[TenTec] Model 238C Tuner Questions

Robert Mcgraw rmcgraw at blomand.net
Tue Oct 16 14:27:35 EDT 2012

For the very reason Jim states, I do not like antennas fed from anywhere
but the center.  Either use a balanced feed system or use a good 1:1
current balun or choke of suitable RF power rating {if in doubt go for a
higher power rating} at the feed point when using a coax feed system.

I know that many say OCF antennas work and work well.  However there is
most always feed line radiation and that means feed line reception too. 
With the feed line being basically vertical, we find that vertical
antennas and vertical feed lines are very prone to receiving local noise.

A balanced antenna feed has and offers a high degree of common mode
signal rejection.  This common mode rejection makes for several dB of
reduced noise.

Balanced feed systems are really quite easy to install and use and are
proven to be the highest efficient means of transferring RF.  I know what
you've been told by others about balanced feed systems.  From my
experience, what I've been told by others is largely at least 90% wrong. 
The other 10% is questionable.  I run a balanced feed from the top of the
tower, down the tower on suitable stand-offs, through the eve vent,
across the attic, and it drops down through the ceiling to the tuner. 
Nope, no coax or balun to get to the outside either.  That defeats the
general purpose.

Yes the procedure in the manual works well.  Start with the lowest
frequency bands first, make a chart and move to the higher frequency
bands.  As one gets to the higher frequencies, the tuning controls are
more critical but the actual tuning range is much wider than many other
types of tuners.

Bob, K4TAX

> On 10/16/2012 4:19 AM, Richards wrote:
>> Does anybody have a simple description of the procedure one should use
>> to quickly find the sweet spot?
> There's a procedure in the manual, which is not in front of me at this
> time, but from memory it goes something like this:
> Using small settings for L and C, switch the center knob from its off
> setting to high and to low, observing which seems to reduce SWR and use
> that setting as a starting point. Now, adjust C and L for reduced SWR.
> When you switch the center knob you are adding capacitance, so when you
> increase the C with that switch you should retune the capacitor to
> minimum and continue.
> The trickiest matches to find are those with small values of C -- it can
> take a while to figure out whether the switch should be left or right.
> In general, high values of L and C are needed on the lower bands, very
> small ones on the higher bands.
> The Ten Tec tuners DO tend to stay matched over a wider range than most
> tuners, but the antenna you describe is not well behaved, and a poor
> choice if you have local noise because it is inherently severely
> imbalanced.  That imbalance puts considerable common mode current on the
> feedline, which really needs a choke, but the severe imbalance puts so
> much common mode voltage across the choke that VERY high values of
> choking Z are required if the choke is to handle high power.  OCF
> dipoles are notorious for frying chokes.
> 73, Jim K9YC
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