Rick,
Thanks for augmenting my post with some important caveats.
YES!, you have to have a room sized to not have the indoor loop too
close to the walls and possible conductors.
I was testing a loop made at our research lab in San Antonio one time,
indoors. It was just a quick test, not at full voltage, to see what the
wave form would be from a pulse source dumping into the one turn copper
tubing loop from a charged capacitor bank.
We were simulating lightning to do near field testing of some sensitive
amplifiers to be used outdoors.
Although my loop set up was horizontal on some insulating stands and one
foot from a metal shelf, and that seemed a safe distance for the planned
"one shot"; when I fired the charged capacitor bank into the loop, I got
"lightning". A one foot arc to the painted, and insulated metal
cabinet, (or at least we thought it was insulated up to that point.)
(The things you do as a junior scientist).
We took the loop and instruments outdoors for waveform testing after
that. It simulated the magnetic field of a lightning pulse quite well
for our purposes. It fired at any charging voltage without doing
anything unusual, except the expected jump, against its supports, from
the transient high field being discharged.
Oh, the reason it arced? The machine shop who rolled the tubing into a
loop for us, had one spot with a non smooth curve, and it formed a high
voltage peak at that discontinuity. It was such a minor imperfection it
had gone un-noticed until it called attention to itself.
As Rick points out, outdoors, even just outside at roof line, you get
dramatically better results from a loop. And, the loop can be made
larger (and more efficient) over an indoor model limited by room size
and contents. Don't overlook estimating the field before you stay close
to a loop.
The highest quality low resistance joints dictate silver soldering, or
brazing anything that connects to the tubing, if you can't weld it.
Other mechanical methods that might work, are to polish the flattened
surfaces to be joined, then introduce conductive grease to protect the
bolt joint from oxidation and maintain the connection. Tubing to be
bolted should use highly conductive washers on the bolt, to distribute a
high loading to the joint, and dissipate any heat build up. Use of
capacitors where the current does not have to flow thru a rotor shaft is
preferred. You can get two big variables twice the capacitance needed,
and put them in series so that the current only travels via the field
thru both rotors, and no mechanical rotating connection is a current path.
WB5AOH used a U shaped tubing "rotor" in his trombone capacitor, so that
the field was between the air gap between the U and the two tubes that
formed the stator. Teflon rings provided a lubricated sliding joint,
and spacers between the tubes. He had a group of paralled capacitors
that could be switched into use for 80m.
Coupling to the loop can be by the use of a small loop at one side of
the transmit/ receive loop.
As Rick said, for low bands 80 and 40, a two turn or more loop allows
the band to be driven more efficiently. (Smaller loop diamter).
I have been doing research on WW2 NVIS early use, and loops were
sometimes used horizontally one meter above the roof of a Scout car.
There was also a meander line dipole used one meter off a car roof, as
well as other designs that seemed to emulate an isotropic source, as
that radiator was a random structure, not resonant, and single wire fed
from a larger transmitter, truck mounted.
W5IFQ, another researcher here, uses the early MFJ multi band loop to
maintain ham radio links when he is on research in distant oceans. He
is able to maintain email schedules with home by the use of a loop
placed above the superstructure of the ships, which typically are 200
feet long or less.
-Stuart Rohre
K5KVH
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