 ratio choke-type or 
current balun) actually belongs at the point where the coax leaves 
the boom, NOT at the feedpoint! 

Fortunately bad installation advice won't always cause problems, 
since results depend on the final installation's common-mode 
impedance at the attachment point of the antenna.

I think it is better to avoid depending on luck and just do things 
right in the first place.
 
> As a practical matter, mine seems to have worked reasonably well
> this past year...pattern as expected.  No evidence of stray radiation
> from tower/etc., due to having my feedline run in what DOES seem to be
> an inappropriate place.

Many times we can get away with feeding a balanced element without a 
balun, or taping a balanced line to a tower leg or other cables. Look 
at the early TA-33, where the manufacturer directly grounded one half 
of a balanced dipole driven element!  We just have to learn to 
separate dumb luck from skillful design or installation.

The job of a common-mode choke or current balun is to allow two close 
points on a coaxial line to have a voltage difference, so the system 
has minimum current flowing along the outside of the shield. If we 
assume the coax shield is connected directly to a lower boom at the 
feedpoint, voltage difference between the boom and shield is zero 
volts. The potential difference comes in when the cable leaves the 
boom, like at the tower. At that point if the boom has 150 volts on 
it, so will the outside of the cable shield.

If the cable and tower impedance happens by pure dumb luck to be 
high, there will be minimal current on the outside of the cable 
shield where it leaves the boom for the run down the tower and the 
system will stay reasonably well balanced, despite the bad feed 
system design. If it is low, considerable current can flow over the 
outside of the braid and excite the tower or feedline shield like an 
antenna.

If the balun is a 1:1 choke balun, it belongs where the feedline 
leaves the boom. If the balun is a voltage balun or an impedance 
transformer, it belongs at the feedpoint but with an additional choke 
where the cable leaves the boom to the tower.

The cable could also be hung in the air either directly in the middle 
of the two booms making up the the log feed, or kept several boom 
spacings away from both booms. This also would apply to balanced open-
wire or open-buss feeds for antennas. In this case the balun would be 
placed at the boom-end of the unbalanced feedline, or directly a the 
feedpoint.

It never hurts to avoid potential problems and do things correctly. 
Especially when it is so easy to correct an obvious design or 
installation problem, and all the hardware is already in place.73, 
Tom W8JI
W8JI@contesting.com