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
balanced.
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
correctly.
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
http://www.ifwtech.co.uk/g3sek
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|