Tag Archives: antenna

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.

The Go Forward Backoff

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.

Backstop

I’ve heard of it being done, but I kinda secretly want to wire my radio and my antenna tuner to a softball backstop fence to see what kind of signal I can get out.

I have no idea if I can match the impedance of a backstop, or if I will be able to deal with the RF coming back down the feedline to my radio. I don’t even know if the fact that the backstop is anchored in the ground with concrete and touching the ground for its entire length will cause it to be its own ground plane. Does it act as a random wire antenna? Is it a longwire antenna? Is it good only for reception? What kind of harmful harmonics does it put out? How does the pair of 45º bends with the 90º straight legs affect the radiation pattern? What’s the optimum frequency band for it?

So what the hell…I gotta try it.

Hey, if someone can make long-distance contacts by wiring their radio, amplifier, and antenna tuner to a cast-iron skillet hanging from a swing set, I can make contacts with a backstop.

Stay tuned.

First QSO, First DX

So today was a crap day, but what happened when I left work and raised my antenna was a soothing balm that made everything alright. The good news is that today I made my very first HF contact (called a QSO) on the 20-meter band with my good radio. That’s a first. Also, on the very same contact, I made my first international contact (called a DX). On the same call!

Here’s a shout out to Gil VE2MAM from Quebec, Canada, who was calling CQ/DX to collect American counties. His signal was coming in strong and clear with a little bit of fading, so I decided to give it a try. I turned up my amp power, keyed up my mic, and responded with my callsign. He eventually heard me in the noise and we had an exchange of signal reports and a few other things like my location and county. I gave him a “59” signal report, meaning he was readable and had a strong signal. He gave me a “555”, meaning I was readable, my signal was fairly good, but there was some modulation on my signal (these can be dealt with).

I’m just happy that I’m finally learning how to make it happen. As it turns out, the modular vertical antenna I’ve been using (which is ultra-portable) isn’t so good for making anything other than regional contacts. It’s easy to set up and tune, but it’s just not that efficient at putting out a radio signal. What i did tonight was string my 20-meter dipole between two trees in Mueller park. It took me a little bit of trial and error to throw the ropes high enough and get enough distance between trees so the antenna wasn’t in the branches, but I figured it out.

Gil, I certainly hope your log of our QSO doesn’t need me to submit a log from my end for you to get credit, but it was nice talking with you. 73, good sir. Merci!

First QSO, 14.289MHz 20170609 0:00 UTC

20-meter dipole strung between two trees