[TowerTalk] CM current and SWR meter changes

Tod Olson tod at k0to.us
Fri Apr 20 10:14:28 PDT 2012


Net impedance at the 'feed point'Š ?

Of the antenna, of the feed line at the station?

I am not sure I understand the use of the term "net impedance' Steve.
Mentally I am picturing two impedances the are combined to give a single
impedance. The individual impedances are associated with the Common Mode
[CM] path and the inside of the coax RF path.

The inside the coax path is the one that the SWR measurement is taken on.
The CM path is isolated from the SWR measurement path [if the SWR meter is
a good one]. No matter what happens with the CM impedance, how does that
change the 'independent' SWR impedance? Maybe they are not independent?

Somehow I think that there are a lot of variables in play here and it gets
challenging to figure out what is doing what and when it is doing it.

Where are the guys who comfortably handle the equations? They should be
jumping in here to set us straight.


On 4/20/12 10:42 AM, "Steve Hunt" <steve at karinya.net> wrote:

>I may be wrong that it is a *direct* effect of the presence of CM
>current. Another explanation would be that changing the coax length
>alters the CM path impedance, that changes the net impedance at the
>feedpoint, and that in turn alters the SWR measured back at the shack.
>Steve G3TXQ
>On 20/04/2012 17:07, Jim Lux wrote:
>> On 4/20/12 7:46 AM, Steve Hunt wrote:
>>> Tod,
>>> Typical SWR meters are affected by any CM current that is present, and
>>> because the CM signal is a standing wave it can cause the meter to read
>>> differently for different cable lengths. In fact if someone ever
>>> to you that their SWR changed significantly when they changed their
>>> length, CM current is one of the likely "suspects".
>>> 73,
>>> Steve G3TXQ
>> What is the mechanism of the change here..
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