Hi Pat,
Here is what MFJ says about their RF isolator which I mistakenly called a
line isolator.

The MFJ-915 RF Isolator is a 1:1 Current Balun designed to be placed in line
with a 50-Ohm coax. The RF Isolator is effective from 1.8 to 30 MHz. It was
designed to improve the performance of all amateur stations, fixed or
mobile.

MFJ-915 will reduce or eliminate stray RF often found on coax. This stray RF
can cause burns and other problems with electronic equipment in ham-shacks
and vehicles. Stray RF also results in a loss of radiated power from the
antenna, which will lead to a great reduction in signal strength. Use of the
MFJ-915 will increase the efficiency of all amateur stations.

Eric
KU3I