Rick - DJ0IP / NJ0IP wrote:
Then that is very easy.
Study the detail pictures of the antenna on that link I sent you.
(http://www.dj0ip.de/my-favorite-antennas/old-folks-antenna/ )
It shows close up details of how to wire the big and small loops.
Then simply replace the capacitor (which consisted of a short stub of RG-213
plus gimmick capacitor) with a variable capacitor.
One thing I did on the foldable antenna shown in the picture at the bottom
was use a small variable capacitor (50pF) and then switch in fixed
capacitors in parallel for changing bands.
I don't think you told us which bands you want it to cover, or if you did, I
missed it.
10-30 MHz.
Actually for receiving we can make it cover all bands but not on TX.
Building one for receive-only is really easy to do; not much you can get
wrong "if" you build it as I show in the detail page.
Pay special attention
to how the small loop is prepared. Note that I removed about 1 inch of the
shield half way around the coax. This is important. Everything else is
pretty easy.
Sorry, link is not working here. Do you mean that 1" of insulation and
shield is removed at the halfway point of the small loop of coax?
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Lee
Sent: Saturday, April 26, 2014 12:27 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] New Jupiter on its Way _ A Few Questions
Yea, that's what I had in mind, Rick, an antenna that is quiet, so that I
can hear more than just the strongest signals.
On Apr 25, 2014 2:53 PM, "Rick - DJ0IP / NJ0IP" <Rick@dj0ip.de> wrote:
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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