Total posts : 45366
This would be a good place to start. Unless you have the CB whip you can save money by using a wire and if you are satisfied with how it works you could then get the whip or a copper pipe.
The idea behind a loading coil is to use the inductance of the coil to cancel the capacitance of the short antenna. When done properly (that is, when at resonance) the only load the transmitter output will “see” is resistive and, in theory, this will provide an effective transfer of power.
Without digging out my AM-25 manual and going only on memory I don’t recall them mentioning tuning the coil/antenna. Even if not tuned, the coil inductance moves the radiating system impedance in the right direction providing the coil inductance is not too high. My experiments with a tuned matching network, not a simple coil, shows a very sharp resonance accompanied by a marked increase in signal strength.
I think you will find an improvement with the coil in the system but if you want to push it further put taps on the coil and try different ones until you get the best performance (usually at resonance). An oscilloscope is a great aid in this adventure and if you have one available I will be willing to post how I use mine as a field strength meter for tuning the antenna (rather than doing so now which would make this post quite long).