Wire Doublet Antenna
I have built and installed a ‘doublet’ antenna for HF use at home. There is some rough details on the doublet at Hamuniverse, but basically it is two pieces of wire, fed from the centre with open wire feeder. Ladder line can be used (and many people use the 450 ohm ladder line rather than 300 ohm TV ribbon). The doublet is not tuned to any particular frequency — I sized mine at 29.5m simply by choosing the largest antenna I could fit between the two support trees. An antenna tuner is used to provide the match to the transceiver. I’ve used a couple of stainless steel thimbles to provide mechanical robustness in the ‘figure of 8 loop‘ knots that join the antenna wire to the guy wire (which runs down from the tree and is tied off at ground level). The thimbles are a little hard to see, but they make the coupling stronger. The guy line is ‘polycore’ clothes line, which has a non-conductive core (most important), a slippery outer jacket (slides through trees easier) and is designed for use outside.
Since the doublet is not tuned, there will be a VSWR mismatch in the feed line. It is for this reason that it is critical that the doublet is fed with a balanced feed line. I’m using open wire feeder that is made by Woodtech Electronics. This has a nominal impedance of 600 ohms and has very low loss, even in the rain because there is very little between the two conductors. A fibreglass t-piece is used to provide the mechanical coupling for the antenna and the feed line, as well as a place to bolt the conductors together.
It is best if the open wire feeder is kept away from metallic objects, such as the gutter on the side of my house. Fortunately the roof is concrete tile, so there is no interaction with any roofing iron. I’ve tied the feeder off to give some clearance. A 1:1 current balun (manufactured by Woodtech Electronics) is used to convert the BALanced feed line to an UNbalanced coaxial transmission line that connects the antenna to the antenna tuner. Coax is very lossy when there is a high VSWR, so it is important that this coax is as short as possible, and fairly chunky too. I have some RG213 to make new leads, but at the moment I am using some cheap & nasty RG58 from a local electronics shop (the light blue one, not the yellow one).
My station is nice and simple, and consists of the Yaesu FT-817 transceiver, LDG Z-11pro auto-tuner, SignaLink USB interface and my old Sony laptop. The computer headset is used to make listening a bit easier (the FT817 supports stereo headsets) and I’ve built an interface that lets me use the mic with the radio. A foot switch makes operating fairly easy too. I use Ham Radio Deluxe v5 beta for logging and to control the radio, with a home made CAT interface.
I’m happy with the performance, and have had a few conversations. I was able to easily monitor the Sydney-Hobart yacht race radio checks on 6215kHz prior to the race starting. This afternoon I had a rag-chew with three or four other hams scattered around Queensland (from Emerald, Kingaroy, Ipswich and Maclay Island), which is a first for me on 5W with the Ft817.
