|Drilling the Zareba insulators for 14 AWG wire|
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 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|
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|
|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 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|
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.