Hi Clay, Long time no see...
When I lived in Minooka in the 60's & 70's our old friend from Kankakee
who was with the TV cable company fixed me up with a lot of "scraps" of half
inch and one inch 75 ohm foam dialectric solid aluminum encased coax. Some
of it had a direct burial covering. BTW, is he still in Alaska?
Anyway, I made a number of shielded loops out of small pieces of the one
inch stuff. I figured the smaller amount of capacitance-per-foot would
greatly improve the Q over the RG58 loops that Doug DeMaw and I had worked on
earlier. I wanted to see if I could get enough signal voltage that I
wouldn't need an amplifier. Amplifiers had always reduced the signal to noise
ratio in my experience. That was likely due to the unavailability of low noise
devices as well as the lack of amp building prowess on my part. Doug was a
lot better at that than me.
Nevertheless, it worked out for me. I built a shielded loop about 10 feet
in diameter. There was a one inch gap in the shield at the top. There was a
similar gap at the bottom to expose the center conductor but the shields
were joined there. The center conductor was cut at the bottom and a series
variable cap inserted. That tuned the loop to 160. I had a lousy way of
connecting it to the coax into the shack... just a series cap from one side of
the tuning cap to the coax center conductor. The loop produced enough
signal voltage that an amplifier wasn't needed.
I compared the loop's performance when hung in a tree to having it mounted
on the side of the two story garage with the shack upstairs. I couldn't
detect any difference, Clay.
Later I made a few three turn shielded loops of similar aluminum one inch
coax and a diameter of about four feet. They used a single turn of bell
wire for a coupling coil to the feedline. They were just as effective as the
big single turn loop.
During this period I had a 130 foot vertical, two half wave dipoles about
1000 feet apart, a 2,600 foot Beverage to the south, a 3000 foot Beverage
to the west and a 550 foot two-wire reversible Beverage to the northeast or
southwest. There were other antennas that could be used for 160 receive as
well.
The loops were occasionally the best antenna to hear a particular
station... just as were all the rest. As I reported in the June '77 QST
article on
the subject, even the connection to the finger stop on the old dial
telephone in the shack worked pretty well at times. I remember 160 contacts
that
could be best made listening on our tri-band beam.
One other anecdote relates to your question, Clay. Stew, W1BB asked me to
make one of the small three-turn shielded loops for use at his "Tower"
station in downtown Winthrop. He said that the urban noise was beginning to
cramp his style, even on his half wave 270 feet above the ocean below. I made
the loop and got it to him. He called later to say that it seemed to be as
noisy as the tower antenna and wasn't helping.
In discussion I learned that he had hoisted it up the water tower to 100
feet. I told Stew that the feedline was acting like a vertical antenna and
suggested he bring it down and mount it on the side of the little shed at
the base of the water tower where he had his rig. He did that and reported
that it was making contacts possible with stations he couldn't hear on the
high dipole. There was no room whatsoever for other types of receiving
antennas at that location. I know a couple guys who used those loops to good
advantage with them located in their basements!
Certainly the small loops are no panacea, Clay. But, as my Grandma used to
say, "Beggars can't be choosers."
If space, restrictions and resources intervene, the small loop becomes an
important option.
I'll look for you on 160, Clay. Joyce sez "Hello."
73, Barry, W9UCW
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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