Topband: Amidon W2FMI UnUn

[George & Marijke Guerin]

Fellow Topbanders,

I recently had a chance to smoke test an Amidon UnUn, Model W2FMI-1.78:1-HMMU50.
It has four windings on a 1.6 inch core with 3 output taps at the top, middle and bottom of the 4th winding (for 50 or 70 ohm feed line) and three low impedance taps at the top of the first, second and third winding, giving various impedance ratios of 1:1.36 through 1:16.
Measuring the transformations with an Autek antenna analizer, the various ratios look very close.  I put a 23 ohm resistor on the low side and measured the high side for 8 of the 9 possible ratios at 1830, 3650 and 7050 KHz.  It is rated through 20 MHZ on all ratios, but I did not bother with higher frequencies.
With the help of W8UVZ, I put a KW to it on 160 with a 50 ohm line and a Battle Creek Special as the 22 ohm load on the 2:3 turns ratio or 1:2.25 impedance ratio and it provided a good match from the inverted-L configuration to the coaxial cable transmission line and did not appreciably warm up or change SWR.

In my opinion, this appears to be a good device for matching resonant inverted-L antennas to transmission lines.

73    George    K8GG