[TenTec] 238 "Tuner" Balun

Rick at dj0ip.de Rick at dj0ip.de
Thu Dec 28 03:12:18 EST 2017

Hi Bob,


It might help if you tell us "which" Ten-Tec balun you mean.

I will assume you are speaking of the Model 238-x range of matchboxes.
They all have a 4:1 Voltage balun which is more or less worthless for that particular task.
REASON:  The voltage balun has very little common mode impedance.  It's basically just a transformer.
Unfortunately in that respect (balun), TT just copied all the rest of the matchboxes that were on the market at the time and got it all wrong.
Even Ten-Tec made a mistake or two in their long successful history.

It is by far better to mount the balun external to the matchbox, unless you have one with a huge case where you can mount the balun with a few inches space between it and the cabinet or components (in all directions).  This is never the case in modern matchboxes.

The problem is, just as stray capacitance to ground degrades the common mode impedance of a symmetric ATU, it does also for a current balun.  A common construction technique is to clamp a toroidal current balun against (or close to) a conductive panel.  This is counterproductive and degrades the performance of the balun.  Unfortunately this is what most matchbox OEMs do anyway because they don't have enough space within the cabinet to mount it properly.

"An unbalanced T match followed by a 1:1 Guanella (current) balun in an external non-conductive enclosure and having high choking impedance, very short coax connection to the ATU, and high voltage withstand is capable of excellent performance in a ‘balanced ATU’ role for general purpose HF application."
(Source: http://owenduffy.net/blog/?p=4578 - last bullet in his Conclusion)

THEREFORE:  The best thing to do with that balun is carefully remove it and store it away.
There might be rare cases where you can use it (external to the case).

What you need is a 1:1 Guanella balun which is simply a specific number of coax wrapped around a specific ferrite core.
Homebrew this yourself; it does not even have to be in an enclosure since it is used indoors.

If you're not going to work 160 meters, use FT-240-43 for the core.
They cost less than $10 apiece.
Use one core per 500w of power you want to run.

Wrap 17 turns of RG-58 around the core (or stack of cores).
(For more than 500w, use RG-142 instead of RG-58).
You will need about 5 ft. of coax for the job.
Before you begin, solder a PL-259 onto one end of the coax.
You should head the instructions from K3LR and W3LPL in their presentation on antennas for contests (@ Contest University) and ALWAYS solder the shield of the coax to the barrel.   Then wrap the core.

Leave about 6 to 10 inches of coax stub sticking out from the core.
Use small wire-ties to secure the coax to the core before beginning to wind.
Simply wrap in one direction, trying to get as many turns onto the core as possible.
You usually end up with 18 turns.
Secure this end with wire-ties too.
This end goes to the antenna.  INSTEAD of soldering a PL-259 to it, solder a pair of banana jacks or whatever means of connection you wish to use for connecting the balanced-line feedline to the balun.

Wrap as described above but use the W1JR (Reisert) method of "cross-winding" the balun.
Electrically both ways are the same, but with this method, the two ends of the coax exit the core on opposite sides, rather than next to each other.
This is usually a more convenient mechanical solution.
For more info on how to do this, see:
The Reisert method of cross-winding is described with a picture at the very bottom of that page.

If you are not yet convinced that the above is correct, then read information on all 6 of the links at the bottom of this page:
(Link Soruces:  G3TXQ, K9YC, VK1OD, W8JI and of course DJ0IP)

In Jim's (K9YC) excellent document, "RF-HAM", he states that you should use a stack of 5 cores for full legal power.
Ignore that statement here.
Perhaps when running 1500w RTTY for long periods of time that is necessary but in this case, you will blow the caps in the 238-x LONG BEFORE destroying the stack of 3 cores!
Most of us only run CW and SSB and less than full legal power so 3 cores is enough here.

I suppose the biggest challenge for most of you guys will be finding the RG-142 and the FT-240-43 (or FT-240-31 for 160m).
One of the OM in our OCFD/Windom group has created a list with US-sources and prices these.
RG-142 is available from many sources, including Scott, W4PA (Vibroplex).

Rick, DJ0IP
(Nr. Frankfurt, Germany)

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Bob Good
Sent: 28 December 2017 03:11
To: TenTec at contesting.com
Subject: [TenTec] 238 "Tuner" Balun

The Ten Tec manual has virtually nothing to describe the internal balun supplied with the unit.  I do not know how to physically look to tell its design.  Is it a 1 to 1 or a 4 to 1;  Current or Voltage design?  Okay, if it is 4:1 or voltage design, please comment on the proper  design in schematic form or picture; what ever it takes for me to wind a current design, 1 to 1 balun.  Can the toroid be reused?  I use it on a ladder line, center fed 135 ft. dipole up 60 ft.  Other antennas are coax fed.  Thanks to all who would like to help this analog old timer.  Happy New Year and ‘73.  Bob K4BG.  (Good on QRZ) _______________________________________________
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