Friday, January 08, 2010

A portable HF receiving whip

Most VHF-UHF HTs (or hand-helds, as they tend to be known over here) nowadays provide receive coverage of the HF short wave bands, though only in AM mode. The Kenwood TH-F7E (or TH-F6A in the USA) is unique in that it can receive LSB, USB and CW modes as well. It doesn't use a narrow filter in those modes, so reception is far from optimum, but it is still a nice little radio that allows me to listen to what is happening on HF when I am away from the shack. Or it would be, except for the fact that it needs a proper antenna - or at least a length of wire - to receive anything. There is an internal ferrite bar antenna but it is ineffective on the amateur bands. Consequently I have made little use of this facility, and had got to the point where I was thinking about selling the radio.

A Miracle Whip or Wonder Wand works fine with the TH-F7E, but it doesn't actually fit on the radio so you need a cumbersome collection of adapters and cables to use it. What I wanted was something like a mini ATX Walkabout - essentially a base loaded whip with tapping points on the loading coil for each of the bands. This afternoon I decided to see if I could make something like that.

I cut a crimped-on BNC connector off the end of a cable, leaving about 1cm of cable protruding from the end of the sleeve. I then cut away the jacket and exposed braid to leave just the insulated centre wire. I stripped a small piece of insulation from the end of this and soldered to it the end of a 5m length of 38SWG enamelled copper wire.

Next, I found a plastic ball-point pen whose body fitted nicely over the crimped end of the BNC connector (a bit of insulating tape helped make a good interference fit.) This ball point pen body conveniently had a small hole at the other end through which I dropped a fixing screw for a short telescopic whip antenna. A blob of super-glue held the screw in place since I didn't have a screwdriver long enough to reach the screw head inside the pen body. Once the glue had set I attached the telescopic whip with a solder tag between it and the pen body. Once tightened, the tag was bent down flush with the pen body.

I drilled a small hole in the pen body that would be the same height as the point where the 38SWG wire was soldered to the co-ax centre, then fed the end of the wire through the hole. When it was all the way through I pushed the pen body on to the BNC connector and secured it with another dap of super-glue. I then wound the wire onto the pen body to form a coil. When nearly all of the wire was wound on, I secured the last few turns with a strip of insulating tape, stripped the enamel from the end and soldered it to the solder tag. I then had an antenna that looked like the photo above.

Miraculously - since the mathematics of designing a base-loaded antenna is beyond me and this had been pure trial and error - I ended up with a whip that with a ground wire attached can be tuned to a good match on 20m just by adjusting the length of the telescopic whip. Although I intended it as a receiving antenna it will be interesting to connect it to a QRP transmitter and see if I can make a contact with it.

The next stage of the project will be to try to establish tapping points on the coil to allow coverage of 17m, 15m, 12m, 10m and 6m. I really had hoped to get coverage of 80m, 40m and 30m as well, but obviously I would need a lot more turns on the coil to achieve this. I have a reel of something like 42SWG solenoid wire so I may try to make another version using this. Alternatively perhaps the lower section could be wound on a short ferrite rod to get more inductance with fewer turns. There is a lot of experimentation to be done with this.

One aspect of this antenna that I'm not too happy about is that I have to use an SMA to BNC adapter to fit it to the TH-F7E. This adapter doesn't clamp down to the body of the radio for additional mechanical strength, so the radio's SMA connector is vulnerable to stress and even damage if you knock the antenna.

Why hasn't any manufacturer come up with a ready-made version of what I am trying to make? There are plenty of amateur HTs and hand-held scanners that cover the short wave frequencies. Surely there would be a market for it?
Post a Comment