A very, very simple way to get a match when the tuner can't do it alone is
to simply add a capacitance or inductance in series with the antenna "hot
lead" right where it connects to the tuner.
The inductor can be simply a coil wound using No. 22 to 24 wire on a 35 mm
plastic film can or something similar. Punch two holes at the top and two at
the bottom to thread the wire through to secure the ends. Every two or three
turns, twist the wire to from a tap loop. Use your hobby (Xacto) knife to
trim off the insulation for a tap. Of course you can use a toroid core, but
if you do use the largest one you have handy. Sometimes high currents flow
in the inductor, even at QRP power levels, and a toroid can saturate and
become lossy. The beauty of air core coils is that air cannot "saturate".
Your only losses in an air-wound coil are ohmic losses in the wire.
If you don't have a variable cap (or don't want to take up the space) just
use a few fixed caps of various values. Something from 50 to 300 pf is
normally what you'll want at HF. One thing to avoid are physically TINY
capacitors. Just like the coil, under some conditions the capacitor may have
to handle significant RF current. Disc ceramics or larger dipped silver
micas are usually FB. If you have some 50 or 100 pf caps, parallel them
until the tuner finds a good match.
Whether you'll need the inductor or the capacitor depends upon the
electrical length of the antenna. If, on the band(s) where you can't get a
match, the antenna is less than 1/4 wavelength long, you're sure to need the
inductance. Inductance "lengthens" the antenna electrically. If the antenna
is close to 1/2 wave or longer on the troublesome band, you'll probably want
to use a capacitor in series. That will tend to "shorten" the antenna
electrically. For a given antenna, you can usually come up with one
capacitor or inductor tap position that lets you tune all the bands
automatically. It simply a matter of moving the impedance of the troublesome
band back into the range of the tuner.
Ron AC7AC
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