An additional source of VSWR error exists. It is of interest to the purist.

If I provide a perfect 50.000 ohm dummy load for typical JAN-C-17 RG
type cable, example RG-9913, I have to design for a maximum VSWR of 1.06
to 1 due to tolerances in the cable!! So, I'll have a minor variation in
observed VSWR as a function of length.

Same applies to the design impedance of your VSWR meter. It has a
tolerance on it's 50 ohms!!!!!

Deacon Dave, W1MCE

W5DXP wrote:

Dr. Slick wrote:

How would you explain what Cecil wrote? How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?


If I were using a 450 ohm SWR meter on 450 ohm Z0 feedline, the SWR
meter would indicate no change in SWR as I change the feedline length.
Assuming the 450 ohm SWR meter is accurate, the indicated SWR and the
actual SWR are the same.

When I use a 50 ohm SWR meter on 450 ohm Z0 feedline, the SWR meter
is NOT indicating the true SWR on the 450 ohm feedline. It is indicating
what the SWR would be if the line were Z0=50 ohms. All it is indicating
is the impedance seen by the transmitter which indeed does change with
feedline length in a system with reflections.

Theoretically, the SWR on a feedline changes only slightly with changing
feedline length and that is because of additional losses in the feedline
as the length is increased.