The problem
Users of some ATUs may have noticed particular sensitivity to hands on the capacitor adjustment knobs. It is a common problem with cheap implementations of the T match as the capacitor rotor is usually at high RF voltage and if that shaft is extended to the adjustment knob, under certain circumstances tuning becomes very sensitive to hands on the knobs.
In some of these implementations, if the users hand touches the metal grub screw in the knob, or the metal panel bushing behind the knob they may get a significant RF burn.
The cause
Let’s use the MFJ-949E as a discussion example. It is a T match, and the metal capacitor shafts in the knobs and panel bushings carry RF voltages.
So why is this only sometimes a problem?
The RF voltage across the coil, and impressed on the capacitor shafts can be extremely high when using loads with small resistance and large negative reactance, more so on the lower bands.
A deeper understanding
Let’s explore a load of 10-j500Ω @ 3.6MHz, it is in the range of what might be experienced at the base of a shortened vertical, but it might also exist at certain points on a multi band antenna feed line such as a G5RV.
Above is a simulation in W9CF’s T Tuner simulator using values typical of the MFJ-949E. The selected inductance is the smallest L tap that will match this load, the values of the capacitors are that needed to match the load. Note the value of the input C is smallish (29.1pF) and calculated tuner loss is 49% and efficiency is 51%. Small input C causes high internal voltages.
If we have knowledge of the relationship between knob calibration and capacitance and inductance, we can make a pretty good estimate of efficiency using T Match efficiency estimator.
Plugging the relevant values into T Match efficiency estimator we get…
It is an approximation, but it is very close to that from the T match simulator.
The efficiency in this case should be very concerning, it means that for 100W average input power, the internal dissipation in T match elements (mostly in the inductor) is 35W… and it will not withstand that dissipation without heating and softening the styrene coil supports permanently damaging the coil.
The calibration curve of the capacitor and inductor knobs can be obtained by measurement.
Above is my measurement of the total circuit capacitance of the capacitors in the MFJ-949E vs the knob calibration. The linear model is C=228-20*setting(pF).
My measured inductance values are above.
A Simsmith model of losses at rated power (300W)
Above is a Simsmith model of the ATU with 10-j500Ω load and 300W input. Capacitor Q is assumed to be 2000 and inductor Q is based on measurement.
Note that the power dissipated in the inductor is 140W, way above what it could withstand continuously.
The voltage across the load is 1944Vrms, the voltage across the capacitors is C1: 3750Vrms, 5303Vpk and C2: 1793Vrms, 2536Vpk, voltage across L1 (and C2 shaft) is 3736Vrms, 5284Vpk. The ATU would not withstand these voltages and would not handle 300W input on the example load, and would be unlikely to handle more than about 30W input continuously to avoid melting the coil supports.
Conclusions
Operation on extreme loads causes extreme voltages and high internal dissipation.
The sensitivity to touch is a hint of a more sinister problem that extreme voltage exist inside the ATU and that may drive damaging coil losses.
The efficiency of a T match on the lower bands can be well estimated using T Match efficiency estimator.