300 OHM
Seems that whenever I try something based on low budget planing I get low budget results. My most recent disappointment is with 300 ohm twin lead. The stuff really does have problems keeping its electrical characteristics when it gets wet.
I was using the good stuff too. The heavier duty twin lead. True, it was spliced in several places and the splices were not taped but I had done the same with 450 ohm window line and never had a problem during rainy conditions.
This morning my 75 meter, 9 AM schedule was interrupted by a thunderstorm. Never mind, I still tried to keep the schedule and fired up the rig. I was using the all band dipole and antenna tuner. The whole setup had been carefully adjusted for an absolute minimum SWR the night before, but now I could not get it to drop below 3:1.
It was pouring rain outside and I never did connect with the target station. He was having storm problems too and had not even tried to keep the schedule. Finally I gave up.
An hour later I fired up the rig again. The storm was over. It was still wet outside but it was not raining anymore. SWR was back down to 1.3:1.
I am sure that the higher SWR was due to the rain.
The whole point to using the twin lead was to avoid having to design an open wire line interface. Or a window line interface using feed through insulators at the window and doing it the right way. Looks like if you don’t do it the right way, you will end up with wrong way performance.
The lesson to be learned here is that it will pay off in the long run to do it right the first time. Go ahead and homebrew your open wire line, design a proper interface through the window or wall, install the tuner in a decent location and don’t forget to run a decent ground to the thing.