Portable and More

I have been working with the 300 ohm TV ribbon to make several antennas and have come to realize that while it is nice and light, the lack of any semblance of strength once the plastic jacket is removed does not make it very suitable for repeated deployments. it is also difficult to remove the plastic coating without losing some wires in the process. I resorted to using a soldering iron to remove the plastic while keeping the thin copper strands intact. I would be really pissed off after walking several hours to a chosen location and find the antenna of my dreams had fallen apart on me after a few minutes of use. Another problem of using ribbon for the feed was to obtain consistent results re SWR and the need of a balun at the end. Ribbon is probably great in a fixed location but problematical when you are operating from a variety places and you may not be aware of where and how you are going to have to run the feed to the radio. Despite the weight of coax my thoughts are drifting back to coax.

Luckily we had a couple of fine days recently one of which was windless so I was able to play around a bit more with the Bi Square. By trimming the twin feed I was able to bring the SWR down to about 1.5 with the 450 ohm ribbon which was much easier to trim than the 300 ohm TV stuff. I kept trimming till I noticed the SWR on channel 1 started to creep up. Need to measure what length I ended up with. I was unable to test the results on air unfortunately, as no one about. But I now have an antenna I can transmit with. I probably have gone about this the wrong way and need to make sure the loop is resonant before playing with the feeder. I also note in the books that the tuned feeder is a stub and shorted at the bottom and the matching points found by trial and error, which is a bit hard with the plastic coated feeders I have on hand. Where does the balun go? Also the books state the Bi Square should be 1/4 wave above ground at the feed point which I am unable to achieve with my 31′ pole. While I am a firm believer in getting as much wire in the air as possible, I think I have to rethink the bisquare. The added wire also adds to the windage and deployment hassles. I like it and it certainly warrants further work, but I will put it on the back burner for now till I get an antenna analyser to bring the loop into resonance before working on the feed system.
Another thing is the wire. I need to be using hard drawn non stretchy proper antenna wire and not the weak stretchy flexible thin stranded normal wire with heavy insulation that is readily available in the shops here. Buying a ready made wire antenna is probably quite worth it, as to build something that will last will cost you! However the purpose here is to work out what works for me first, and then make it properly!

BI Square pictures

So where to from here.?

What works well so far? The conventional inverted V. And the recently completed 1/2 wave coaxial vertical. I like the vertical. Easy to deploy. Doesn’t need guys and coax all the way to the radio means consistent results in the field.

Which brings me to the conventional single quad loop. Can be fed with  a quarter wave of 75 ohm coax back to 50 ohm coax to the radio. A bit of gain, bi directional, closed loop, and can get the feed up to approx 1/2 wavelength so a little bit more out of the way.
Lots of  information on making and feeding quad loops on the internet.
Diamond shaped.
That’s the next project. I think it could be used vertically without too much hassle running the coax feed down one of the guy lines that pulls the loop away from the pole.

About David Donaldson

64 and Live in Jakarta, Indonesia. like to ride my bikes.
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