[TenTec] Model 238C Tuner Questions

Robert Mcgraw rmcgraw at blomand.net
Tue Oct 16 20:32:20 EDT 2012

The feed impedance on my 160M 1/2 wave wire is about 38 ohms as I recall.
 The currents in each side of the feedline are equal to about 8% or
better.  I'd consider the system to be therefore reasonably balanced. 
And I surely could not consider this impedance as "high impedance".  At
the same time the feed currents again are equal, but for 80M operation
with this antenna the currents are only about one tenth of the value for
the same power on 160M.  In this case the impedance is quite high.

Now there may be some installations where circumstances do not permit
good installation in terms of optimum balance.  I think all need to
understand, just as you said, a balanced feedline does not make
automatically a balanced antenna system.

At the same time, I've never been satisfied with the approach of
connecting a piece of coax direct to the center of two equal lenghts of
wire in dipole fashion.  Looks like this approach is connecting an
unbalanced feed to a reasonably appearing balanced antenna.  Hence the
ultimate need for a choke type balun.

Bob, K4TAX

> On 10/16/2012 11:27 AM, Robert Mcgraw wrote:
>> Balanced feed systems are really quite easy to install and use and are
>> proven to be the highest efficient means of transferring RF.
> Substitute the words "high impedance" for "balanced" and I'll agree with
> that.  Two-wire lines do NOT make a system balanced. Most REAL antennas
> have some degree of imbalance, and that makes the feedline prone to
> common mode current. Antennas are unbalanced by their surroundings --
> trees, buildings, the nature of the earth under them, ground slope,
> unequal heights, and unequal lengths. Antennas fed two-wire line need a
> serious common mode choke just as much as those fed with coax.
> 73, Jim K9YC
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