View Single Post
Old 10-12-2011, 01:10 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
Join Date: Jun 2006
Location: Larimer County
Posts: 6,613

Originally Posted by Rezarf View Post
Thanks Dave-

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.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche
Reply With Quote