The T-Network Antenna Matching Unit was constructed for use with my all-band HF dipole with the added feature of switching various antenna and radio combinations . Install July 2012.

Due to the operational limitations of my Z-Match unit that had been in service for number of years an alternate antenna system matching was required. The Z-Match had shown to be problematic on a number of bands with particular antenna configurations that manifested as arching across the variable capacitors. While the Z-Match design is sound the availability of variable capacitors that suit the Z-Match circuit configuration that had sufficient plate spacing to deal with the voltages that can be generated across them are very difficult to source. As it turns out I had accumulated most of the key components over the years to construct a T-Match antenna matching unit capable withstanding very high power operation, well within the Australian power limitation of 400 Watts.

The T Match antenna matching unit is a very accommodating circuit that is well capable of matching almost any antenna and feed line combination impedance that the radio amateur is like to confront.

With the need to match a wide range of complex antenna loads between 1.8 and 30Mhz at moderate power levels it was decided that a simple T Match network was the most suited to the task and that all key components had been collected over the years.

While the T Match has a wide matching range the major draw back with this design is that it can act as a high pass filter and therefore offer little suppression of transmitter harmonics. While this would be a good additional feature it is regarded generally as not necessary for modern radios due the very high emission standards imposed on transmitting equipment these days.

Theory

The T-Network Antenna Matching Unit as do other types of Antenna Matching units match a transceiver with a fixed impedance of typically 50 ohms to the feed line and antenna combination load impedance.  The feed line and antenna impedance for the most part is unknown and complex often with significant reactance.

To achieve an ideal match the ATU brings the feed line and antenna combination into resonance there by removing the systems reactance and then correctly loads the transceiver to allow maximum power transfer.

Construction

This has been a long term project that has had the key components such as 500pf variable capacitors and a nice roller inductor procured at various ham-fests over the years. The variable capacitors with 3mm plate spacings are well capable of withstanding voltages of several kilo volts. Both capacitors are adjustable by vernier planetary 6:1 reduction drives to achieve very fine value adjustment. The roller inductor is adjusted by a rotation counter crank handle.  

Figure 1  Schematic of the T-Network Antenna Matching Unit including switching feature.

Two additional switches have been included in the circuit. The first is rotary switch to select one of three radios and another rotary switch to select one of three antennas.

No balun has been included in the T-Match tuner as it is viewed that an outboard balun gives the flexibility of trialling various baluns. Operationally some balun ratios may be more suitable in matching more extreme impedances presented on some band antenna combinations.

The T-Match tuner has been assembled on a aluminium base as part of an aluminium enclosure designed specifically to house matched component layout.

The components were all scale drawn in a CAD (Computer Aided Drawing) package to determine the ideal physical positioning to achieve the least possible component interaction, layout practicality and control functionality. While not essential for the construction of the T-Match tuner the CAD approach allows for various component configurations to be precisely positioned and accessed for practicality with accurate construction measurements determined from the final layout. 

Figure 2  CAD (Computer Aided Drawing) to determine the ideal physical layout and control functionality.

Photo 1  T-Network Antenna Matching Unit roller inductor and tuning capacitor fitted.

Photo 2  Front and top view of the T-Network Antenna Matching Unit.

Photo 3  Side view of the T-Network Antenna Matching Unit.

Photo 4  Rear view of the T-Network Antenna Matching Unit.

Photo 5  Top view of the T-Network Antenna Matching Unit.

With the key components fitted the values of the Variable Capacitors and Roller Inductor were measures with the AIM 4170C antenna analyser to plot of relationship between the tuneable components values and the front dial indications.

Figure 3  Plot of relationship between variable capacitor values and front dial indication.

Figure 4  Plot of relationship between roller inductor values and front dial counter indication.

Operation.

Tuning technique

The above matching technique described by Andrew S. Griffith W4ULD in his article Getting the most out of your T-Match Network Antenna Tuner has always produced excellent results with my T-Match Network and antenna system. 

Video of the T-Network Antenna Matching Unit, features, construction and operation.

References  

Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Antenna_tuner#Ultimate_Transmatch  

This is a simulator for the popular, series capacitor, parallel inductor T-network tuners. The three knobs across the bottom adjust the three components.

http://fermi.la.asu.edu/w9cf/tuner/tuner.html  

Getting the most out of your T-Match Network Antenna Tuner by Andrew S. Griffith W4ULD

QST article January 1995  

www.arrl.org/files/file/Technology/tis/info/pdf/9501046.pdf

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