eHam
eHam Forums => HomeBrew => Topic started by: WT1V on December 29, 2022, 05:03:13 PM
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QST Jan 2023, pp 92-93 has an article about an antenna matching unit for QRP by Doug DeMAW. The original article was posted Feb 1973. The article shows the schematic and parts list including C6 and C7, dual section air variable capacitors, manufactured by nonexistent Millen Mfg Co. All the other parts seem to be available on EBay etc. These dual section, air variable caps, however, are the only components that are not made anymore since Millen Mfg no longer exists. C6 is 140 pf per section, C7 is 100 pf per section. Any one have any ideas where I might look around for these items?
73 wt1v
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You actually don’t need dual capacitors.
C7 has both sections in parallel, so a single 200pF
(or something close) will work.
C6 is a remnant of the “Ultimate Transmatch” circuit
that was in vogue at the time, but efficiency is
better if you leave off C6B and make a regular T
network tuner instead.
For QRP use, the “polyvaricon” (variable condenser
with polyethylene dielectric) type as used in analog
transistor radios has been popular.
Exact values aren’t critical: larger values improve
efficiency on the lower bands, while smaller ones
make tuning easier on the higher bands.
Note that both ends of both capacitors are hot
to ground, so they require insulated mounting.
I find lots of variable capacitors at hamfests,
but another source is MFJ - the ~208 pF capacitors
that they use in many of their smaller tuners
should be quite adequate.
I’m sure there are other sources as well - you
might check VK3YE’s tuner articles and see if
he gives any links.
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Yes,
The good part of the 1973 design was the matching bridge used to protect QRP finals, and the SWR meter made for QRP levels. For the matching unit, a standard T type would be superior. A reverseable L type would also be interesting. Good variable caps can often be found in old AM/FM radios often seen headed for the landfill.
73
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You don't need exact parts for this for the tuner portion of the circuit. And with QRP you can use receiver plate spacings on the capacitors and wire gauge can be minimal with the air wound coil.
If you plan 160m operations use about 400-500pf caps and about 29-32mHenrys for the coil. If just doing 80-10m use about 200-250pf caps for easier tuning.
For 100w+ plan on a least 1000v spacing for the caps.
For full power, 3.5-6kV spacing caps recommended, and 12-10ga. wire in the coil. MFJ does sell parts, but you could probably find a better price at Digikey or Mouser.
Space wound coils are easy to make yourself.
-Mike.
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...but you could probably find a better price at Digikey or Mouser.
Good luck with that...
I just went through the DigiKey listing. Anything over
about 50 pF is pretty sparse. The Sprague-Goodman
film trimmers are obsolete and limited to 100 to 150 volts
or so, but still sometimes available if you can manage
to glue a shaft onto one. There are some Voltronics
high voltage trimmers that go up to 200 pF and a
few thousand volts that were made with a panel-mount
shaft, but they ran over $400 each, and are also
obsolete and no longer in stock.
That's why I recommended the capacitors from MFJ, as
they will sell any part that they use in their own tuners.
Or you can look at the inventory at Surplus Sales of
Nebraska (https://www.surplussales.com/index/Air-Variable-Capacitors-Shafted.html) - not as cheap as a hamfest, or all sized
for a QRP tuner, but you might even find a replacement
for the original types if you want to maintain the original
circuit...
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I learned something new today. Very interesting reading. Thanks WB6BYU
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More background on why the ultimate transmatch is no longer so ultimate
https://ve7sar.blogspot.com/2018/02/the-ultimate-transmatch.html
Also the original ultimate transmatch
http://www.arrl.org/files/file/Technology/tis/info/pdf/7007024.pdf