Weekend-before-last's success with this antenna has invited a few questions about how to build one, so here we go. Some time ago I built a couple of bike & hike antennas based on WX2NJ's Jurassic Duck design. They were... OK. They certainly worked, but for whatever reason the dimensions given there were about 8" too short. Maybe it was variation in the CPVC pipe plastic from what he used to what I could get locally. The results were blogged about here, but the bottom line is that it was a pain to make and too fragile for backcountry use. Also, the 1/2" CPVC whipped all over the place, even though it was solidly mounted on my pack or bike rack. And one just flat broke during a hike. But the Mk I was a good start, and I'm grateful to WX2NJ for designing and writing it up, and to KK4SNA for pointing it out.
OK, let's ditch the scrawny 300 ohm twin-lead and upsize to relatively rugged 450 ohm window line. Window line will slip-fit into 3/4" SDR21 PVC pipe – that's the thin walled stuff, not the thicker Schedule 40. Take a little piece of window line with you to the hardware store, make sure it'll fit. The heavier components are key.
By the way, if you're not familiar with j-pole antennas, that's what we're building here so go familiarize yourself over at Wikipedia. The ladder line section forms the "j" matching stub. The rest is just a length of wire.
Also, you'll need an antenna analyzer that works on the 2 meter band. If you cut to exactly the described dimensions, it'll probably still work, but you'll be risking burning out a transceiver. If you don't have one, get a friend with one to help tune it before you put any power through the antenna.
With the basic design down, here's the shopping list:
- 3/4" SDR21 PVC pipe. It comes in 10' lengths. The antenna part will take up about 4.5', but you'll want at least 3' for a mounting stub below that. Cut it off to whatever you feel like, I chopped mine at 8.5'. That's a good compromise between "too long" and "too short to mount on a pack."
- cap for top of pipe.
- 17-1/4" 450 ohm window line. Wait! Cut it an inch longer at the bottom so you can short out its two legs, and make the "hot" leg an inch long so you can fold it over to form an eye to attach the radiating wire.
- 37" of any wire you have handy, in the 12 to 20 gauge range. Wait! Cut it 6" too long, so that you'll have some to trim back when you tune it.
- about 10' of 50 ohm coax cable, with your favorite connector on one end, bare wires at the other.
- a couple of small zip-ties.
- about 10' of thin string of almost any sort. Paracord is too thick, but one of the inner strands would be about right. Light fishing line would be perfect. Use whatever you have laying around.
If you've modeled this out with an online j-pole program, those dimensions will seem a little short. That's because they're not accounting for the PVC enclosure. Don't worry, I've done the cut-and try part, so you can just start building. Here are the steps:
(1) Strip the bottom inch of the window line, short the ends together, solder them.
(2) Strip tap spots on the window line 2-1/4" from the bottom.
(3) Solder the loose ends of the coax onto the window line. Be sure to have the "hot" center connector soldered to the side with the extra length where the radiator wire will be attached.
(4) Loop over the bare loose "hot" leg on the window line, and attach & solder the radiator wire as you see fit. Maybe slip some shrink-wrap over the solder joint to give some strain relief, or wrap it with electrical tape. This isn't critical, it's just some reinforcement to take the bend off the solder joint.
(5) Loop over the top inch or so of the radiator wire and loosely zip-tie it down. Tie the string through this loop, and snake it up through the PVC pipe, gently pulling the whole thing up to the top. Slip the cap on to pinch the string and hold it all from falling back through.
(6) Hold it vertically somewhere clear outside and apply the analyzer. It should resonate at a frequency a little too low. Trim the radiator wire back a 1/2" at a time, repeating step #5, until it resonates in the middle of the band. It should cover the entire 2 meter band.
(7) Clean it up. The antenna part is only about 4.5'. You may want to trim out some of the excess PVC on the bottom end. I cut mine at 8.5', which leaves a 4' stub to zip-tie to a pack frame or a bike rack rack. Remember, you don't want the bottom end of the antenna anywhere near, well, anything. Especially not a metal frame or rack. Zip-tie the coax coming out of the end of the pipe about 6" back up on the pipe, both to give strain relief for the soldered tap points inside and to make a drip loop.
Ta-da! Done. Spray paint it if you want. I didn't, but may eventually. Here's mine, zip-tied with unlockable ties, to my backpacking pack:
How's it work? It easily reached the W5SGL repeater 20 miles away from the Big Biloxi campground, and 25 miles away from my back porch. The old Mk I antenna barely reached W5SGL from 15 miles away, so this is way better all around: easier to build, more rugged, and better performance. Call it a success, call it a day.
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