A Most Unusual Experiment

Several years ago, I used to see ads by an antenna company called Comdel and they were selling a most unusual looking antenna. It consisted of a large coil bolted to a magnet mount with a couple of small telescoping whips attached to the top and sticking out at right angles to the coil. They also sold some DDRR antennas for 2 meters that worked well. A local ham had one on his car for some time. I think it was stolen after he left it on his car in a not too secure area, as it is called.

But the two meter antenna was not the object of my interest, the HF antenna got my attention, as it was a very low profile antenna. I think it was a foot tall at the most. There was an interesting product review done on the antenna and the person doing the review stated he was very pleased with the way the antenna worked on the bands that he tested. I seem to remember he tested it on 20, 15 and 10 meters. There was some mention of several DX stations being worked, but I don’t remember where they were, mainly because I was interested in the amazingly small size of the antenna.

I mentioned this to another local ham who was always interested in strange antennas and thought we could possibly come up with the stuff to build a copy of that antenna. Several weeks passed before we did anything, but eventually we got on to this project with some diligence. Not having any tubing handy, a roll of 1-inch aluminum TV cable was found and pressed into service. We took his rig, SWR bridge, a scruffy looking length of coax cable and a card table into the back yard.

Next, we made a coil out of the roll of TV cable. What we did first was totally goofy. Instead of making a coil and cutting it off from the roll, I grabbed the inside end of the roll and pulled it straight up, making a tapering spirally wound cone out of the cable. After propping it up with a short length of PVC, we ended up with a 2 1/2 ft. tall cone sitting in the middle of the card table. So just for grins, we hooked the ground shield of the coax to the bottom end with a large battery clip and then started tapping up the spiraling portion with another battery clip connection to the center conductor of the coax. Here’ how it looked in Figure 1.

After a while of tapping up and SWR checking we finally got the SWR to 1:1. Then, we listened to see if anyone was on our frequency. Not hearing anyone, we then ran the power up full to check for corona or other nasty things. When we did this, we saw a tiny bit of reflected power. We adjusted frequency with the power cut down until we had absolutely no reflected power. Then we noticed the rig was drawing more collector current than it had ever done before. This was very intriguing. So, the next thing we did was look for someone to give us a report.

After tuning around slightly, we heard a CQ coming from a ham in San Antonio, Texas. We called him and he gave us a report of 10-20dB over S9 with QSB. This was a real shock because we were on 40 meters and San Antonio is 150 miles away. And, all we were using for an antenna was a roll of aluminum coax pulled up into a 2-1/2 foot tall spiral that was about the same dimension across the base of the roll. Not exactly what you would expect as an optimum antenna for 40 meters.

It appears this is another one of those antennas which bears more experimenting. Perhaps it falls into the same category of the magnetic antenna, but not that certain, as this was the only time that we did anything with that homebrewed antenna. I do not believe there was any more than 16 feet of cable in the roll. So, it was not going to resonate naturally on 40 meters. It seemed to be extremely quiet on reception with very little noise being heard. It did pick up the SW broadcast stations that infest 40 meters in the evening and night. Maybe another possibility here for the SWLs?

Even though we had not hooked it up to a ground rod, there was no RF on the rig. This tell me the RF was going into the air like it should have. The connecting cable was 20 feet long between the rig and antenna. NO radiation there. We had checked with a neon light and a florescent tube. The only place we could get any indication of RF was from the goofy antenna that shouldn’t have radiated anything much into the air, but did. Connecting the coax shield to the ground rod, which was 20 feet long, didn’t seem to make much difference. The received signals didn’t seem to change nor did our transmitted signal strength.

Unfortunately, that was the only time we did anything with this antenna. Other things took priority, and it was resigned to the back of the garage and forgotten until a recent discussion of the magnetic loop came up and I remembered that strange coaxial spiral antenna. It needs more experimentation and someone might find a new antenna design here. I certainly intend to do some more work on this antenna as soon as time allows me. It’s just too interesting to ignore.

Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF

Last Updated : 28th March 2024