2020-11-23, 12:46:13
First design thoughts:
The tuning must be as simple as possible. Although a variable capacitor seems like the best solution, high voltage air-spaced variable capacitors are becoming very rare since manufacturers stopped producing tuned RF radios (replaced with VFOs and rotary encoders). One possible solution being considered is to use a fixed inductance and a fixed capacitance that are close to resonance then adjusting the length of the whip for fine tuning.
For a dual band buddistick for 20m and 40m, a 20 microhenry coil in series with a short coax open stub will theoretically work. For the 20m band, 3 inches of RG-8X coax will provide a capacitance of approximately 6.25pF. For the 40m band, 12 inches of RG-8X will provide a capacitance of approximately 25pF. In each case, the core of the coax will be connected to the coil and the braid will be connected to the whip. The coax stubs would be plug-in to facilitate easy band switching.
The inductance and capacitance calculations are based on using a Buddipole 9ft whip (which is actually about 9ft 4in long). If the L-C circuit is resonant at a slightly lower frequency than required, the whip can be shortened to bring the antenna resonant frequency up a little.
The tuning must be as simple as possible. Although a variable capacitor seems like the best solution, high voltage air-spaced variable capacitors are becoming very rare since manufacturers stopped producing tuned RF radios (replaced with VFOs and rotary encoders). One possible solution being considered is to use a fixed inductance and a fixed capacitance that are close to resonance then adjusting the length of the whip for fine tuning.
For a dual band buddistick for 20m and 40m, a 20 microhenry coil in series with a short coax open stub will theoretically work. For the 20m band, 3 inches of RG-8X coax will provide a capacitance of approximately 6.25pF. For the 40m band, 12 inches of RG-8X will provide a capacitance of approximately 25pF. In each case, the core of the coax will be connected to the coil and the braid will be connected to the whip. The coax stubs would be plug-in to facilitate easy band switching.
The inductance and capacitance calculations are based on using a Buddipole 9ft whip (which is actually about 9ft 4in long). If the L-C circuit is resonant at a slightly lower frequency than required, the whip can be shortened to bring the antenna resonant frequency up a little.