My assumption from looking at the cheaper swr meters that have this defect
is that the cases are inadequately bonded. In a typical cheap cb type meter
its two halves with only a couple sheet metal screws that may not make good
contact due to paint. The meters are also commonly plastic faced and
mounted right to the pc board providing a big gap in the front of the case.
On good meters the circuit board is separate from the meters and may
actually use coax from the connectors to the board instead of relying on the
case as the ground. Also the measuring circuit may be isolated from the
case and separately shielded using transformers to feed out the samples.
In the Daiwa meters I use now the measurement circuit is in an unpainted
plated box with the connectors on it that is mounted inside the case that
has the controls and meters.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: Jim Lux [mailto:jimlux@earthlink.net]
Sent: Friday, April 20, 2012 16:07
To: towertalk@contesting.com
Subject: [TowerTalk] CM current and SWR meter changes
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
> reports to you that their SWR changed significantly when they changed
> their coax length, CM current is one of the likely "suspects".
>
> 73,
> Steve G3TXQ
>
What is the mechanism of the change here..
Say I have a box with a coaxial connector on each side. there is no
connection between the inside of the box and the outside of the box except
"through" the wall. Inside the box (i.e. inside the shield) I have a
current and voltage probe, from which I can determine the direction and
magnitude of power flow, reactive and active power flow, SWR, etc. ). The
ends of the system are also boxes with the generator contained entirely
within the shield, as is the load. So there's no common mode path.
If I change the load or generator impedances, the I and V will will change,
and my box can measure the SWR (or any other related parameter) jus tfine.
If I now put some current on the "system" that flows along the outside of
the coax (i.e. a common mode current), that doesn't change the I & V I
measure inside the box (because it's only measuring between the center
conductor and the inside of the coax)
ANd if I change the load impedance so the power flow changes, it should be
exactly the same as in the case with no common mode.
Now... the contention is that "real" devices don't do this (something
I'll readily accept). The question is "why".. it implies that the box
is somehow also measuring the current on the outside of the shield.
I'm not sure, though, what kind of design defect would cause this..
Maybe something like leakage capacitance?
If we are looking at a balanced line configuration could easily see an
imbalance of leakage C from internal components would result in a
transfer of current from outside to inside. But how would that happen
in a coax type design..
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