Saturday, March 20, 2010


In the 1930s, engineers at the Radio Corporation of America[1] introduced what they called "six-wire transmission line," a neat dodge that used U-shaped supports and a handful of insulators to turn ordinary wire into a good approximation of high-power coaxial cable (scroll down for drawings and photos) without all the trouble and expense of using large-diameter copper tubing: in cross-section, four wires apart occupied the corners of a square about twelve inches on a side while two more, very close together, were set in the center. The outer wires were all grounded (just like the outer shield of coax) and the center two, connected together, made up the center conductor. (Here's an example of genuine American six-wire line at WGY).

It got a little attention; the technology of the day used what's known as a "balanced two-wire line" between transmitter and antenna, both wires "live" and rather sensitive to weather and temperature, or even dodgier methods. But about then, a little war broke out and the engineers suddenly had different priorities.

The WW II development of fancy plastics, smaller concentric line and the boom economy of the 1950s left that Depression-era trick in the dust -- real coax made out of big copper tubing was just the ticket for the 250W to 5kW stations that made up the bulk of broadcasting in the U.S. and they could afford it.

...Other places don't do things the same way; in the rest of the world, fewer stations and higher power were more usually the case and in cases where one country sought to, ahem, get to know the neighbors better, very high power was the norm. Nobody makes coax for that and even if they did, most outfits -- many governments! -- couldn't afford it. But some wire, insulators and few custom castings? That's well within the budget and it leads to images like these.

The young gentlemen in the lower photograph are standing in (or nearly in) a ring that supports the outer wires of a scaled-up version of RCA's penny-pinching 1930s six-wire line. Call it five and a half feet in diameter, at least. Once completed, it carried a 1.2 million Watt signal from the transmitter to the antenna. Not bad for something an RCA guy dreamed up to avoid the cost and trouble of turning two sizes of water pipe[2] into coax, hey?

This is why it pays to remember old tricks.
1. At the time, not the "RCA Corporation," a name-change that was the knee of a long curve down for the former technology giant.

2. Honest. Early on, the commonest size of "rigid line" with copper-pipe inner and outer had a non-standard impedance (determined by the ratio of the diameters of the inner and outer conductors, times a fudge factor) because that's what you got with the off-the shelf pipe sizes. To this day, the outer conductors connect using standard large size high-pressure water-pipe flanges. But the inner conductor's now a size that, oh, gosh, a size that works out to a more slide-rule friendly number.


  1. Makes me want to sell my QRP stuff and go QRO .. OOO!

    But maybe I should check my neighborhood C&Rs about the 2,000 kilowatts. And the Part 97 rules, too.

    Or maybe I'll just turn the power down a little? (Maybe they won't notice.)

    Let's see, if I come out the backside of the rig to a frame to set the shape of the six wire line, I'll need to fashion a 90 degree corner to turn right to go over to the side wall and then through the wall of the house -- need that long drill bit again for the six holes. Then another 90 up to the roof -- how do you do a rain drip, I wonder? Then another 60-70 degrees to match the 30-20 degree roof slope, and a final matching 30-20 degree turn when it reaches the mast so it comes in square.

    Now, for 14,100 KHz using 18 gauge copper-coated steel wire the spacing is gonna be ... ... darn it, who's got my slide rule?

  2. Impressive. Now, how to feed it through my (rented, humorless landlord) apartment's horizontally-sliding window... 73 de ka9vsz
    WV: ableene- a suburb of Abilene?

  3. Well, here are some hints: WikiHow or a tool supplier's website (haven't bought from them, just popped up on a search). Making the landlord happy about it, that's another story.