The use of differential capacitors in an ATU offers important advantages. Mike Grierson, G3TSO, explains
In the January 1987 RadCom I described a general purpose antenna tuning unit(1), based upon the 'T-Match' variant of the popular 'Ultimate Transmatch' ATU that has been featured in the ARRL Handbook for a number of years(2). The Transmatch has undergone a number of circuit revisions over the years and the T-Match variant is one of three fundamental circuits, the most recent being the 'SPC' Transmatch(2).
Many commercial ATU designs feature the T-Match variant of the circuit which has the advantage of being the easiest to build with only three major components. As the T-Match is a high-pass filter network, it offers no harmonic rejection and serves purely as a variable impedance matching network, a task it performs very well over a wide range of frequencies.
The use of a roller-coaster type of variable inductance achieves flexibility in matching, but has the disadvantage of making band changing a little tedious. In addition, the adjustment of three interdependent tuning controls with more than one combination of settings for a matched condition can appear a little daunting to many of today's black-box operators.
Fig 1. Differential connection of Cl and C2.
After careful consideration of the T-Match circuit and some experiments using the original ATU, I decided to connect the two capacitors C1 and C2 (Fig 1) together differentially. Differentially means that C1 will be at a minimum value when C2 is at a maximum value and vice versa. At the halfway position both capacitors will be of equal value and 50% of each individual capacitance. This has the effect of providing a continuously variable tapped capacitive reactance between the input and output of the ATU to which the variable inductance can be connected.
The T-Match functions by matching the series network Cl, L1 to the input impedance whilst the output impedance is matched to the series network C2, L1, where L1 is common to both circuits. By making the ratio of C1/C2 continuously variable with a single rotating shaft, the requirement for two interdependent controls has been eliminated. For maximum effectiveness the inductor L1 must be of the roller-coaster type. A switched inductor will not give a perfect match, because two conditionally varied adjustments are always necessary.
Tests have shown that the differential T-Match is capable of matching a similar range of impedance to that of the traditional T-Match - somewhat to my own surprise - but with considerably easier operation. The time taken to tune up is reduced, making band changing far less tedious. In addition, there is now only one possible setting of the controls for a matched condition.
Differential capacitors are not very common and should not be confused with split-stator types. Two Eddystone 250pF medium-spaced transmitting-type variables with a shaft at either end were obtained new from a rally for about £5 each. They are easily ganged together in tandem using a brass spindle coupler and may be arranged back to back or simply in line, phased 180° apart. The capacitors should ideally be of the linear type and not 'E Law' or logarithmic.
Most transmitting capacitors will be of the linear type (with semicircular plates) so this is not likely to be a major problem. Suitable capacitors and roller-coasters can be purchased from at least two companies who advertise regularly in RadCom, and the 250 + 250pF types can easily be modified for different Operations. The roller-coaster used by the author contained 28 turns, making it suitable for operation from 80 to 10 metres. For 160m operation at least 35 turns would be required.
It Is Important to use heavy gauge wire for interconnections in the ATU, keeping leads as short as possible. The earthy end of the roller inductor and the sliding tap should be grounded with a heavy-duty braid to reduce any stray inductance which may prevent 10m operation. The photographs show a suitable layout. Operation of the ATU is relatively simple. Set the capacitor to the 50% position and adjust the inductor for maximum noise on receive or for minimum VSWR on transmit. Now adjust the capacitor for a further improvement in noise or VSWR, and repeat the operation alternately using the inductor and the capacitor. Two to three adjustments are all that is normally required. Tuning is quick and precise and the use of slow-motion drives is not necessary. The majority of matching will occur with the capacitor between the 25% and 75% rotation positions. To change bards simply adjust the inductor, in creasing for lower frequencies and reducing for higher frequencies and repeat the procedure described above.
Fig 2. Complete circuit of tuner.
Additional items such as a built-in VSWR bridge and a balen will enhance the ATU design and can be found in my previous article(1). Fig 2 shows the circuit of the complete ATU. Switches have been avoided to keep the size down and the balun can be brought in to circuit by the use of banana plugs and sockets on the rear panel. The balun may be used as either a 1:1 or 4:1 depending on the interconnections.
After a number of experiments with the traditional T-Match and the differential T-Match the following advantages have emerged.
The differential T-Match is easier to tune, making band changing less tedious and necessitating less time on the air during tune-up. There is only one position of the controls for a matched condition compared to the two or more with the traditional circuit arrangement. By installing two capacitors in tandem the width of the unit can be reduced, making it more compact and more suitable for portable operation.
G3TSO, Mike Grierson