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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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