[BC] open wire

[Clive Warner]

Mike Gideon wrote:
> We still have open wire lines to four of the towers at WDIA/Memphis.
> Scheduled to be replaced next year. Believe it or not, the current
> system has excellent bandwidth.

I arrived in Nicaragua to install a replacement 10KW (Siemens PDM) AM
transmitter for Radio Catolica.
The old unit was a vintage US-made transmitter, with 300 ohm output.
I walked the feeder to the antenna and was amazed to discover the old
open-wire line, that had long since rotted away and fallen to bits, had been
replaced recently.
Because Ronald Reagan was sending terrorists into Nicaragua at the time the
country was in a siege condition, and the local engineer had not been able
to buy any insulators. So, he had obtained a lot of four-inch PVA drainage
pipe and the thickest copper wire he could find. He measured the wire with a
micrometer and found the formula to calculate the impedance in an ancient
radio book. Then he cut lots of short pieces to act as spreaders and glued
vertical pieces into the middle for supports. He made holes in the ends of
the tubes using a hot icepick. Then he threaded the bare copper through all
the spreaders, and put posts in the ground at suitable intervals to support
his feeder.
Of course, I was unable to resist the temptation to check his work even
though I was installing new 50 ohm 7/8" Kabelmetal coax. So I terminated the
feeder in a 300 ohm resistor and put the OIB on it. The feeder was so close
to 300 that I really couldn't find any error.
- Clive