LF: Antenna tuning tip
Sat, 14 Mar 1998 18:49:46 -0500
> >From Peter Martinez G3PLX Kendal Cumbria.
> Here's a little idea for adjusting the antenna tuning if, like me, you don't
> have a variometer, and it's a bit of a nuisance fiddling with turns on the coil.
> Build the coil to tune the antenna at or just below the bottom of the band.
> To bring it up in frequency into the band, connect a capacitor in series,
> between the coil and the antenna. The value you will need is obtained by
> multiplying the antenna capacitance by the antenna resonant frequency
> and dividing by the twice the frequency shift you want. For example, my
> antenna is 330pF and resonates with my fixed coil at 135 kHz. To bring it
> up to 136.5 kHz I need a series capacitor of 330 * 135 / (2 * 1.5) =
> 14850pF. A 15nF is close enough. I have a 22nF, a 15nF, and a 10nF and
> clip the antenna onto whichever I need to get me to the bottom, middle,
> and top of the band. The capacitor doesn't need to be a high voltage one,
> but check it doesn't get too hot with the current. I use the biggest ones
> I can find from the junkbox. I would steer clear of ceramic capacitors of this
> value as they usually have incredibly high negative temperature
> I think this is probably a lower-loss way of adjusting the antenna to
> resonance than parallel-tuning with a capacitor. If you have an isolated link
> winding for the feeder, the capacitor can go in the bottom end of the coil,
> which would make it easier to fit a switch.