You can have a dipole where each leg is equal length has an identical
impedance to ground, and yet still have unequal currents.
Picture such a "perfect" dipole fed with coax. The outside surface of
the braid forms a "third" antenna wire which is capable of flowing CM
current quite separately from the balanced Differential-Mode currents it
is delivering to the feedpoint. That "third wire" connects to just one
side of the dipole and has the potential to unbalance the currents.
Depending on the length of the coax braid, how it is routed, and how it
is grounded at the shack end, it's possible for almost all the current
intended for one dipole leg to be "diverted" along the coax.
A balanced antenna system doesn't guarantee balanced currents once you
attach a feedline.
Steve G3TXQ
On 19/01/2011 17:53, Hans Hammarquist wrote:
> You can, if you want, view the dipol as two pieces, each with one pole at the
> feeder and the other pole at ground. If the the two part are equal, as we
> intend to have them, the two "hot" poles, will have the same current. If, on
> the other hand, we have an off-center feed the impedances will be unequal and
> the only way to keep the current in he two poles equal is to "force" it with
> an isolating balun such as a current choke.
>
> The current between the antenna wire and ground is, in scientific term,
> called "displacement current". You actually have a current without a
> conductor. That's the same type of current you have through a capacitor.
>
> Hans - N2JFS
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|