On 8/9/2010 1:46 PM, Jim Brown K9YC wrote:
> On Mon, 09 Aug 2010 13:29:44 -0500, Stuart Rohre wrote:
>
>> His antennas were in the clear, which can affect RF on the shield,
>
> RF on the shield IS common mode current. Common mode current is the
> result of antenna imbalance, mostly with respect to ground and grounded
> conductors. For example, a tree, wiring in a house, the ground slopes
> under the antenna, the antenna slopes, ground conductivity varies under
> the antenna. Using coax is only PART of that imbalance. A common mode
> choke simply kills that current.
And the feed line not leaving the antenna at exactly 90 degrees.
>
> The virtue of coax is that it is EASY TO PUT AN EFFECTIVE COMMON MODE
> CHOKE ON IT, whereas with parallel wire line it is not.
>
> The choke does not BALANCE the antenna, but it DOES reduce the common
> mode current to near zero, so all the antenna current flows in the
> antenna. That DOES tend to force the current to be equal on either side
> of the coax connector. The antenna may still behave asymetrically
> because of those imbalances noted, but the coax is no longer carrying
> common mode current, either on RX or TX. In other words, the choke
> effectively causes the coax to "disappear" -- as far as the antenna is
> concerned, the only effect of the feedline is to connect the antenna to
> the transmitter and add a bit of loss. The choke does not add loss
> (unless it's a lousy choke), it's only the loss in the coax.
>
> 73, Jim K9YC
>
>
That's what the choke balun is all about.
73, Jerry, K0CQ
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