Saturday, February 17, 2018

The Ultimate Transmatch

The Communicator Revisited: September 2011

A few participants have asked me about the “Ultimate Transmatch” used at Field Day in conjunction with our “vee beam”.  

This device would today be called an “antenna tuner” which is really a misnomer because the device does not tune the antenna, rather it provides the transmitter with a 50 ohm resistive load by compensating for capacitive and inductive reactance present in the transmission line and antenna.  The SWR protection circuits in modern transceivers require an SWR of 2: 1 or less if they are to deliver the full rated power of the transmitter section. If the forward power is all radiated from the antenna and none of it is reflected, the SWR will be 1:1 which is ideal, but seldom achieved.  

The Ultimate Transmatch became popular nearly 40 years ago when it was introduced by Lew McCoy in QST of July 1970.  It is mainly renowned for its ability to match a wide range of impedances, far greater than the average “tuner”.  In conjunction with the use of open wire feeder which is close to “lossless” compared with any transmission line you can name, it is an extremely useful device particularly when the antenna (in the case the vee beam) is non-resonant, i.e not cut for any particular band or frequency.  I used it at home for several years to match my transmitter to an 80 m inverted vee fed with ladder line, on bands in which the SWR likely exceeded 50:1.  Try that with your average tuner.  The high SWR doesn’t much matter if the transmission line losses are low.   I used the 80 m vee successfully on 80, 40, 20 and 15 m to work the world with 100 watts despite an SWR that was off-scale on my meter.  If my feedline was coax instead of ladder line, the losses would have consumed most of the power and very little would have radiated.  

Alas, the Ultimate Transmatch is no longer considered “ultimate” as the circuit has been supplanted with other variations that are considered superior.  Modification of my unit to a more modern version would be a simple matter, and I intend to do it one of these days.  The figure shows the basic circuit which is a basic T-network with an additional shunt capacitor placed across the transmitter terminals and ganged to the transmitter-side capacitor in such a way that the two capacitors move together.  Its chief disadvantage is that the T configuration is that of a high pass filter, which means that it does little to suppress harmonics or spurious signals that are higher than the desired frequency.  Although not a perfect device, it saved the day for us at Field Day when the MFJ auto-tuner could not do the job.  

Here’s what David Knight G3YNH and Nigel Williams G3GFC say about the Ultimate Transmatch in Impedance Matching. Part 2: Popular Matching Networks: 
"An inauspicious fate awaits anything which presumes to call itself 'The Ultimate'; although to be fair, it was probably so called because of its enormous matching range in comparison to the then-prevalent networks intended for use with co-ax fed antennas. The Ultimate transmatch must now be regarded as an obsolete circuit; but to those who still own and use such units, it should be obvious where to put the wire cutters. With the offending C3 removed from circuit, it should still be possible to match any impedance with occasional recourse to a dual-capacitor matching strategy; and by connecting the spare C3 across C2 it should be possible to increase the efficiency still further, provided that the minimum capacitance does not become too large for matching high-impedance (high |Z|) loads at high frequencies."

VA7XB’s home-built Ultimate Transmatch using roller inductor and two high-voltage variable capacitors, with  SWR meter at left rear and 4:1 balun at right rear.

The original article was written by John Brodie VA7XB and appeared in The SARC Communicator in September 2011. 

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