One of the drawbacks with the antenna system I took to French Island was the time and effort to change the squid pole antenna from a 20m quad loop to a 40m ground plane. It takes about 15 minutes and that is 15 minutes more in the sun. If you need to change back then it is another 15 minutes.
Today I trialed using a trap near the mid point of the main element to see if the antenna would tune and work on 20m as well as 40m. I was using my FT7 on a 7 Ah SLA battery sitting in the boot of the car.
Now, in theory this should work as I use the same principle on my 80/40/20m trapped dipole. Using one of the 20m traps from my dipole I tried a couple of wire lengths and settled on the dimensions shown to the right.
The trap is made with 42mm diameter PVC pipe with 9 turns of wire on it. In the middle of the pipe is a length of open ended coax acting as a capacitor. Details for making the trap are shown here.
With the wire lengths shown and supported by my 9m squid pole, I got SWR less than 1.3:1 on 7.100 MHz and 14.200 MHz.
While checking the 20m SWR I heard a fairly strong signal near 14.300 MHz and, after checking my SWR was good, I tuned down and found Rick, VK2AAH, calling CQ. I answered his call and Rick came back to me with a 5 X 8 signal report into Sydney. Not a bad effort for a first contact with only 10 Watts. As the sky was getting a little dark and rain was coming along soon I didn’t talk long with Rick.
I gave the 40m band a quick look and heard Ray VK2FW calling another station. I listened and Ray called a second time. Still no answer. After his third call I called him and, after short introductions, he gave a 4 X 5 signal report. Not as good as my 20m report but I was still pleased to make a contact so quick.
By now I was starting to get wet so I signed with Ray, put on a rain coat, and packed the antenna away.
So, there we have it. A trapped vertical portable antenna for 40 & 20 m.