Trying to measure transmitted signal strength with a dummy antenna, getting unusually high readings.

Thread Starter


Joined Sep 3, 2010
Several years ago I made a dummy antenna for my old CB radio and it worked fine. When my DMM was connected I measured 0V when not transmitting, as it should be. Recently I began replacing old caps in the radio and I noticed I have around 50 V on the chassis wrt earth ground. Amperage is very low. My thought is that a cap is leaking to ground somewhere. Anyway, now when I connect my DMM to the dummy antenna I read several dozen mV when not transmitting and I can get as high as 36 V when transmitting when I tune for max signal strength. SWR can be as low as 1.3:1 but usually tuning for highest signal out does not give lowest SWR so a compromise has to be made. If I measure about 19.5 VDC (plus 0.8 V for diode drop), I calculate I'm transmitting 4 watts. But if the 36V is correct, I'd be transmitting 13.5 watts. I can't believe this. So I'm wondering if some of the high AC voltage on my chassis is coming through the diode in my dummy antenna and giving me a false high voltage reading. I don't know if it's even possible for the radio to transmit that much power. I used to struggle to get 9-12 V from this old radio but now I have too high an output, it seems. No tubes are glowing abnormally and nothing is burning up. The resistor in the dummy ant is hotter than I've ever felt it before though.
Further info: Dummy ant resistance = 50.1 ohms. Power calculation: Voltage measured + diode drop = X p-p voltage. X * 0.707=RMS voltage, RMS^2/50.1 ohms = wattage delivered to dummy ant.