[TowerTalk] EFJ Matchbox Question

[K7GCO@aol.com]

In a message dated 11.06.00 18:26:43 Pacific Daylight Time, 
kh7m@hsa-kauai.net writes:<< 
 Hi,
  Am fooling about with an old kW Matchbox.  I have just
 yesterday put up one of W7FG's  "true ladder line"
 fed 80 meter doublets.  Uses #16 gage wire,  spaced
 about 3 3/8 ",  or about 580 ohms or so line Z.
 
 Have the doublet up as an inverted-V;  apex at about
 55 feet,  ends about 15 or 20 feet,  and the angle 
 between the legs is around  130 degrees or so,
 broadside more or less oriented on toward the US
 mainland.  Worked like "gang busters"  last afternoon
 on 40 meters,  at least about Hawaii;  fellows on the
 other islands said the signal was as strong or perhaps
 more so than what they receive from the Force 12
 EF-140S dipole.  Closest other island station was about
 120 miles away;  more distant down on the Big Island,
 about 325 miles or so.  Maybe we are really comparing
 NVI performance on 40 in the latter afternoon hours.
 
 Anyway,  my question about the Matchbox,  the "output"
 capacitor is a dual differential job.  I find that on both
 of my most frequently used 75 and 40 meter frequencies,
 for local KH6 land contacts,  that this cap wants to be
 set at the highest dial reading,  and then even a bit
 more!  Scale goes to 100;  but seems to give the
 best match at 108 (estimated).  Assume this just means
 the knob has slipped on the shaft or something over the
 decades of use.  But at max setting of this control on the
 kW Matchbox,  is this dual-differential variable at a
 max value of C on one section,  and a min on the other,
 or what??  The Matchbox manual indicates more typical
 settings,  when coupled to a 600 ohm resistive load would
 be a setting of around  32 on 75 meters,  and about 45
 on 40 meters.  Of course,  that is to  a pure R load.
 Just seems odd that I would wind up with this dual-diff
 cap at a max setting (whatever max means using a
 dual-differential variable).
 
 Per the original instruction manual,  the "tuning" capacitor
 is a dual variable which is used to tune each half of the
 secondary coil of the coupling inductor to resonate the
 coupling circuit.  The "matching" capacitor is the dual-
 differential variable which acts as a voltage divider to
 match the antenna load.  The secondary coil of the
 coupler is tapped on each side of the balanced output
 circuit;  different taps provided for the five bands
 80 up to 10 meters.  Note on 30 meters,  the 20 meter
 tap position works fine;  the 15 meter taps seem best
 for the 17 meter band.
 
 On the modern T-tuners,  the most efficient operation occurs
 when either the input or the output variable C is at max
 capacitance;  but do not believe that is true at all when
 using the Matchbox -- an entirely different circuit.
 
 Anyway,  were the case of the Matchbox not held on with
 absolutely dozens of sheet metal screws,  it wouldn't take
 too long to remove and have a look inside,  hi;  but as it
 is...too much trouble,  so just thought I would ask in
 case someone on the reflector knows how things are operating
 inside,  hi.
 
 BTW,  this Matchbox has been modified a bit,  per the notes
 on LB's,  W4RNL web site.  There is now a series tuning
 variable in the primary or "link"  input circuit;  and each of
 the primary coil turns has been tapped.  This allows the
 primary circuit to be more quickly and adequately set
 to resonance or the needed input Z,  and also provides 
 rapid "tweaking" as the two secondary circuit resonating 
 and the voltage divider caps couple reactance back into 
 the primary "link" circuit when they are  adjusted.
 
 I always set this tuner up using the 259B antenna analyzer
 before the rig is connected to this antenna system.  The
 Matchbox circuit can develop some extremely high voltages
 within if running even low power in to tune it up.  E. F. Johnson,
 in the original instructions, caution to NEVER tune up the
 Matchbox with live RF.  They requested that an SWR bridge
 always be used for tune up (manual,  pg. 4).  Interesting...
 
 Thanks for any info about the dial vs.  capacitor settings
 within the old Matchbox.
 
 73,  Jim,  KH7M
  >>
    It sounds like the coupling has shifted on the shaft.  If it peaks at 108 
it should also peak at 92.  I'd take it apart and completely disassemble the 
MB and check for arc pathes and even burned off wires, check each switch 
contact and remake or clean all connections etc.  I found that Johnson on a 
run of 275W MB's connected the 50 ohm tap to the 300 ohm 5th turn link tap 
and the 300 ohm link tap to the 2nd turn--they goofed.  If an antenna loads 
up with the dual-differential Xc full open or closed (0 or 100) it may not be 
properly tuned.  Change the length of the feedline.  Unless you have my magic 
length of open wiire line the settings can be way off and even some arcing.  
I use a series Xc in the link only if a load is hard to match and I also 
added a switch to select also the 3rd, 4th & 5th tap on the link also.  I had 
a 40M Lo-Z load that caused arcing on the KW model even with 1:1 SWR.  I 
reversed the connections to the differential Xc and with 1:1 SWR got more 
output, no arcing and about 20% less tank coil was used.  I have a big knife 
switch in there I added to do this quickly.  I use plug in RF ammeters in 
series with both output legs and added another SO-239 for balanced 100 coax.  
It's one of the best systems going.  K7GCO

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