At MJF-986 ATU – flashover analysis I mentioned that the MJF-986 ATU differential T ATU uses smaller capacitors than a good T match, and that gives rise to higher loss and more importantly higher vc2 in the example scenario.
MJF-986 ATU Differential T model
Now let’s model the scenario in SimNEC.
Above is a SimNEC model of a similar scenario, the components are not exactly the same as F8DRA’s, but similar and allow discussion of the risk of flashover for that system if is as he describes and there were no faults.
The first thing to note is the dipole is approximately a full wave and the center feed impedance will be very high, perhaps as high as 4400Ω depending on proximity to ground, 2200+j0Ω is assumed here.
That feed point impedance is transformed by the two wire line, the input impedance to T1 is 43.34-j124Ω. This is the load seen and matched by the ATU.
C1, L1 and C2 make the MFJ-986 ATU. It is a differential T circuit, so C1+C2=300pF. The ATU has been adjusted for a match.
The model G block is set to supply 1000W to a matched load so that voltage calculations apply to 1000W input. Note that all voltages and currents shown are RMS. The model also calculates vc1 and vc2, the voltages across C1 and C2.
Generic 500pF T match model
Now let’s model the scenario in SimNEC.
Above is a SimNEC model of a similar scenario, the components are not exactly the same as F8DRA’s, but similar and allow discussion of the risk of flashover for that system if is as he describes and there were no faults.
The first thing to note is the dipole is approximately a full wave and the center feed impedance will be very high, perhaps as high as 4400Ω depending on proximity to ground, 2200+j0Ω is assumed here.
That feed point impedance is transformed by the two wire line, the input impedance to T1 is 43.34-j124Ω. This is the load seen and matched by the ATU.
C1, L1 and C2 make the 500pF T match ATU. It is a conventional T circuit with 500pF variable capacitors. The ATU has been adjusted for a match.
The model G block is set to supply 1000W to a matched load so that voltage calculations apply to 1000W input. Note that all voltages and currents shown are RMS. The model also calculates vc1 and vc2, the voltages across C1 and C2.
Analysis
Some key points:
- vc2 is 777VRMS or 1099Vpk which is high, but lower than 1060VRMS (1500Vpk) for the MJF-986 ATU;
- power lost in the roller inductor is 70.5W, still quite a lot of power to dissipate over the small part of the roller inductor that is in use (perhaps around 6 turns) but 25W less than the MJF-986 ATU;
- 49W is lost in the feedline (open wire lines are often thought of as nearly lossless);
- 877W reaches the feed point, somewhat less will be radiated due to conductor and ground losses; and
- in both cases, the voltages inside the ATU can be greater than the antenna terminal voltage.
Conclusions
This is just an analysis of one band, albeit a likely flashover case. Other bands may be worse. The analysis is of the stated scenario, and even small changes might alter the results.
Operating voltages and losses are significantly lower for this scenario using a generic 500pF T match compared to the MJF-986 Differential T ATU.
The convenience of a Differential T ATU can come at a significant performance cost.