[TenTec] Tuners

[Ten-Tec Inc. Amateur Radio Sales]

At 09:51 AM 6/4/98 -0700, you wrote:
>In reading the Antenna Book I came across tuners. The L network (hi and low
>z) and pi-network tuners are very efficient but the T-network is about 10
>times more inefficient. MFJ makes only T network and while Ten Tec offers
>both the L network and the T network they don't have any inbetween prices.
>I wrote MFJ but they declined to answer. Any hunches about why the T
>network is so popular and so inefficient?
>

Hi Jim (and all):

There is a reason why the T network tuner has enjoyed more popularity
than an L network tuner like the #238 2KW roller inductor tuner that
we presently build. 

First, the advantage to an L network tuner as opposed to a T:

Because there are only two variable components, there is only one
setting of each which will provide a perfect match to a given load
impedance, and this unique setting automatically provides the 
lowest network Q possible.  Low Q means low circulating currents,
hence low loss, and it also provides the widest frequency bandwidth
of operation before re-tuning is necessary. 

Since the inductor is always parallel, the L network always provides
a two-pole lowpass response to provide harmonic rejection.

In English:  You can more easily achieve a 1:1 SWR with an tuner
other than an L network, but is more likely you're dissipating RF
output within the tuner itself.  The T network is not as broadbanded 
as an L network.

The reason the L network, despite its' distinct superiority to a T
match, does not enjoy wider use is this:  The L network requires
two configurations to match all possible antenna loads.  One, for
impedances greater than 50 ohms, requires the capacitor to be
across the antenna.  The other, for load impedance of less than
50 ohms, requires that the capacitor be placed across the transmitter.
Keeping in mind that any SWR other than a perfect 1:1 match indicates 
a load impedance that could be either above or below 50 ohms, you
will need a switching system to reverse the configuration of
the tuner (which is used in the #238).  

The T network is more widely used due to a misconception about
what constitutes an efficient match between transmitter and antenna.
An SWR of 1:1 is meaningless if there is no efficient transfer 
of RF energy to the antenna system.  The T provides easy matching
capabilities, and is a simple circuit to construct and use. 

The switching system we use in the #238, in concert with fixed 
capacitance, allows 5 different tuner configurations.  We refer to 
the #238 as a "modified L" for this reason.  Our web site has a
picture of the tuner, simple schematics of the configuration
possibilities, and a more detailed description as well.  Between the
#229 and its' newer brother #238, we have been manufacturing this
"modified L" tuner for 18 years and are well satisfied with the
performance.     

I've said this before and I will say it again:  No antenna tuner,
ours or anyone else's, will match 'any' load impedance on 'any' band
at 'any' power output.  The important decision, when purchasing an
antenna tuner, is to accurately assess your needs and to have an
idea of what value of load you are trying to match.  A little reading
in the ARRL Handbook or the ARRL Antenna Book will give a wealth
of information for this. 
 
Sincerely,

Scott E. Robbins, W4PA
Amateur Radio Product Manager 




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