Over the Chinese New Year’s holiday I managed to start and (almost) finish a project I have been brooding over for months: a Cobweb antenna. Lightweight, small, multi-band, no tuning needed, in one-word “ideal.” There was a very interesting thread over at QRZ.com about building one and I read all the other literature I could find on the subject. I finally settled for the alternative design by G3TXQ using single wire dipoles which would be easier to tune, but required a balun for impedance transformation.
I’m still on a very tight budget, so the antenna had to be build on the cheap. That means thinking outside the box and starting with taking a good look around you. Fishing rods are expensive here. The cheapest I could find were almost 400 NT dollars per piece. But hey, we’re in Asia: bamboo is cheap, but it is also strong and lightweight. So I spend 200 NT dollars (5 euro’s or 7.50 USD) on bamboo instead of 2000 NT on fishing rods. Aluminium is hard to get in Taiwan, because people find it too soft. Stainless steel is widely used, so the local iron monger cut me two pieces for a little less than 200 NT dollars. U-bolts were also cheap at the local hardware store. Twin lead costed me 400 NT dollars for 50 meters and after two days of work it all looked like this….
The G3TXQ version requires a 1:4 balun to bring up the impedance to 50 ohms, as square folded dipoles have an impedance of 12 ohms (which every one knows, right?). The ferrite of choice is the Amidon FT140-61, but ferrite is not easy to get in Taiwan. Over in Taipei there is a little shop called RF-Parts, run by Mr. Dong, so I went to see what he got. He only had the FT-114-61, so that would mean two stacked together and that twice. Suddenly the Cobweb started to get expensive, because when I left Mr. Dong my wallet was NT$720 lighter. Add another 160 NT dollars for two meters of RG316 coax and the cost of the balun gets close to NT$1000, That’s almost more than the rest of the materials for the antenna together! But the final result looks like this….
After Chinese New Year we had two days of mild weather. Great for pruning the antenna on the balcony. Centimeter by centimeter I started cutting the wires until I had the SWR 1:1 on the frequency I wanted. The five dipoles influence each other, so it was going back and forth from 20 to 10 meters, measuring and cutting. The balcony was full of little pieces of wire afterwards, which refused to be swept up, so they were picked up by hand. But after two days the job was finally done and I could start enjoying my new creation.
NOT! Where are those sun spots when you need them? The bands were pretty empty and my first QSO was with 4W0VB in East Timor on 30 meters, at night. I know, that is not one of the band the Cobweb covers, but the TS-440S build-in tuner did it’s job well. And then it started to rain and I started to cry. Because the rain caused my carefully tuned 1:1 SWR to wander off two to 300 kHz below the resonant frequency that I tuned it for. Again, the TS-440S tuner could manage that, but why? Why does a little bit of rain de-tune my new baby? I don’t want to use a tuner. That’s not why I build the Cobweb.
I did manage to work some nice stations during the last week though: C31HA twice, 4W0VB twice more, XW3DT, PY6HD, CX3TQ and N7DR who mailed me to say I need to be patient, because conditions are indeed pretty lousy. I put up the CB whip to compare signal quality and indeed the Cobweb is quieter than a vertical. Signals are also a bit weaker, but only by one or two S-points.
So, am I happy now? No, because after the rain the wind started to play games. Longtan is not Port Martin, but it comes close. Winds howling all day long and gusts that make you and the house shiver. Longtan is situated on a plateau about 300 meters ASL and the area is known for wind and moisture. The Cobweb is light and strong, but the wires on my version were sweeping and swaying so much that SWR was not stable for a second. And the sectioned fiber glass pole that the Cobweb is mounted on was turning too much and the coax curled around it.
I need to start thinking about a sturdier construction. I saw some springs in a hardware store and I’m thinking about using them to keep the dipoles tight. That is the easy part, but I also need to figure out why moisture is influencing the SWR so much. Could it be the bamboo? Or is it the heavy reinforcement in the house?
I’ll keep the antenna up for another couple of days and then put the 10 meter vertical back up again. I miss roaming the upper bands at night and I want to keep my CW streak going. But a fun project it was and I learned a lot from it. And that’s what it’s all about, not?