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Old 10-12-2011, 12:10 PM
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DaveInDenver DaveInDenver is offline
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Join Date: Jun 2006
Location: Larimer County
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Quote:
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.
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