On Feb 12, 3:18 pm, "KE5MBX" wrote:
Hi,
What's all this business I hear people talking about blasting people's
car stereos or killing cars altogether with high-power RF? I run 2m,
10m, and 11m in my jeep and I'd like to sort out the fact from the
legend and find out what kind of power I can use without risking
damage to my jeep or cars around me. At what power level is front-end
overload to a nearby radio likely? At what power level is damage
likely, and to what componets?

I am currently using only 50 watts on a 5/8 wave (2m), 25w on a 1/4
wave (10m), legal 4w on a 1/4 wave (11m)

Thanks,
Nelson KE5MBX


you might have more issues with RF exposure to you and safety,
particularly on the 2m band, depending on where the antenna is.

But, as far as killing your car radio (or one next to you) goes, you'd
have to look at the coupling from your antenna to the car radio's. 2m
is going to be the worst case because the victim receiver is close to
that frequency, so the coupling from your antenna to its is highest.
The easy case is the car in the next lane (because it's reasonably far
away.. more than a couple wavelengths), so you could do some sort of
rule of thumb free space path loss kind of calculation...Say you're 5m
away at 144MHz.

Loss = 32.44 + 20*log10(d in km) + 20*log10(f in MHz) = 32.44 -46+43
dB = about 30 dB loss. You're radiating 50 W, and let's assume your
antenna is 3dBi, so your EIRP is 100W or +50dBm. The victim is seeing
+20dBm (a tenth of a watt)... about 2 Volts into a 75 ohm load.
Assuming the victim radio has protection diodes across the input,
you're probably not going to kill it. Run a kilowatt and it's another
story...


As a practical matter, my 2m antenna is about 1 meter from the car
radio antenna, and when I transmit at 50W, the FM car radio blocks,
but recovers just fine. Likewise, transmitting at 100W on HF. Your
mileage may vary, etc.

Jim, W6RMK