George, W5YR w5yr@att.net
Sun, 22 Oct 2000 13:44:12 -0500

Not necessarily, Bob. 

SWR bridges of the conventional Bruene design will read 1:1 ONLY when
they see a 50-ohm resistive load in the output port. In your 100-ohm
example, the meter will read 2:1 not 1:1.

The Bruene design "assumes" that the input will be a 50-ohm coax
connected to a transmitter that wants to see a 50-ohm resistive load.
The application is such that either the antenna system will present
approximately a 50-ohm load for the transmitter - about 1:1 on the SWR
meter - or a tuner will be in the feeder line to force a 50-ohm
resistive load for the transmitter and hence for the SWR meter.

Even using a Bird 43 for SWR measurement requires that the coax be 50
ohm. "Equal input and output impedances" just doesn't enter into the
picture as bridges and power meters are designed for 50-ohm lines.

Your point about common-mode current is well taken, and I know of no
bridge or analyzer, even the MFJ, that can identify this component and
deal with it. With our comparatively simple SWR meters and analyzers, it
is up to us to ensure that no common mode current is present when we
take readings if we expect much accuracy. 

Common-mode current can "mess up" the readings of most analyzers as
readily as it affects SWR meters. It is perhaps worse with the MFJ with
its metal case since if you stand there on the ground holding the
analyzer case in your hands, you are providing a path for the
common-mode current to ground through your body. This changes the
common-mode current magnitude and path(s) and can provide results
different from when the meter case is not "grounded." The AEA/Tempo
CIA-HF analyzer gets around this by using a very think and heavy plastic

Common-mode current is the most common cause for "screwy" SWR readings.
A good way to identify the presence of such current is to watch the SWR
meter while you move your hand - loosely grasping the coax - along the
line for several feet. If the meter varies at all, you have common-mode
current on the outer-braid of the coax. This is a major source of RFI in
the shack and IS NOT avoided or disposed of by "a good ground."

But, don't get me started on that one!    <:}

Getting rid of common-mode current almost always involves use of one or
more appropriate current-mode chokes in the feedline, usually one at the
antenna feedpoint and another at the shack end.

Interesting stuff . . .

72/73, George   W5YR - the Yellow Rose of Texas      NETXQRP 6	       
Fairview, TX   30 mi NE Dallas in Collin county	     QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)

Bob & Linda McGraw K4TAX wrote:
> George & Steve & co:
> Most SWR bridges will read 1:1 when equal impedances are on each side of
> the bridge.  Feed one from a 100 ohm source and terminate the output in
> 100 ohms and you will have 1:1 SWR indicated.  I think what is missing
> (actually present) is current flowing on the outer conductor.  Often,
> simple bridges do not know how to or for that matter even deal with this
> condition.  One of the MFJ analyzers should give you a reasonably true
> picture.
> Feedline Z is another issue and is only part of the equation when
> feeding a load.  Yep, it's most always complex even when feeding a
> resistive load equal to the source impedance.
> 73
> Bob K4TAX

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