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Re: [TowerTalk] current balance in ladder line?

Subject: Re: [TowerTalk] current balance in ladder line?
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Wed, 19 Jan 2011 09:04:14 +0000
List-post: <">>
jimlux wrote:
>On 1/18/11 8:46 AM, Andy wrote:
>>> Why is the current into the antenna balanced?
>> Well, it isn't always.  You are right to question that assumption.
>If there's only two terminals, the current is, by definition, balanced
>at that point.

But there aren't only two terminals at the output from a transmitter/ATU 
into a "balanced" feedline. There are always three terminals - the third 
being the transmitter's ground terminal. It is the ground connection 
that creates the possibility of common mode current on twin feeder (or 
currents on the outer surface of coax).

Twin feeder is not self balancing - it actually has very little 
capability to equalize its differential currents and suppress 
common-mode currents. You always have to *do* something to *make* it 

Or to put it another way: you cannot buy "balanced feeder" on a reel! No 
matter what the label may say, what they're actually selling you is 
simply twin feeder. Only *you* can make it balanced, by installing it 

That leads us into the use of chokes and balanced ATUs, all of which are 
aiming to create a high impedance in the common-mode path to ground, so 
that the system behaves <as if> it had only two terminals.

>> An OCF (off-center fed) dipole does not have balanced currents at the
>> feedpoint,
>Not true, from basic circuit theory.  *at the feedpoint* the current
>into one terminal will be exactly the opposite of the current into the
>other terminal.
Sorry, no. There is nothing at the feedpoint of any antenna to enforce 
equal and opposite currents into the feedline - and certainly not the 
feedline itself.

Once again we have that hidden third terminal, the unwanted pathway for 
common-mode current.

In any real-world installation, currents on the two legs of an antenna 
will never be exactly equal. With twin feeder, the difference between 
those two currents will appear as a common-mode current on the feeder 
itself. In coax, that difference in currents between the two antenna 
terminals will spill onto  the outside of the shield.

The advantage of using coax is that it's quite easy to place a highly 
effective common-mode choke at the feedpoint itself. This suppresses the 
common-mode current at the feedpoint and enforces equal and opposite 
currents at the antenna terminals, which in turn influences the current 
distribution across the whole antenna.

If the antenna and its environment are asymmetrical (which in real life 
they always are, to some extent) there will then be some 'regrowth' of 
induced common-mode currents further down the feedline, but with coax 
that is easily treatable by adding another choke.

The major weakness of using twin feeder is that it's very difficult to 
create an effective common-mode choke for use at the feedpoint, 
especially in multiband systems where the voltages and currents at the 
feedpoint can vary enormously. Instead, we are reduced to enforcing 
current balance at only one location, the bottom of the feedline.


73 from Ian GM3SEK

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