Originally Posted by Rezarf
I am running a 4' Wilson Firestick with adjustable tip.
So one of those fiberglass jobs, probably electrically 5/8λ. Does it have a matching coil at the bottom? My guess would be if it's like the Firestik it does not and will probably be sensitive to coax length.
Nathaniel did a 2m ham antenna build that would demonstrate the principle, there would be a tapped coil at the feed point and the antenna would look DC grounded. For 2m a 5/8λ would look something like this.
The Firestik is a 5/8λ but doesn't have a matching coil, so they have to use the coax to match the impedance. Problem is they (IIRC) say to use 18' of RG-58. Assuming it's an ideal transmission line that's going to just mirror the feed point impedance at the other end. But in the real world coax has a lower-than-lightspeed propagation (called velocity factor, which is roughly 0.66 for RG-58), which means the coax is going to be some electrical length around 3/4 wavelength, which makes it act like a transformer. In that case the length is critical. You could use around 6 feet if you want to experiment.
Ham antenna manufacturers build the coil into the base so that the length of coax is not critical.