My MFJ 259 bottoms out at about 1780 kHz, well within tolerance of the
specs. On some occasions, especially when building notch filters and
bandpass filters for 160, it would be desirable to be able to tune down to
the offending frequencies involved (AM BCB).
In order to accomplish this, I added another 3 position mini-rotary switch
just below the tuning and bandswitch knobs, and centered. To it I attached a
tapped coil made from 8" of 26 Ga enamel wire wrapped around an FT-50-J
core, for 10 turns with the tap at 7 turns.
By cutting the trace between L1 the 160 M band coil and the bandswitch
terminal it was easy to add the additional inductance in series with the
160M coil.
With all 10 turns (255 uH extra) in series, the frequency range becomes 500
kHz to 1 MHz.
With only the first 7 turns in series, the frequency range is 750 kHz to
1.76 MHz.
Two extra positions covers the whole Broadcast Band.
It is possible to add even more inductance, and 1 mH brings it all the way
down to the Lowfer band, but oscillation seems to be unreliable below 300
kHz.
Happy Building, Geo>KØFF
http://homepages.dstream.net/K0FF
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|