In September 2023, I wrote a review about the 7-band Cobweb antenna manufactured by AWK in Poland. This symmetrical, or balanced, antenna is fed with asymmetrical, or unbalanced, coaxial cable. To prevent the coaxial cable from becoming an active part of the antenna, it is necessary to place a 1:1 balun at the feed point.
Built a 1: balun
I built a 1:1 balun based on the design by DG0SA. I had some PTFE insulated silver-plated copper wire AWG18 left over, which I had purchased from AliExpress some time ago for an experiment. Since I also wanted to experiment with the Cobweb on the 6 and 4-meter bands, I used an FT240-52 toroidal ferrite core. I then directly connected the balun to the feed point with a fixed PL-259 connector. Everything worked well.
Reception would drop a few S-points
During the winter, I encountered a problem. Sometimes, the reception on the Cobweb would drop by a few S-points, as if someone were shielding the antenna. When I briefly transmitted, the reception would return. I suspected a faulty connector or moisture in the feed line somewhere. Inspection of the coaxial cable up to the mast revealed nothing, so the problem had to lie within the antenna itself. After many weekends of rain and wind, I finally managed to climb onto the roof and bring down the Cobweb. First, I attached a dummy load to the end of the coaxial cable. The last section of coaxial cable also proved not to be the issue.
The SWR on 10 meters was almost 2:1
Next, I removed the 1:1 balun and connected the coaxial cable directly to the 1:4 impedance transformer, temporarily using a coaxial balun in between. In the following days, there were no more reception problems. The 1:1 balun must have been the culprit. At first glance, there was nothing visibly wrong, no moisture in the housing or corroded connections. However, a measurement with the VNA revealed that the SWR of the balun itself on 10 meters was almost 2:1! I had never noticed this before.
Wire from AliExpress
This was very strange because I had built this type of balun many times before and had never seen an SWR higher than 1.1:1 on 10 meters. When I compared the toroidal core with windings to one I had built before, I noticed that I had made the old balun with 11 windings instead of the usual 12 windings. But that shouldn’t have made much difference to the SWR. I had built the old version with wire from AliExpress, which is slightly thicker than what I usually use, hence only 11 windings on the toroidal core instead of 12 windings.
A new 1:1 balun
Since I had another FT240-52 toroidal core lying around, I built a new 1:1 balun, this time with 12 windings of the wire I always use. I housed it in a casing with one SO-239 sockets on the bottom and side. The SWR up to 70 MHz was below 1.1:1. I placed the new balun near the feed point of the Cobweb, connecting it with a 30cm (12”) piece of RG-400 PTFE coaxial cable, which I had leftover from a previous project.
The dipole usually performed better
Not only was the problem with reception solved, but I also had a strong impression that the transmission performance was better than before. Now, making a comparison was not difficult because I had previously measured the performance of the Cobweb on 10 meters compared to a dipole. The dipole usually performed better then. Now, I conducted tests again in WSPR, FT8, and SSB. However, the difference was minimal. Sometimes the Cobweb performed better, sometimes the dipole. No longer were there the larger differences in performance I had seen before.
Was the wire form AliExpress the problem?
What caused the earlier reception dropouts and reduced transmission performance, I am still not entirely sure. It must have been the poor SWR of the old balun itself. My suspicion is that the wire from AliExpress was the cause. Despite testing the wire showing no issues, could there have been a small break in the wire? In any case, the little wire I had left from AliExpress ended up in the trash.
The good news: Renewing the balun had led to a true resurgence of the Cobweb.