[TowerTalk] Antenna Coupler Capacitor Settings

[Bud Hippisley, K2KIR]

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