[TowerTalk] Fwd: current balance in ladder line?

[Robert Harmon]

Hi Jim,
This is a very interesting discussion.
Suppose you are feeding a full wave delta loop with ladder line and
one side of the loop is in close proximity to a large storage building with metal roof and siding.
Do you think this could cause an imbalance in the loop which would contribute to unbalanced currents
in the feedline ?

Bob
K6UJ



On Jan 19, 2011, at 12:09 PM, jimlux wrote:

> 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).
> 
> 
> 
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