Antenna Time

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Lee Aldridge G4EJB relates the continuing saga of his return to amateur radio.

 

 

Lee Aldridge G4EJB relates the continuing saga of his return to amateur radio.

 

Upon my return, my plan was to build the matching unit for the random wire antenna. Around that time, I had been looking at the G-QRP Club website and decided to join. I thought this would assist me in my minimal cost re-entry and my interest in using low power effectively. It was a good decision. For example, the SPRAT DVD covering all back issues to the 1970s really gave me a lot of reading to do. The PW Radio Book Store hadn’t gone unnoticed, including the PW Archive CDs, though I was going to have to save for these. I was also finding numerous amateur radio websites covering all types of construction, theory and experiences – this too was a great source of knowledge that was not around years ago.

I knew the antenna was less than a success and it spurred me on to look through a box of cable given to me by one of my sons. There were some lengths of heavy-duty copper speaker cable – I thought that might be useful for a 20m dipole if only I had some coaxial cable. Then I remembered an off-cut from a reel of PF100 75Ω coax, kept after not fitting a terrestrial TV antenna on the house. At least the coax had a solid copper foil and meshed wire sheath (it seems to have a similar specification to WF100 cable). So, time for a change of antenna. How was that going to be constructed? Even though I have a couple of antenna handbooks, I looked online and found the half-wave dipole length formula along with plenty of theory and considerations:

http://rsgb.org/main/get-started-in-amateur-radio/antennas/your-first-antenna-the-half-wave-dipole

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What could I use as insulators? I found some old plastic cabinet blocks, Fig. 1. I used two of these, one at each end of the dipole using the steel garden wire to splice the copper wire. Another block was used as the centre and I worked out a way of clamping the connected coax to take the strain off a choc-bloc connector used to terminate the dipole. Then with some self-amalgamating tape, I waterproofed the centre section and finished off with some PVC tape. The antenna was tied with rope and tensioned as far as the metal pole at one end would allow. The coaxial cable was weighing the antenna down a little so I found a way of supporting the cable and that helped the height of the antenna at the centre. (A wooden pole at the centre with the coax attached would probably have done a better job). I even drilled out the inner ferrule of an old BNC plug to take the PF100 cable and made the end of the cable off. The photo, Fig. 2, shows my shiny new dipole.

The antenna really improved reception on the old Howes 20m DC receiver. I could hear W1, 2, 3, 8 and even W6 (California) stations perfectly during the afternoon. The receiver drifted a little, had a little AM breakthrough and lacked sufficient selectivity but I hadn’t planned on doing anything about it at that time. I thought the matching unit could wait because the dipole was probably a reasonable match to the receiver. I knew the antenna was low, very basic and probably not what I would finish with but it was a move in the right direction. Longer term, my intention was to cut the antenna for resonance, look further into coaxial cable feed, balance the feed with a 1:1 balun or use 75Ω twin feeder, to study antenna efficiency more than I had ever done in the past and to raise the antenna height. But past experience with neighbours had taught me, let them get used to seeing an antenna then gradually make upward adjustments. Anyway, all that would have to wait; everything was a case of one step at a time.

 

 

This article was featured in the July 2018 issue of Practical Wireless