On 1/19/11 10:07 AM, Steve Hunt wrote:
> 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.
>
Exactly.. because the feedline is part of the antenna, now. If it's
asymmetric with respect to the antenna, it will have currents induced in
it (whether common mode in the case of a balanced open wire line or on
the outside of coax).
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|