[TenTec] Model 238C Tuner Questions

Jim Brown k9yc at audiosystemsgroup.com
Tue Oct 16 12:03:23 EDT 2012

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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