jimlux 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.
"Two terminals" is fine on paper. But you never have two terminals.
You always have "ground", and who knows what else.
Take your balanced feedline and move the wires together so they touch,
and now you have only one wire. Only one terminal, so "by definition"
one terminal alone can't have a current, right?
But we can and do have current into just that one wire, exciting the antenna.
Common mode current on "balanced" feedline is just like that.
There are all kinds of ways to introduce imbalance into the
antenna/feedline combination. It is not just one or the other that
causes imbalance, and there are many things that can make it happen.
>> 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.
The problem is that the voltage on the two feedpoint terminals is
swinging with respect to ground. If you don't put something there to
isolate that from the feedline, your feedline will also be swinging
(have a common-mode voltage on it), making it radiate, and getting CM
current in the feedline.
Rarely can we isolate things in reality as well as we can conceptually on paper.
Andy
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