Sunday, July 28, 2013

Homebrew Ladder Line for my 200 Foot All-band Doublet

Drilling the Zareba insulators for 14 AWG wire
Drilling the Zareba insulators for 14 AWG wire
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, which came to least three 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. 


Back of the Zareba fence insulators package
Back of Zareba Package

As I'd mentioned in the previous posting, I 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 are 86 mm apart.




Ladder line spacers being slid on
Ladder line spacers

 Next, I stretched out two 40 foot lengths of the wire and started sliding on the insulators. At an approximately 10 inch spacing on the line, it took 45 of them. 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 I normally do, so the glue would be less viscous.

Close-up of glue gun filling spacer tube
Close-up of glue gun filling spacer tube
When I'd read about using these insulators for ladder-line, I remembered the person gluing around the wire at the hole. It occurred to me that a better technique would be to send the glue in from each end of the hollow tube. The tip of the glue gun was a perfect fit. 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. We've got plenty of those kinds of critters here in Costa Rica. I glued one entire side first then made small eyeball adjustments as necessary to make sure the spacers were perpendicular to the line and parallel to each other. As I put each in final position, I applied glue via the tube hole on the opposite side.

Finished spool of homebrew ladder line
Finished spool of homebrew ladder line

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 have been edited out. I'm sure brewing up your own open feed-line went out of vogue some time ago.

Characteristic impedance chart for ladder line
Characteristic impedance chart for ladder line from '63 Handbook

Using both the chart and the formula, 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. I had 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.
Johnson Matchbox Interior
Johnson Matchbox Interior

I probably should have made an empirical measurement, 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 my 300W Johnson Matchbox. Update: The ladder line is now in service ahead of a Heathkit 2060 ATU and Ameritron ALS-600, taking north of 400W on CW.

3 comments:

  1. Ken - KB7H26/1/12 16:34

    OK... you've been off the air long enough. We miss you!

    ;0)

    ReplyDelete
    Replies
    1. Just got back from a couple days trip to Lake Arenal, visiting with TI7/AA2UP and his XYL. Should be back on the rig tomorrow one way or another!

      Delete
  2. Ken - KB7H27/1/12 19:08

    OK... you're forgiven as long as you guys had fun.

    ReplyDelete

Thanks very much for your comment! 73, Casey

Related Posts Plugin for WordPress, Blogger...