200 Foot Doublet - Ladder Line

After my last post there was still another day and a half of trimming trees to get a clear path for the doublet. I cut or trimmed at least twice what I thought I was going to when I started, at least 2-3 pick-up loads of trimmings. It was difficult work to say the least, running up and down rough terrain and tending to the hoists at each end of the antenna, which is about 100 meters long including the support ropes. Finally, Saturday, the rope and wire had a clear shot and I was able to pull it all the way up in the clear. 

In the middle of that I took a break to start making the homebrew ladder-line with which I would feed the doublet. 

As I'd mentioned in the previous posting, I'd ordered some Zareba (which I always remember as Zebra) fin insulators, a tip I'd picked up on an eHam page. These are small, lightweight, made from UV-stabilized polyethylene and just 10 cm long. Perfect. They come in packages of 50 for $4. I set up a quick jig on my drill press and made holes on each end 2 mm wider than the insulated 14 AWG solid conductor copper house wire I would use for the feed-line. Center to center the holes were 86 mm apart.

Next, I stretched out two 40 foot lengths of the wire and started sliding on about 45 of the insulators, figuring an approximately 10 inch spacing on the line. Once they were all on I stretched the wires and secured the other end. The fit was such that the insulators slid on easily but there was enough friction to hold them in position prior to fastening them down. I spaced them along the wire using a 10" stick as a gauge.

When the spacers were all in place I fired up the glue gun, setting the temperature higher than normal so the glue would be less viscous.

When I'd read about using these insulators for ladder-line I remembered the person gluing around the wire at the hole. But, it occurred to me that a better technique would be to send the glue in from each end of the tube. The tip of the glue gun was a perfect fit and one squeeze of the trigger just filled the hole with a little glue extruding from the wire hole. In this way, too, the tube is blocked for any small insects who might think of building a nest inside. I glued one entire side first then made small eyeball adjustments as necessary to make sure the spacers were straight as I glued each one from the other side.

Voilá, 40 feet of ladder-line done in about an hour and a half at a cost a little north of $10. I wasn't terribly concerned about the exact impedance as you might have guessed, but I did dust off a 1963 ARRL Handbook to see what the calculation would be for this spacing and wire size. Somewhere between '63 and my next most recent Handbook (circa 1990) this formula and chart seem to have been edited out. I'm sure brewing up your own open feed-line went out of vogue some time ago.

I used both the chart and the formula and I was puzzled at first why I was getting two different answers (480 ohms by formula vs. about 550 on the chart). Finally, I checked a different source (Wikipedia) and realized my lack of reading comprehension was at fault as I'd used the wire diameter (1.5 mm) instead of the radius. That error corrected I came up with 568 ohms Z, which corresponds nicely with the graph. I probably should have made an empirical measurement, at least for the exercise, but the wire is already up in the air now doing its duty as a low-loss carrier of radio energy under highly variable SWR conditions! The radio end is attached to the Johnson Matchbox. More about that in the next installment.