[TowerTalk] Johnson Matchbox Question

[Rob Atkinson]

On C2 the top sections are in series with the balanced RF coming from
the coil.  If those sections are fully meshed you have run out of
capacitance.  Because 80 m. is 500 kc wide, the balanced line from the
load, which should be a half wave center fed dipole, will go through a
range of R + J.  Therefore, you may hit a rough frequency where it's
hard to get a match.  You can increase the length of the feedline
which is what I did.  But that just moves the tough frequency around.
Add enough length and maybe you'll move it out of the band.  Adding
line is a lot of work.  Or, you can experiment by clipping additional
capacitance across those top series sections which is a lot more fun.
Find a pair of 100 pF disc ceramic caps and clip them across each top
section and put a vswr analyzer on the input to the Box and see what
that does.  Add or subtract added capacitance until you can tune out
that -132 reactance.  Once you find an added fixed value that allows
you to cover the band, you can find capacitors that can handle the RF
voltage and current.  I opted for a pair of 50 pF vacuum caps which I
mounted on upside down yogurt cups with cutouts in the bottoms to hold
the caps.  I cut copper strap and bolted the caps in with silver
plated knife switch clips on the top strap pairs, allowing me to clip
them to the floating rotor frame when I need them.  This solution
worked for me because I run my matchbox out of the cover, something I
always do with my matching networks.   If you want yours to stay
inside the cabinet, you'll have to be more creative.  I've always
found being able to actually see how my air variable caps are meshed
to be helpful when tuning in a frequency on one of my dipoles.

73
Rob
K5UJ

On Mon, Mar 14, 2022 at 9:50 AM Tom Hellem <tom.hellem at gmail.com> wrote:
>
> Rob-
> I think you are on to something. My tuner still has the link (primary) coil, but the relay has been removed, and the lone tap is one turn or so from the grounded end, which as you suggest is for a coaxial input according to the schematic.
> I am using the Matchbox to tune an 80 meter dipole fed with a roughly 90 foot piece of 450 ohm window line. My VNA indicates the impedance at the tuner end of the line to be 63-j132 on 80 meters, where I seem to have the most trouble getting a perfect match with the tuner. With the cover off I can see that C-2 is maxed on the upper (ungrounded) section. My best match is around 1.5:1 swr, and it gets worse from there depending upon the weather.
> Thoughts?
>
>
> Tom
> K0SN
>