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How to balance your
antenna
By Tom Rauch, W8JI
Flag and Pennant mail list, November
3, 2000
The
ICE 180 transformers comes in small 2" x 3" aluminum boxes,
as does their termination resistor, model 185A.
I hang
the resistors on my Beverages out in the open air.
People are really getting fancy!
The only thing special I do is put a spark gap across the insulator.
All of my resistors are just hanging there (I do coat them with
Krylon spray) and a few have lasted about 30 years now, despite
being moved to three different locations.
On a more serious note, the box won't hurt a thing on the
terminated loops. The objection to the metal box on the feedpoint
end are mostly due to the fact these terminated loop antennas
don't have much signal sensitivity, and they have a high input
terminal impedance.
At the feedpoint the metal box, even if not tied to the loop or
the feedline shield, will increase capacitance between the loop
and the feedline shield. Since the common impedance of the loop
is reasonably high, this can upset the voltage division between
the loop terminals and allow the loop to respond more to common
mode (where it acts like a short longwire) signals. Less-than-
perfect-isolation along with a bit of unbalance will also allow
the feedline to couple any signals or noise it "picks up" into
the antenna.
The Flag or Pennant will not be perfectly balanced even
if the feed method is, because one side of the antenna is closer
to ground. Using an isolated winding allows the feedline connection
to look like a perfect ground independent current source, so the
voltages from each loop terminal to ground can seek any value
they like to cause even current distribution.
You may or may not have problems with transformers without very
low primary to secondary capacitance, depending on how that capacitance
is distributed. If the capacitance is in a ratio that happens
to cause the correct voltage division on the output, you'll never
have a problem even with a fair amount of coupling. The antenna
won't respond to feedline currents, and the feedline won't cause
additional unbalance to the antenna.
Beverages and EWE's would be a different story, because
any common mode from one unbalanced system to another can be a
problem, but the saving grace is the ground system at the feedpoint
offers a low impedance so you can tolerate more common mode coupling.
As a matter of fact is the ground system is perfect the shield
of the feedline can share the same ground connection as the Beverage
or EWE without introducing unwanted signals.
In many cases the grounds on Beverages and EWES are very poor,
so the feedline shield (if connected to the Beverage ground) acts
like the "other half" of the antenna!
Even if this doesn't introduce noise it can reduce system directivity...and
directivity (as opposed to gain) is all that matters with
S/N ratio on weak DX.
That's why I favor electrically non-connected windings on all
antennas, and low capacitance windings on high Z "balanced" antennas.
Small loop (non-termainated) antennas at times solve this
balance issue by using a shield over the "loop". The shield becomes
the actual loop antenna, and IF it is grounded at the exact electrical
center the loop is perfectly balanced and will not be susceptible
to feedline common-mode currents. The shield, contrary to popular
folktales, does not shield the antenna from electric fields. The
shield simply allows a ground at the proper point to balance the
antenna.
If you do not ground the loop shield at the electrical center
point, you can be worse off than not having a shield on the loop.
To some people, this all may appear to be "fussy".
But unless you know how to tell if you have a problem, you may
not know you have it. Sometimes an ounce of prevention is worthwhile,
and knowing how it works will allow to understand potential problems.
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