Hi Coyote,
As others have already mentioned having a good choke on the feedline is the key.
When operating mobile (40 and 20 meters) with a ham stick or hustler mobile antenna I use approximately 9 of the Fair-Rite 0431167281 clamp on cores and place them on the coax right before it enters the passenger compartment by taping that section of coax to a rail on the roof top luggage rack to keep them from banging around, or I tape them to the coax right after the coax enters the passenger compartment. I have also at times used a single FT240-31 toroid core with at least 8 turns of the coax wound on it. My test to see if I have enough of the clamp on cores on the coax is to touch the NanoVNA while watching the SWR or touch the coax anywhere in the passenger compartment and if the SWR changes drastically (noticeable shift in resonate frequency, etc.) I then add more cores.
Mike (W9RE) and I (WD8DSB) operated CW mobile in the Indiana QSO party last year and we used the 9 clamp on cores as mentioned above on our 40 and 20 meter hustler antenna feedline to stabilize the SWR. We also tried running a trailer hitch mounted 80 meter hustler mobile antenna and without clamp on cores we could only run 30 watts as our keying would lock up (computer running N1MM with a K3 transceiver) so we installed some clamp on cores on the 80 meter antennas coax external of the passenger compartment and that allowed us to run full power (100 watts) without any keying problems.
Using a good choke on the feedline really helps provide a stable and repeatable system and as an example I would never cut the stinger on a mobile antenna based on resonate frequency measurement without first having a good choke on the feedline.
73,
Don (wd8dsb)
