Low Frequency Electro-Magnetic detection

Move the filtering up the signal chain as far as you can -- for example, turning your ferrite coil pickup into a parallel resonant circuit, then using bandpass filters for gain.

As an example, you might google for Polar heart rate monitor receivers

-- the Polar system generates 5 KHz pulses. The typical receivers follow the scheme described; a tuned-circuit pickup followed by gain stages incorporating filtering.

namaste--

Do what the guys suggest, but keep in mind that the field from your transmitting coil is probably dropping off as the cube of distance, and will be zero at certain orientations.

John

Unless the transformer has been designed for this task, it may be losing as much signal as it gains. I would wind the middle third of the ferrite rod with several layers of fine magnet wire, and resonate that with a capacitor at the detection frequency before connecting it to a high input impedance amplifier (to keep the Q up). 1200 Hz takes a lot of turns on the rod to maximize signal voltage. Adding a big ferrite bead on each end of the rod (like you find around the cables on monitors to block EMI) would increase its inductance and sensitivity, also. You need only a small overlap between the rod and the bead. Rod length is signal.

I totally agree with the resonance method, as well as keeping the loading to a minimum. I would use an opamp in gain follower mode as the first gain stage. I do not think that ferrite beads on the ends would be of any material benefit.

Another trick one could try is to arrange four ferrite rods into a square form and to connect the windings on each one inseries aiding, to make a ferrite version of a "quad" antenna. That would increase sensitivity and increase directional capabilities (RDF??).

I'm doing (well the PC is) a low level format, writing zeros to the drive, using a seagate Disk Wizard program, at the moment.

Next I'll do a high level format ,writing ones to the drive.

sorry, coudlnt resist

martin

I immediately recognized that, but most "beads" are rather small compared to larger sized ferrite rods. That is why i suggested the "quad" configuration.

The beads I am thinking of are made to fit around 1/4" to 3/8" cables and are about an inch in diameter.

A coil of wire can preform quite well. Take a look at:

Although the noise graph hits its bottom below 1000Hz, it is still quite good at 1200. The only thing that is going to do better will be a low-temp SQUID.

10fT/sqrt(Hz) puts it below the earth's background noise.

How long do you have to do the detection and how accurately is the frequency known? Around 1200Hz could mean within a Hz or within 100Hz. It could also mean 1201.234567 Hz exactly or something at drifts around in the 1200Hz area.

If the signal doesn't drift or only drifts very slowly, there are lots of methods like "stacked PSDs" that can pull the signal out from the noise. The biggest problem will be the mains noise right at 1200Hz.

If you know the field strength at distance that is at least, lets say, 5 times the size of the source, you can use the simple 1/R^3 rule to figure out where it will drop below what you can detect. You may be surprised at just how far it is.

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kensmith@rahul.net   forging knowledge

Ken,

You're idea was pretty intriguing and I looked at it some. However the BF-10 antenna is over 4' long and 2.5" in diameter. For the purposes of this demonstration the receiver has to be covert (unless of course the reception could be done from outside a building at some distance).

It was difficult to estimate how much signal we'd have at 100 feet but I'm guessing AC line noise would overwhelm us.

See my more general comments, in my other post, on the signals in particular.

I agree that a low-temp SQUID device seems ideal, but cost/temperature issues are problematic.

The security case we're trying to protect against would be a low-cost (

The quad antenna w/ or w/o inductors sounds like exactly the kind of case we're looking to reproduce / defend against.

The following scope plot shows an example of the signal

The green trace is the FET on the target device driving the coil.

The darker trace above is the signal out of the band-pass filter (using our existing ferrite rod antenna at 6" or so).

• ~2.2KHz signal since the edges are much sharper and we're not dealing with sine waves here. The low-pass filter is set about 10-15KHz. If we get much below 10KHz we round out and flatten the signal and drive it into the noise.

The modulation is non-traditional with one flux transition per bit-time for a 0 and two transitions per bit time for a 1. In this case a bit time is ~430us.

What would be the easiest way to get a rough idea of what this quad antenna might look like? What kind of rod? How many turns? Inductors, etc...

Alternatively, if someone out there feels they have a good grasp of this issue and could deliver an antenna w/ or w/o preamp w/ significantly more range than we currently are getting we'd certainly paying for time and materials.

In any event, I'll eagerly await posts on this site or feel free to contact me directly at my email below (replace the with an @)

Thanks, Dave. david.pariseausbcglobal.net

Get some of the software that amateur radio operators use (with PC and sound card) for weak signal operation. They have FFT based displays for such things as detecting morse code signals bounced off the moon.

Mark Zenier snipped-for-privacy@eskimo.com Washington State resident

So you think I couldn't hide a 4' lone 2.5" diameter object near your system? I'd put legs on it and a couple of hooks and hang a coat on it. :)

I haven't made measurements up at 1200Hz lately so I can't tell you and real solid numbers but: At 180Hz there is usually something like 100nT in a normal building. As the harmonic gets higher, the amplitude decreases about proportionally or perhaps with N^2.

[...]

Also they require more management than just a coil of wire.

Other ideas:

Drive a fluxgate magnetometer with a 1800Hz drive so that its frequency responce goes up high enough. Making a fluxgate good to about 5nT is within the range of mere mortals.

Use many magneto-resistive devices. MR devices and GMR devices really suck in terms of performance but they are fairly small and low cost. Averaging the outputs of several would get the noise down.

There is a company called PNI that makes electronic compasses for cars. They are magneto-inductive sensors that are very low cost.

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kensmith@rahul.net   forging knowledge

Try resonating the rod you have to 11 times the 2.26KHz or about

24.9KHz, with a "Q" to make it almost critically damped (slight ringing; experiment on that).

Hi Dave,

I think you are trying to receive the magnetic field.

Long pieces of ferrite can be concealed in a plain old aluminum walking cane, not sure if stealth is a requirement. I was probably attempting to do the same thing as you are, but it was several years ago. Had limited success. But, the cane very stealthy.

I was also interested in spoofing as well, which is transmitting too...that's another story alltogether.

Also, be sure to 100 percent shield the device from RF. Some low frequency rf fields are very strong and you can have problems with rf being picked up and amplified. Of course, this would be unknown unless you looked at the spectral output of the chips, and even then it might not be apparent.

But, you can gain a very large dynamic range boost by shielding the receiver with a 100 percent copper shield or aluminum. The VLF users find this tactic to be very successful and use it all the time.

You might also run the problem by the loopantennas mailing list (yahoo) and the VLF_Group (also at yahoo). Both groups are highly knowledgeable and require high sensitivity receivers and antennas.

Regards,

M