Why can I use coax between the tuner and the 102-inch whip in order to keep the feed line from radiating?
A. The capacitance between the center conductor and the shield will essentially bypass the signal around the high impedance antenna making for a very inefficient system. On the other hand, if the antenna is resonant on the band you are operating in (a 75M Hamstick operating in the 75M band, for example) then the feed impedance will be reasonably close to 50 ohms and the coax will work fine. In this case the tuner serves only to “trim” the match a little but you loose the real benefit of using a tuner to jump from band to band without getting out of the vehicle to change whips. If you are using a wide range tuner to match a single whip to bands where it is not inherently resonant then you MUST NOT use coax for the feed line between the tuner and the antenna. Use a well insulated single conductor wire, keep it as short as possible, and keep it spaced well away from surrounding metal.
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