Last night I got an email from John Isabella, K1IF, stating that he’d read my blog regarding my Diamond X510, and was wondering if I knew what voltage the capacitors inside were rated at.
I confessed that I didn’t, as mine did not seem to be in need of replacing, but with a bit of math we were able to ascertain some suitable ratings, namely 300V rated 2pf capacitors. If you’re interested in the math, continue reading.
The Power Law states that the power dissipated in a device is inversely proportional to the squared value of the voltage across it:
P = V2/R
John had informed me that he only wanted to run 100 Watts through his antenna, so we did a few calculations.
Firstly, we ascertained that the voltage across the capacitor at 100 Watts, with a SWR of 1:1, would be 70.7 Volts.
The calculation was then performed again, assuming an SWR of 10:1, and is as follows:
- Power = Voltage Squared / Resistance
- Power x Resistance = Voltage Squared
- 100 Watts * 500 Ohms = V^2
- 50000 = V^2
- The Square Root of 50000 = 223.61 Volts
Therefore, it was concluded that any capacitor rated at 300 Volts or higher should do a mighty fine job indeed. Just for reference, the value of the capacitors used is 2pf.
Note: I’m no expert, and I expect that in the world of High Frequency, Alternating Current Resonant Circuits with Complex Impedences (rather than purely resistive loads), Ohm’s Law and its DC variants / derivatives don’t tell the whole story. That said, this is amateur radio, and what do we have to lose? A few burnt 2pf capacitors. That’s a gamble I’m willing to stake on Ohm’s Law. Someone please correct me if the above conclusions are misguided and/or bad advice.