Re: Cores for rod antennas
- --- In VLF_Group@yahoogroups.com, "Herbert E. Plett" <cachureos@...>
> --- John Popelish <jpopelish@...> wrote:
> > My coil is similar to a Helmholtz, except that it has 3 coils,
> > of 2 (and with fewer turns on the central coil).(snip)
> > I posted pictures of it in the ferriterodantennaexperimentersI moved copies of the field coil and the iron wire rod to the photo
> > group photo area. It works well up to beyond 60 kHz.
> could you, please, post a link to the specific file?
section of this group, in the popelish album.
--- In VLF_Group@yahoogroups.com, "Michael Hebert" <qrpbear@...> wrote:
> Using two 3# spools of #32 wire and one 2# spool
> of the same in series on a 15" Stormwise SLF-15 ferrite core
> overlaid with a layer of metglas produced a very nice
> sensitive system.
> Using the same 3 spools of wire on a 15" core made up
> of a few sheets of Conetic mumetal produced the
> same results but with better apparent "signal" to noise ratio.
> Inductance of either of these sensors was not measurable
> with my meter which only goes to 40 Henries.
What is the total DC resistance of this coil?
> My current setup employs a recycled 24" diameter degaussing coil
> that measures 24mH with a DC resistance of 12.4 Ohms.
> This is used in conjunction with a negative impedance preamp
> to further cancel inductive reactance effects.
It cancels some of the coil resistance, I think, extending the L/R
> With the Conetic cored solenoid and a THAT1512 instrumentation
> amp I can get a fair signal at 45' and a detectable one at 60'.
> With the loop feeding an AD743 in negative impedance configuration
> I get a strong signal at 45', fair at 70' and still detectable
> at 85'.
> I don't think it's so much a case of the signal being stronger
> as it is one of the SNR being better. The loop/AD743 system
> has much a lower background noise level.
Ive looked over the data sheetd for the THAT1512 and the AD743. The
problem with the THAT1512 data sheet is that they give a voltage noise
only at 1 kHz, without giving the corner frequency below which the
noise rises at a 1/f rate. I assume this is because it is optimised
to sound quiet to the ear, and low frequencies are hard to hear.
The AD743 data sheet not only shows the voltage and current noise
versus frequency graphs, it specifies several frequencies down to 10
Hz. It looks like its 1/f voltage noise corner frequency is a low 20
or 30 Hz.
Another good amp for an input with a low impedance, like your degauss
coil would be the LT1128, with voltage noise at 1 Hz about 1/20th of
that of the AD743. Its current noise is a lot higher, but that gets
lost into the low impedance input.
--- In VLF_Group@yahoogroups.com, "John Popelish" <jpopelish@...> wrote:
With regard to the 3 spools of #32 on the Conetic core...
> It cancels some of the coil resistance, I think, extending the L/R
> time constant.
The cancellation of a portion of the coil resistance is also how I
understand the Negative Impedance circuit operates. As you point out,
that will extend the L/R time constant. In spite of that the
improvement in transient response time of my 24mH loop as compared to
my 40+H solenoid is like night and day. A further benefit of the
Negative Impedance configuration is that the current flow through the
coil is increased due to the resistance cancellation. If one believes
Sutton & Spaniol's research that has the effect of increasing the
aperture or "capture area" of the loop considerably beyond its
> Another good amp for an input with a low impedance, like your degauss
> coil would be the LT1128, with voltage noise at 1 Hz about 1/20th of
> that of the AD743. Its current noise is a lot higher, but that gets
> lost into the low impedance input.
The LT1128 is a chip that I would like to try. Unfortunately Linear
Technology is not so generous with samples as Analog, TI or THAT Corp.
I am slowly building an order for bits'n'pieces from Digikey and plan
to purchase the LT1128 as part of that order.
Note in my references to Sutton & Spaniol's work that I am running
only the Negative Impedance circuit _not_ the Black Hole sensor with
the Howland V2C converter and feedback coil... experimental work with
the Black Hole will come later. I am more interested at this point in
time with going to even smaller values of sensor inductance. I have a
suspicion that lower impedance sensors will also prove valuable at
VLF. One of my dreams is to be able to hear sferic tweaks (maybe even
the occasional whistler) from my home location in the heart of Honolulu.