Tag Archives: FM

Blast and Blow

To catch you up to date: when I set my radio output to 100W full power in the car with the new mobile antenna, the SWR is so bad that the radio lowers its output power to 40W to save itself.

I experimented with a few hypothetical fixes:

  • Run on 12V battery instead of the car’s power system, in case the power system was actually not able to provide full current; no change. Power system is functional.
  • Disassemble antenna mount and scrape paint layers between adjoining parts for better conductivity; no change. DC conductivity is not the same as RF conductivity.
  • Adjust the ground strap and tighten connections; no change
  • Coil more feedline into 4″ turns as a poor-man’s feedline choke; no change
  • Clamp on a bigger ferrite choke outside the trunk lid and run more turns through it; no change
  • Run the feedline directly into a dummy load (a resistor with a heatsink); success at 100W. The antenna is to blame, and the radio is OK.
  • Insert my manual antenna tuner between the radio and antenna and adjust for best SWR: success at 100W

So, yeah, my initial assumption that I’ll need an automatic antenna tuner unit (ATU) holds up. The issue is that the antenna, the car underneath it, and the ground underneath the car together have the kind of reactance (impedance to alternating current due to inductance or capacitance instead of pure resistance) that causes more energy to get stored in the antenna system and then sent back down the line to the radio. The antenna tuner injects enough of the right kind of reactance to reduce feedback and make the radio happy.

Once I got the manual tuner set right, the radio was able to pump a full 100W on CW. Success. An automatic tuner will make that a breeze.

So tonight I was out and about on a drive. Pulled off the road to raise the antenna and make some tests and contacts on 40 meters. SWR was bad, so I inserted the manual tuner. Instead of digging into my radio box for the pushbutton I typically use to trigger the CW tone, I decided to flip over to FM modulation because, like CW, it has a 100% duty cycle (a solid signal to test power output). With a few keydowns resulting in low power output, I reached over to tweak the tuner knobs and get better SWR. Suddenly, the radio pumped out full power — for 4 seconds before it blinked off and died.

My initial concern was that the radio was fried, or worse, I had fried the custom power circuit I had installed, or even worse, I had killed my car and was stranded in BFE — none was the case, thankfully. The culprit was the 10A fuses I had installed in the inline fuse blocks (I designed the system for 25A fuses, but downgraded to 10A for paranoia reasons).

10A X 13.8V = 138W

138W is realistically more than my radio should consume, but there are losses inside the radio, voltage drops in the power line, and FM modulation spreads the signal out over a 3KHz bandwidth and consumes more power than CW signals.

So there you have it. I fried my first fuse.

Once I made it home in silent humility, I replaced the fuses with 15A units which should be able to take the heat and still be safe. I think that should be enough, but time will tell.

I learned a few things this week.