I too agree with Carl and George. Most all SWR meters are "relative"
devices. And yes, if there is any current circulation on the outside of the
coax, changing the length of the line will change the apparent SWR.
One source of this current is an induced value from the radiating device
itself. (Feedline in the presence of the antenna.) Now, should the line be
terminated in a non-radiating load, moving the bridge to a different
position in the line will have little if any effect on the indicated SWR
value, except the nominal SWR induced from the insertion of the bridge
itself and the combined line loss. The lower loss the line, the less effect
one will see with regard to SWR changes. The induced SWR from the bridge
itself is a constant provided the frequency is a constant as well.
A second factor is the actual source impedance of the device producing the
RF. Ideally, we'd like to think the output of a transmitter is 50 ohms.
Most are actually lower. Ideally one should isolate the RF source from the
bridge by a suitable device as a 6 to 10 dB attenuator of 50 ohms Z. In
this condition, the bridge is actually seeing 50 ohms, or the Z of the coax,
on one side and the line/antenna load on the other. For critical antenna
adjustments, on VHF I've always used about 50 ft of RG-58 as a "lossy line"
between the transmitter output to the bridge. Then the feedline with the
antenna can be connected to the output of the bridge with reasonable
assurance that antenna adjustments can be accurately made. This is a bit
tougher to do at HF.
Good reading folks. Keep up the good work.
73
Bob, K4TAX
----- Original Message -----
From: "Carl Moreschi" <n4py@earthlink.net>
To: "tentec" <tentec@contesting.com>
Sent: Wednesday, January 29, 2003 9:06 PM
Subject: Re: [TenTec] Jupiter SWR, and VSWR generally
> George,
>
> I agree 100 percent with your analysis. That being said, I've never seen
a
> real world situation where the SWR that you read didn't change when you
> changed the coax length. It makes me think that the SWR meters we have
are
> only rough approximations and don't really computer SWR properly. Also, I
> think currents on the outside of the coax shield are more prevalent than
we
> think. This current could be a major contributor to SWR changing when the
> coax length changes. Anyway, it's an interesting discussion. The
> theoretical verses real world does seem to be misaligned in the SWR case.
>
> Carl Moreschi N4PY
> Franklinton, NC
>
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