BC-306A Low Frequency Antenna Tuning Unit

[Tom WA3KLR]

Over the last 6 months I have heard a number of people remark that they have the WWII BC-306A Antenna Tuning Unit but do not know anything about it.  I have one also and doped out the unit 12 years ago.  Two photos below also.

The unit consists of a tapped inductor, switch (E) and a variable inductor (variometer F) and 2 ceramic terminals.  The fixed inductance and variable inductance are in series, and built on the same coil form.

This is the inductance values I came up with for the unit:
                 Variometer
Switch E   F=0       F=100
   1    Shorted between terminals.
   2         85 uH    303 uH
   3        265 uH    595 uH
   4        550 uH    915 uH
   5        835 uH   1210 uH.

There is some overlap on ranges.  If you find in use that you need a value of inductance in the overlap region, use the lower range setting as this will have the variometer itself at a higher inductance setting which will be higher Q.  The killer of variometers is that at low inductance where the two coils are bucking each other, all of the wire loss is still there with little resulting inductance, hence poor Q.

Have fun lowfers.