[TowerTalk] To get a truly balanced antenna feed

[Ian White]

>Any suggestions how to apply this concept to ladder line/open feed?
>
>Ed McCann
>AG6CX

(Apologies, Ed, I was tied up yesterday with Radio Club Secretary work.)

The answer to your question is "With great difficulty!" All the things that can be done relatively easily with coax feedlines - making effective CM chokes, measuring CM current and so on - are either very difficult with open-wire or physically impossible.

CM chokes at the feedpoint are very difficult. Obviously you couldn't wind a choke from the open-wire itself, and whatever alternative line you choose, high voltages at the feedpoint on some bands would cause great difficulties with broadband ferrite chokes.

About the only *practical* thing you can do to reduce common-mode current in open-wire line is to insert a CM choke in the coax at the transition point, or at the ATU. But even that will prove difficult with a multi-band antenna because of the high voltages that appear on some bands. 

Whenever you insert a high CM impedance in the form of a choke, there will also be a regrowth of CM current a quarter-wavelength away. If this proves troublesome, with coax you can simply insert another CM choke farther up the line, but with open-wire that isn't possible. 

Measurement of common-mode and differential-mode currents on open-wire line are also very difficult because they require a complicated arrangement of current transformers and baluns. It can be done [1] but I have never seen it described in an amateur radio publication.

That is why, out of the many thousands of amateurs who claim to be using "balanced feedline", there are few or none who can actually *prove* it. All the rest are really just expressing a hope or a wish which will almost never be granted.


[1] A Google search for
 <  measure "common mode" and "differential mode" impedances  >
will produce lots of references, but all of the measurement techniques rely on idealized baluns within the test fixture itself. The measurement errors in a high-Z situation remain completely unknown.


73 from Ian GM3SEK


>-----Original Message-----
>> On Jun 23, 2016, at 12:48 AM, Ian White <gm3sek at ifwtech.co.uk> wrote:
>>
>> VE7RF wrote:
>>
>>> ##  Ian mentions the current.  If that’s the case, why not just
>>> measure the current with a clamp on ammeter.... at say
>>> several random places on the coax feedline ?
>>
>> That is exactly what I try to do - see my website: "Clamp-on RF Current
>Meter - *the* most useful tool for RF interference troubleshooting."
>> http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm
>>
>> A perfectly functional instrument can be thrown together in half-an-hour.
>The passive diode detector will typically read from about 25mA up to 1A. It
>is also possible to build a much more sensitive log-scale instrument based
>on an AD8307 detector chip (or similar) which will read down to 1mA, but
>with reduced resolution.
>>
>>> Then we have yet another issue to deal with, and thats
>>> what if the coax braid is  bonded to the top of the tower ?
>>> If a lousy CM choke balun  is used at the ant feedpoint,
>>> and  braid also bonded to top of tower, we can then have
>>> a case where  CM current is now flowing down the tower.
>>
>> Well jes' don't do that! Don't use a lousy CM choke at the feedpoint, use a
>good one; and if in any doubt, use another CM choke (which doesn't have to
>be so good) just above the point where the braid is bonded to the tower.
>>
>> Or best of all, measure the CM current both before and after.
>>
>>
>> 73 from Ian GM3SEK
>>
>>
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