Is it possible to predict the terminal voltage of an installed antenna with feed line? discussed prediction of an intended antenna using an NEC model.
ATU terminal voltage
This article shows how to predict ATU terminal voltage based on VNA or analyser measurement of the antenna system looking into the feed line. We will also look at an example of the voltages within a T ATU.
Above is a calculation of peak voltage at the ATU antenna terminals at 100W calculated from a saved .s1p file made looking into the feed line from the ATU position.
Above 3MHz, these voltages are modest at 100W. These voltages are impressed upon the balun that sits between feed line and ATU via 300mm of RG-400. Realise that at 1600W, the voltages would be four times that in the graph, ie up to about 3000V at some frequencies.
Let’s look inside the ATU.
Above is a SimNEC model of a T match ATU that uses 200pF variable capacitors (eg the ubiquitous MFJ-949E) at 100W out. Also calculated are vc1 and vc2, the voltages impressed on the two variable capacitors. All voltages and currents are RMS.
So, although the terminal voltage at 100W is 465Vrms or 658Vpk, vc2 is 577Vrms or 816Vpk.
The voltage impressed on ATU components can be higher than the ATU terminal voltage, double is not out of the question.
Those voltages are ok at 100W.
Note the coil loss of 18W @ 100W out. Not a problem for low duty cycle modes, but the active part of the coil is just the end turns (perhaps 6t or so), and it does not take a lot of dissipation to soften the coil support material (a common problem with these ATUs) at high average power.
Ok, let’s get serious… with a bigger ATU.
Above is a SimNEC model of a T match ATU that uses 500pF variable capacitors at 1000W out. Also calculated are vc1 and vc2, the voltages impressed on the two variable capacitors. All voltages and currents are RMS.
So, although the terminal voltage at 1000W is 1472Vrms or 2082Vpk, vc2 is 1607Vrms or 2273Vpk.
The voltage impressed on ATU components can be higher than the ATU terminal voltage, double is not out of the question.
Those voltages should be ok for most high power ATUs at 1000W.
Note the coil loss of 155W @ 1000W out. Not a problem for low duty cycle modes, but the active part of the coil is just the end turns and the resultant rise in temperature may cause damage for high average power.