Roy Lewallen wrote:



tim gorman wrote:
. . .
Unless you are using rg174 the 7 foot of extra cable should not make this
much difference unless the input impedance of the amplifer is not 50ohm
resistive. If it is not purely resistive then changing the cable length
can impact the SWR seen at the transmitter end significantly. . .

Changing the cable length won't change the SWR on the cable regardless
of the kind of load impedance and, if the SWR meter is designed for the
cable's Z0, it won't change the SWR meter reading, either. Except, of
course, that cable loss will always lower the SWR -- but that shouldn't
be a significant factor with such short cables.

Changing the cable length *will* change the impedance looking into the
cable, whether or not the load is purely resistive. The only exception
to this is if the load is resistive *and equal to the line's
characteristic impedance* in which case the impedance looking in will be
Z0 for any length cable.



Roy Lewallen, W7EL


I repeat, changing the cable length will change the swr *seen* at the
transmitter end significantly. As w7el says, changing the cable length
will change the impedance looking into the cable. Since most common,
inexpensive swr meters are calibrated for use with a 50ohm load, the actual
swr readings will be dependent upon the impedance presented to the swr
meter while looking into the cable. Although the actual swr may not change,
the swr that you see may very well change. A 7 foot stub represents an
eighth wavelength all the way down to 60m. This is enough to present an
impedance change.

tim ab0wr