Recent articles discussed flashover risk at 7MHz in a ATU for a dipole with two wire feed line configuration, and that raises the question whether it is feasible to predict the terminal voltage of an installed system.
This article give an example based on measurement of the impedance looking into a 1:1 Guanella balun + 11m of DIY two wire feed line + G5RV inverted V. Note that the measurement configuration preserved the common mode current path, it the outer conductor of the input connector of the balun was ground exactly as it is in operation.
The measurement was made using a Rigexpert AA600 in September 2023 during a routine maintenance check.
Above is a plot of peak terminal voltage @ 100W at the balun coax connector calculated from measurement from 2.1 to 30MHz. Voltage at 1600W is four times that.
The graph shows the nature of voltage vs frequency and is specific to this system, being sensitive to the antenna length and feedline type and length. Similar plots would result from measurement of similar antennas, the location of the minima and maxima is dependent on those lengths mentioned, and so maxima can be steered away from operating frequencies.
3.6 MHz: 272 Vpk 7.1 MHz: 486 Vpk 10.2 MHz: 687 Vpk 14.2 MHz: 209 Vpk 18.1 MHz: 493 Vpk 21.2 MHz: 105 Vpk 24.9 MHz: 209 Vpk 28.5 MHz: 323 Vpk
Above is a list of voltages @100W in the HF ham bands. This antenna system is not intended to operate on 1.8MHz, extrapolation of the graph data suggests terminal voltage in excess of 5000V @ 100W. Nevertheless people report trying it… and resulting problems in the connected ATU, what a surprise!
The .s1p file was analysed in Jupyter using Scikit-RF lib.
p=100.0
vpk=2**0.5*(p/nw1.y.real[nrange,0,0])**0.5
for fr in [3.6e6,7.1e6,10.2e6,14.2e6,18.1e6,21.2e6,24.9e6,28.5e6]:
index=np.searchsorted(f,fr)
print('{:0.1f} MHz: {:0.0f} Vpk'.format(f[index]/1e6,vpk[index]))
Above is the Python code for performing the calculation.
Note that this calculates the terminal voltage looking into the feed line. It is quite possible that voltages inside a driving ATU might exceed this value.