Application Note #10
Subject: On Taking the Rainbow to 20m
David Larsen WB7QMR asks ...
"I'm ready to put my Rainbow Tuner together, but would like to put it on 20 meters. I read all the technical articles on your site, but didn't find my answer. What should the value of C7 be for 20 meters?
Joe replies ...
Actually for 20 meters, just changing C7 may not give you much tuning range. The whole tank circuit needs to be modified. Probably the easiest way to do this is by reducing the value of inductance.
You *could* do this by winding a new toroid for L1, but getting lower inductance and the correct impedance-matching taps might be tough.
The easiest way to do it is to add another inductor of equal inductance across the GND and ANT terminals. This will halve the inductance and allow you to tune 20 meters with the original C7.
The additional coil can be another T68-2 toriod with the same number of turns as L1, but not taps. The taps on the on-board inductor will work just fine.
Hope this helps!
Joe E., N2CX
from Southern New Jersey, y'all