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ABOUT ANTENNA CAPACITANCE:
Antenna capacitance is a function of both the length and the diameter of the antenna. The RadioShack CB whip is actually 102″ long and .2″ in diameter. This calculates to be 24.5 pf. A 102″ long 1/2″ copper pipe (actually .625″ OD) calculates to be 30.3 pf. These are calculations. Actual measured capacitance may be slightly different.
If the copper pipe antenna is length adjustable by plus/minus 4 inches you will get 29.3 pf at 98″ and 31.3 pf at 106″. When used with a loading coil with taps spaced properly, the whole antenna can be tuned by selecting the best tap and then peaking the tuning by adjusting the antenna height.
If your transmitter doesn’t have an internal test point for a voltmeter to show the tuning peak, the absolute best way to tune is with a Field Strength Meter (FSM). An FSM has a short whip antenna. It should be positioned as far as possible from the antenna within range to get a usable reading and no closer than about 2 ft and should be supported in a stable position. The meter will read highest when the antenna is tuned to resonance.
MFJ sells these inexpensive FSMs: