Al, I agree on the caps and the coils.
The usefulness of the Johnson design took a big dive in the 1970's when
transistorized rigs and then the WARC bands came in. The TX side of its
tank circuit was no longer sufficient for the job. As a result, you really
have to play with feedline lengths a lot when you try to use one on all
bands.
There is a simple mod you can make on the Johnson that will bring it back
into being a useful matchbox.
You only have to add a third variable capacitor in series with the coax line
to the TX and use a 4 position switch to tap the coil on the TX side of the
link. After that, it provides an excellent match over a much broader range
of impedances than it previously did. [Both sides of the new variable cap
must be insulated from ground.]
You can read more on this here:
http://www.dj0ip.de/antenna-matchboxes/symmetrical/viking-vs-annecke/
73
Rick, DJ0IP
-----Original Message-----
From: Al Gulseth [mailto:wb5jnc@centurytel.net]
Sent: Saturday, February 16, 2013 4:58 PM
To: tentec@contesting.com
Cc: Rick - DJ0IP / NJ0IP
Subject: Re: [TenTec] OT: Openwire Balanced Antenna Tuners (QST Test)
Rick,
TNX for the spreadsheet - it's an interesting read. What I gleaned in a
quick analysis is that while they aren't as convenient, the "old school"
link-coupled tuners (in this case, the Johnson Matchboxes) seem to hold
their own pretty well against the newer technology, especially when it comes
to efficiency. Maybe there's something to be said for the use of air-wound
inductors and good quality variable caps (as opposed to toroidal inductors
and fixed caps) when it comes to antenna tuners?
73, Al
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|