Tag Archives: power

Moving Ground

I did some rethinking on the power circuit for my two-way radio in the car. I had read multiple sources that connecting the 12V and Ground wires directly to the battery was the best way to minimize engine noise in the radio, so I went with that guidance and wired things directly to the battery terminals.

As it turns out, my assumption was half correct. The incorrect half could have set me up for disaster. According to an article written by W8JI, a situation could happen where — as a worst-case scenario — my radio is destroyed.

In the case that the radio is grounded directly to the battery, there are two connections coming from the negative post. One is a copper strap that goes from the battery post to somewhere nearby on the chassis body. The other connection is the ground lead to the radio.

So, if the chassis ground strap becomes loose, corroded, or disconnected, then everything in the car that’s powered will try to ground through the only battery connection left: the radio. This will result in instant destruction of its circuit traces and will damage the cable assembly until something either opens, melts, or catches fire.

The correct method for grounding the radio is to bolt the ground lead to a convenient place on the car body as close to the battery ground strap as possible. That way, if the strap breaks, the entire car loses its grounding.

I also learned that modern cars have sensors in the battery cable assemblies that measure how much current is being drawn from the battery. This signal goes to the ECU so it can command the alternator to produce more current. If the radio ground is connected directly to the post, it is upstream from this sensor and the ECU gets an incorrect measurement of current draw from the battery. Putting the radio downstream from this fixes that problem.

Foreground: battery ground post with current sensor. Background: new position of radio ground lead (blue) next to engine ground strap.

With these facts in mind, I made an adjustment to the circuit. I pulled the radio ground connector from the battery post and moved it down to a bolt that holds the ECU bracket. This bolt is mere inches away from the ground lead that goes to the engine block, so I know the connection is solid.

As an aside, I also see some guidance that I should take the fuse out of the ground lead circuit, but I’m not so sure about that. I’ll have to think more on it to figure out what will happen if the ground fuse blows. Will the radio have a floating ground? Will it continue to ground through the ground strap I’ve lashed to the radio body? Will I blow a circuit trace between the radio ground lead and the board’s internal connection to the radio body? I’m not sure.

More improvements to follow.

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.

Mobile Power Project: Ham Radio on the go! By KG5RHR

Mobile Power Project: Ham Radio On the Go!

Intent:

It seems minor, but installing the power cables for a ham radio in a car is a major project if you want it done right. After months of thoughts and tactics, I made good on my plan to install a 25 Amp circuit into my car to power my portable amateur radio equipment. I’ve been needing this capability for a while. It gives me another option for operating away from home, allowing me to drive to any convenient spot and make HF contacts, as well as allowing me to make VHF contacts while on the road. I’ll have radio power for as long as I have gas in the tank.

It took me a while to get around to doing it, but once I started, it was two evenings well-spent. Here’s the details. Continue reading

Pure Power

O’Brien: “Now I will tell you the answer to my question. It is this. The Party seeks power entirely for its own sake. We are not interested in the good of others; we are interested solely in power, pure power. What pure power means you will understand presently. We are different from the oligarchies of the past in that we know what we are doing. All the others, even those who resembled ourselves, were cowards and hypocrites. The German Nazis and the Russian Communists came very close to us in their methods, but they never had the courage to recognize their own motives. They pretended, perhaps they even believed, that they had seized power unwillingly and for a limited time, and that just around the corner there lay a paradise where human beings would be free and equal. We are not like that. We know that no one ever seizes power with the intention of relinquishing it. Power is not a means; it is an end. One does not establish a dictatorship in order to safeguard a revolution; one makes the revolution in order to establish the dictatorship. The object of persecution is persecution. The object of torture is torture. The object of power is power. Now you begin to understand me.”

— George Orwell, “1984”