A DIY Ducky for 2m
I recently decided to save wear and tear on the SMA socket of my TH-F7E dual-band hand-held transceiver by permanently installing an SMA to BNC adapter or "HT saver". I felt much happier swapping antennas on a BNC socket instead of an SMA which is not designed for repeated connections and disconnections. The trouble was that the only BNC antenna I had was a quarter wave telescopic whip. I wanted something a bit shorter. I started to look through ads on eBay for something suitable and then I thought: why not make one? So I did.
I used the body of a crimp-on BNC plug, 50 inches (125cm) of enamelled 28SWG wire, a 5in (13cm) piece of sheath from some co-axial cable and some heat-shrink tubing.
The sheath is used as the former for the helically wound antenna. I used rubber sheath from some flexible microphone cable, but the outer from some RF cable such as RG-58 could probably be used too. The main thing is that it fits snugly over the part of the plug that you would normally crimp on to.
Although I used enamelled copper wire I see no reason why you couldn't use plastic covered hookup wire, as long as it has a solid core.
The end of the wire was cleaned and soldered to the centre pin of the BNC plug. It was pushed into position, and then a short piece of insulation from the centre of the coax pushed over the wire and down into the plug body to hold it firmly in position. The idea being to prevent the pin being pushed back up into the plug body if you try to mount the antenna on to a rather tight socket.
I made a small hole in the sheath, at a point that would be just above the top of the crimp-on part of the plug after the sheath has been pushed all the way on. I fed the end of the wire through the hole, then slid the sheath down the wire and pushed it on to the plug. If it is not a firm fit, a bit of insulating tape may be used.
Now the wire is wound on to the sheath in a helical manner. You should try to keep the turns evenly spaced. Wind all but the last inch (2.5cm) on to the sheath. The last inch should stick upright as a tuning stub. Use a bit of insulating tape if necessary to hold the turns of the helix tightly in place.
For the next bit an antenna analyzer is very useful. Hopefully the antenna as you have just made it will be resonant below the 2m band. Without an antenna analyzer you will just have to trim a bit off the tuning stub and observe whether the SWR falls. If it rises you have cut off too much.
Bring the antenna to resonance by trimming a tiny bit at a time off the tuning stub. I do mean tiny. Even 1mm can make a big difference. You should be able to get close to a 1:1 SWR at 145MHz.
If the tuning stub is reduced to half an inch, pull off a whole turn and straighten the wire, then continue trimming. Don't worry if you slightly overshoot the optimum point, as I found that after the heat shrink tubing was added the resonant frequency lowered a bit. You might want to fine-tune the antenna after shrinking the tubing on.
Before shrinking heat-shrink tubing over the antenna I pushed a bit of wire insulation over the stub to give it some extra thickness and strength. You might also put a bit of black insulating tape over the top of the stub to form a cap. The heat-shrink tubing keeps the turns in place and gives the antenna a professional appearance. Mine would probably look a bit better if I hadn't shrunk the tubing quite so tightly.
The total length of the finished antenna is about 6 inches (15.5cm) which is about one-third the length of a quarter wave whip. The performance belies the short size, however. It is excellent.
On receive I could see no difference between the quarter wave whip and the DIY Ducky on the S-meter readings of local repeaters and could hear very little difference either. I could open a repeater 50 miles away just as easily with either antenna. On transmit I was told there was very little difference between the quarter wave and the DIY Ducky. In short, there seems to be very little benefit to be gained from using the longer antenna at all.
For those who may find these results hard to believe, based on experience of commercial rubber duck antennas, the SWR curve may help to reassure. The DIY Ducky is tuned spot on the 2m band, because you made it so. Many commercial rubber duck antennas - including the Kenwood one supplied with the TH-F7E - is resonant quite a way out of the band. This obviously doesn't help performance at all.
This was one of the easiest antennas I have ever made, as well as one of the cheapest to build. The results were certainly worth the effort. Why not give it a try? Real hams use home-made antennas, after all. :)