At 08:01 PM 2003-01-30 , N4KG wrote:
>The SIMPLE solution is to take off the cover
>and LOOK at the output capacitor. Adjust
>the knob until the plates are fully meshed,
>and NOTE the position of the POINTER.
Well, "easy to say" but not so simple to actually DO with a Johnson Matchbox.
To take the cover off requires you to first remove the knobs on the capacitor
shafts, so you instantly lose any pointer reference position you might have
had. Furthermore, it's next to impossible to put the cover back on without
disturbing the capacitor shafts AFTER you have so carefully rotated them to
fully meshed or unmeshed positions.
I've found no better way than to leave the cover on, peer through the
perforations with lamps or flashlights shining in from very specific
directions, and re-position (if necessary) the knobs on the shafts -- all while
everything is buttoned up. Even then I'm not 100% sure of when the plates are
exactly 0% or 100% meshed.
An alternative method is to use the flashlight method above to get reasonably
close to 100% or 0% mesh with the cover and knobs on, then watch the SWR of an
arbitrary antenna as you tune each individual knob through a small range on
both sides of its "0" or "100" setting. Most of the time, the ACTUAL 0% mesh
and/or 100% mesh positions will be obvious by the way the SWR varies as you go
through the region of minimum or maximum capacitance. If there's a single dip
or peak and it doesn't exactly coincide with the "0" or "100" mark on your
capacitor scale, your knob pointer is not correctly set. The same is true if
you find that there are TWO dips or peaks NON-symmetrically located near one
end mark or the other -- your knob pointer is not correctly set.
The biggest error factor with the Johnson Matchbox (and many other couplers,
too) is ultimately how good you are at tightening down the knob's set screws
without disturbing the settings you just found by the methods above.:-)
Bud, K2KIR
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