[ICOM] AT500 problem on 18 and 24 Mhz

[Adam Farson]

Hi Bert,

The Icom 0 ~ 8V band-selection voltage is ambiguous. 18 and 21 MHz share one
voltage value; 24 and 28 MHz share another. As a result, the AT-500, AT-100
and AT-150 share preset positions for 18/21 MHz and 24/28 MHz respectively.

A good way around this is to calibrate the AT-500 with a 50-ohm resistive
load on 21.2 MHz, and adjust the preset pots until the 4 LED's are dark.
Then, calibrate in the same way on 28.5 MHz.  Check on 18.11 and 24.93 MHz
to verify that the tuner matches the dummy load at these frequencies,

Check again with the antenna on 21.2 and 28.5 MHz; the reflectometer in the
tuner will detect any reflected power, and cause the tuner to re-match.
(This assumes that antenna SWR < 3:1 at these frequencies).

Now, when you select 18 or 24 MHz, the tuner should match the antenna
(again, assuming VSWR < 3:1). You may need to recalibrate the presets with
the antenna if the AT-500 capacitors are too far out of range when you
change bands, or recalibrate on a lower 15m and/or 10m frequency with the
dummy load.

An alternative is to start by calibrating the AT-500 in midband on 18 and 24
MHz with the dummy load. 

Cheers for now, 73,
Adam VA7OJ/AB4OJ




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