Unless some people told you, you’d never know they’d been on vacation.
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.
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.
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.
Went to Belton, TX for the Ham Radio Expo held there twice a year. Big swapmeet and convention. Picked up a trunk-lip antenna mount for the car and a multiband antenna for HF — 80m through 2m. Cool thing about the antenna is that instead of using lossy traps to keep the antenna resonant for each band, it features jacks along the length so you can use the included jumper cable to manually select each band.
I took the new kit out yesterday for a test run, and unfortunately I’m not impressed. When connected to my antenna analyzer, the SWR on the antenna looks like it could go down to 1.25:1 after tweaking the length of the whip, but the moment I connect it to my radio and push 100W, the SWR gets so bad that my radio automatically backs off its power output to save itself. 40W is the limit, apparently.
My biggest suspicion is that the antenna mount isn’t fully grounded to the car frame. The frame is an essential part of the antenna system (it functions as the ground plane for the vertical antenna), and if it’s not properly connected, that RF energy goes back down the feedline and into the radio.
The mount has a rubber pad to protect the paint on the topside, and the clamp on the underside has four set-screws, an unpainted metal plate for protection, and that’s directly against the painted underside of the lip. What I need to do is take my roll of steel grounding strap and run that under the lip clamp, over to a bolt hole under the trunk decking, down to the trunk mount, and then underneath to the frame with some sort of lock washers and bolts to dig into the paint a little. Electrical conductivity is not the same as RF conductivity, and it’s for that reason that the ground strap is necessary.
If I still don’t get any better results, I’ll have to get an ATU (Automatic Tuner Unit) and put it electrically close to the antenna mount and run a line from that to the radio. Maybe even rewire the mount with a thicker cable. As long as the ATU can get a good tune, the radio won’t care and I might be able to run full power.
All antennas are compromise antennas. Too bad I can’t run a dipole on my car, eh? More news to follow.