[TowerTalk] Optimum/Efficient Matchbox Operation

[Jim Reid]

Aloha to all,

Got long;  delete if not interested.

I received  excellent input about using the Matchbox
from many.  Perhaps the most complete came
from Russ,  NM6DX,  who also has a kW Matchbox
with the same added link series C and selected
taps on the primary link coupling coil.  I have followed
his set-up instructions to the letter,  and am quite
pleased with the results.  He also allowed that I
might share his experience/instructions and my results
in following them with the reflector folks who might be
interested.

First Russ's set up instructions which would apply only to
the W4RNL modified version of the kW Matchbox:

"I will describe the proper tuning of a modified matchbox 
such as the one you have. This tuning results in the least 
amount of power lost in the tuner and maximum bandwidth 
with your specific load.....

It should be confirmed that the tune capacitor has maximum 
capacity at 100 on the dial. I use about 20 watts for initial 
tuning. Select the band being tuned with the band switch 
(in this case 40 meters). Set the input coil link to the 3 turn tap. 
Set the link capacitor to half capacity. I use a 270 pF
capacitor for the link capacitor. This provides the low capacity 
needed for 10 through 15 meters and has been sufficient 
for 80 meters. Apply about 20 watts to the input of the tuner 
and adjust the match and tune controls for minimum reflected 
power. 

The VSWR should be less than 4:1 or so. If this
is not so adjust the link capacitor to reduce the VSWR 
while retuning the match and tuning caps. It should be 
possible to get a very low VSWR (unity is typical) at this 
point. If this is not so change the link coil tap to
another position and repeat the process. Record the link 
tap and the matching and tuning cap settings."

I followed Russ's instructions to the letter and obtained
a swr of 1.0:1 immediately at 7.088 mHz,  my particular
set up test frequency.  Tuner settings were much "better"
than what I was using yesterday.  His suggestion of using
turn 3 of the link coil resulted in the Clink being set to about
20 degrees of a full 180 degree mesh;  Matchbox Tune C
reading of 31;  and the Match C reading of 79 resulted in
the unity swr reading.

But this is not the end!  Russ now has instructions for optimizing
the settings for best operating bandwidth,  and highest efficiency
within the coupler!  As follows:

"It is probable that several link positions will result in tuning 
that produces minimum VSWR. The way to get the lowest 
loss setting combination is to measure and record the 
number of kHz between some low value of observed
VSWR with a small fixed power input. I use 20 watts input 
and 1.3:1. By setting the tuning capacitor to a lower capacity 
and temporally increasing the VSWR slightly and then retuning 
the match and link caps to minimum VSWR it is possible to 
keep unity VSWR and increase the observed 1.3:1 VSWR
bandwidth. The idea is to get the maximum bandwidth possible 
with unity VSWR. This should also be checked at different link 
tap settings. The maximum bandwidth settings result in the 
maximum RF current into the open wire line. This is the lowest 
loss in the tuner settings."

So I proceeded further with his instructions.  My initial resulting
BW,  for in my case 1.28:1 swr bandwidth was 57 kHz.

I reduced the Tuner C by three scale divisions,  and the swr 
at my center 7.088 frequency increased to 3.6:1.  Tweaked  
the link C just a few degrees,  and increased the Match C 6 
scale divisions,  and the swr was again 1.0:1.  This time 
the 1.28:1 BW increased to 66 kHz.

Reduced the Tune C another 3 scale divisions;  swr increased
only to 1.46:1 now at the center F.  Link C raised a tad to 30 
degrees and increased the Match C to its max value of 100,
swr was again 1.0:1.  1.28:1 BW had increased to 88 kHz!

Now Russ gives one final instructions to see what is 
happening to the tuner efficiency,  whether it is worth
going on,  stopping,  or even going back a setting:

"Now disconnect the antenna feed lines from the tuner,
reapply your RF power and measure the vswr.
You should see greater than 15:1 VSWR with 20 watts 
input and the antenna disconnected. The directional couplers
used in ham stuff are usually not very good at high VSWRs 
but the measurement can be done with a high quality coupler. 
The higher VSWR measured the lower the antenna tuner loss 
for that tuner setting. I had a little less than a 3:1 VSWR
observed before I used the tuner with adjustable link coupling 
and that increased to about 18:1 after the use of the tuner
tuned as previously described. The VSWR reading can be 
related to loss by converting to return loss etc."

Again,  I followed Russ's directions,  used my Bird 43 with a 100
watt slug to do the forward and reflected power measurements.
Rho, the reflection coefficient,  is the square root of the ratio of
reflected to forward power.  Bingo,  I measured a Rho of about
93%;  or a swr of over 25:1.

Decided I didn't like leaving the Match C at max value,  so went
back to the settings of the tuner where the BW between the
about 1.3 swr points was "only" 68 kHz,  down from the 88.
Repeated the BW measurement,  and this time came up
with 78 kHz with the Match C at 89.9 again!

And,  the swr measured,  with the antenna feed disconnected
from the tuner,  to be 27:1.  Sounds just fine to me,  given
Russ's information!

So the settings above will be  used  within the allowed
SSB band of 7075 to 7100 out here in KH6 land.  Good
for DX;  have worked ZS and the Falklands around 7090
from here.  Also covers the FISTS calling frequency of
7058.  Could not be happier with these results here on 40.

Thanks to Russ and each of the others who sent along
very useful info about the old E. F. Johnson Matchbox,
and to LB,  W4RNL for his writings about improving this
historic design.  But,  you know,  I still am not sure what
and how works a "dual differential" variable capacitor
as is used as the Match C component in the Matchbox,  hi.
One of you suggested I buy a powered screwdriver to
disassemble the Matchbox case;  that way I could
see inside and learn about that dual thingy.

73,  Jim,  KH7M
On the Garden Island of Kauai


 







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