What is the advantage (if any) to using co-ax v. open-wire feed for
above HFBC example?
Coax has the fields contained within the cable, open-wire does not.
Structures, people, and critters in proximity to coax don't matter.
It's a different matter with open-wire. Open wire usually requires a
BalUn, as well.
Coax has the fields contained within the cable, open-wire does not.
Structures, people, and critters in proximity to coax don't matter. It's a
different matter with open-wire. Open wire usually requires a BalUn, as
well.
The WWII, British, CH radar (circa 50MHz), used open wire feeders to the
transmitting antennas. The transmitters were a Marconi-EMI television
design, of a few hundred kW, modified for pulse modulation, one of the
reasons for killing the early British TV service.
The guy I knew who'd worked on it described how the routing at one site,
around the edge of the field, with several corners, to conceal the feeders
from the air, couldn't be made to work, and they had to go direct, making
a beautiful sighting line to the camouflaged transmitter.
Perhaps Coax wasn't available?
This was 1942 Britain.
That station is long gone,
and the site it sat on is now a golf
course.
I got the nickel tour of that station in '70.
SOme of the
switching was done in underground vaults at that time.
They were in the
process of a total upgrade of that plant at the time. 10 new 50 KW
custom National .5 to 30 MHz transmitters.
A complete new control room,
and they were replacing the wood poles with small towers.
The only
thing that wasn't replaced was the Curtain antenna that was aimed at
Europe & Russia.
In sci.electronics.misc dave said:The BBC used motorized balanced feeder switches that were about two
stories tall. They actually convey a lost sense of whimsy that once made
pre-CAD technology interesting.
The original transmitters at that site were made by Crosley. Green
metalflake paint and big glass doors in front of the finals. Beautiful
examples of '30s industrial design, but built before Television. They
tried numerous methods to eliminate the TVI, and were replacing them
with custom designed National radio transmitters. When you changed
frequency, the transmitter reduced power and self tuned every stage.
All 10, or any combination could be fed from one MFO for power levels
from 50 to 500 KW. The self tuning was done with detectors to measure
the signal and large servos to drive the variable capacitors & roller
inductors.
