06-25-2006, 09:40 PM
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Join Date: Apr 2005
Location: Louisville, KY
That adjustment is to minimize the SWR between the radio and the amplifier, NOT on the antenna side.
When the amplifier is keyed, the radio isn't feeding the antenna any more, it's feeding the two driver tubes. The input adjustment is to fool the radio into thinking it's driving a 50-ohm antenna with a low SWR, when the Varmint is keyed.
Requires you put a SWR meter between the radio and the amplifier. Keeping the jumpers short, like 3 feet or less tends to help. The internal SWR meter on a base radio may be accurate, or it may not.
With the amplifier keyed and tuned up for max modulated power, the input match is adjusted for minimum reflected power on the radio-side meter. You'll probably notice that turning it ALSO affects the max modulated power coming out of the Varmint when you turn that knob, as well.
In a perfect world, the spot where it shows minimum input-side SWR is is the same knob setting that shows max wattage coming out of the Varmint. In the real world, max drive may be at one spot on the knob, and least SWR may be at another spot. This calls for setting that knob for a "compromise" between the two knob settings. Generally, you should be able to get the input-side SWR under 1.5 without sacrificing more than 5% or 10% of the power coming out. If the amplifier's input SWR is much over 2 to 1, the radio's final transistor may get worked too hard.
You'll also fid that the Varmint's input SWR will change when switching from "Supermod" to "High". The tubes in the Varmint tend to run cooler on the Supermod side.
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