Friday, May 5, 2017

Hanging Dr. Fong

Hanging Dr. Fong

By Andrew Ellis, NO6E

Long before WB6IQN's latest article, “A Tri-Band Antenna Without Radials For 2 Meters, 1.25 Meters and 70 Centimeters” in the March, 2017 issue of QST, the remarkable Dr. Edison Fong has been designing inexpensive and well-performing VHF and UHF antennas for ham operators. His DBJ-2 , a roll-up dual bander for 144/440 mHz appeared in the March 2007 issue, and the venerable DBJ-1, the subject of this article, dates all the way back to QST of February, 2003.

Ed's antennas are easy enough to build, and he always shares all the dimensions for those who want to construct them on their own. But tuning the antennas is best done with a network analyzer, which most of us don't have. As an alternative, Ed has some of his graduate students at the University of California Santa Cruz Silicon Valley campus build the antennas, which he sells at very reasonable prices from his website . Proceeds of the sale go to support the work of his students.

Ed is an old friend from my SF bay area days. He is whip-smart technically, but kind and generous with his time and efforts. (His antennas are patented, but he allows hams to build them for their own use). He is a wonderful individual in every way.

I own several DBJ-1's, which are available in both amateur radio (146/446 mHz) and commercial (152/457 mHz) versions. The antennas ship with everything except the PVC pipe to house them, and finding the right “Schedule 200” PVC can be tricky. The much-more-common “Schedule 40” PVC is not rf-transparent enough for the job.

Many of us would like to have large steel towers to support our antennas, but, like network analyzers, few of us have them sitting around. Trees, though, are much more common. After running a DBJ-1 on the back deck of my home for some time, it occurred to me that it could be hung from a tree, as wire antennas are.

An early DBJ-1 iteration at NO6E broke when the 20' PVC mast supporting it (not recommended!) toppled over. So, while the antenna could certainly be mounted to a tree, it was not quite sturdy enough on its own to survive long there.

My solution was to cable-tie the antenna to a piece of “one-by-two” lumber to provide a convenient hanging point, cable strain relief and stiffening:

The board is ordinary pine 1x2, painted with white primer for waterproofing and to match the color of the DBJ-1. (All the Schedule 200 pipe I've found has been white).

Since the antenna is around 5 feet long, I used a 6 foot piece of wood. Cable ties (black for better UV resistance) secure the antenna to the board. Note that the end caps on the pipe are slightly larger diameter than the pipe itself, so don't overtighten the ties immediately adjacent to the caps. (If you buy the antenna kit, Ed supplies both caps, one with the coax connector pre-mounted).

At the top of the board, a 3/8” (9 mm) hole allows me to attach the rope support. For the support itself, I use parachute cord ("Paracord"), which will support a couple of hundred pounds and is inexpensive, especially in large spools

Cable ties are spaced every 18” (45 cm) or so. At the bottom, connect your feedline before attaching the antenna to the board and weatherproof the connection with your choice of tape, sealant or whatever.

Add a couple of additional cable ties below the antenna to secure the feedline to the board. This is especially important when one is hanging a fair length of coax off the antenna, since coax connectors do not provide strain relief. The ties support the cable:

The last step is hoisting the antenna into place. I chose a long-leaf maple near the rear deck of our house. Here's the assembly ready for hoisting:

When fully lifted, the antenna is perhaps 40' (12m) up in the tree:

It won't come a shock to readers that hams are a persnickety lot. In anticipation of objections and questions, I've prepared a few replies:

Q: How do you know that the pine is rf-transparent?
A: I don't, and it likely isn't. The antenna works a lot better than it did lower down, though.

Q: And the tree? Doesn't it absorb some of the rf?
A: I'm sure it does.

Q: Won't the cable ties deteriorate in time and need to be replaced?
A: Yup.

Q: How the, uh, heck, did you get the paracord over the tree branch for hoisting?
A: With a tennis ball cannon. Powered by flammable hairspray, these probably-not-too-safe things will toss a tennis ball, trailing, say, 15 pound (7 kg) test fishing line behind it, hundreds of feet. Once the fishing line is over the tree branch, tie the paracord to it and pull it back.

Q: Do you vouch for the safety of those launchers?
A: Absolutely not. I don't recommend using one. If you choose to, you're on your own.

Q: What's that body of water in a couple of the pictures?
A: Evans Creek, in Rogue River, Oregon.

Q: Is Dr. Fong paying you or asking you to write this?
A: No. He doesn't know about it.

For me, amateur radio is about making do. Sometimes, your work will be 100% mil-spec impeccable. Mine never is. The idea is to have fun, and I do. I always ask myself, “How good does this NEED to be?”