Stuart pretty much nailed it.
The only thing I wish to point out is, all he has said has to do with
building loops which are suitable for transmitting and receiving.
If you only wish to receive, this is really simple Simon.
No worries about resistive losses in connections.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Stuart
Rohre
Sent: Friday, April 25, 2014 11:40 PM
To: tentec@contesting.com
Subject: Re: [TenTec] New Jupiter on its Way _ A Few Questions
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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