Sensing the VLF E field?

[TH Jim]

Hi folks,

I have been wondering if It is possible to sense the presence of a nearby MD TX by picking up the E Field instead of the usual M field? Anybody have any experience of doing this?

Regards, Jim.

[dfbowers]

Absolutely. I remember the first relic hunt I did with another guy that had a Whites back in the early 80's. I could not hunt within about 40 yards of the other guy! He was using a 5000 DI and I had a 2DB series 2.

[golfnut]

The orthogonal field to E is H - the magnetic field.

A pickup coil would on a ferrite rod is worth a go for H
or
a whip antenna and a fet front end Rx for E

S

[TH Jim]

Hi all,

I've done some quick tests with some untuned MD RX front ends as receivers but don't get much distance between the two rigs before the noise swamps any signal the TX is putting out. I want a receiver that will cover the whole VLF band - so no tuned tanks as sensors. Hence my thinking that a whip and high z input might be a better choice. Seen the VLF radio guys doing this and wondered if anyone on here might have some experience of similar kit ?

Cheers, Jim.

[Davor]

You'd need a huge coil or a big chunk of ferrite for a magnetic component pickup. If you are lucky you might hear some whistlers.

[golfnut]

What do you want to do with this?

[TH Jim]

Hi Steve,

I want to detect other metal detectors, or more precisely detect what frequency they're using so i can avoid it. Think Minelab FBS machines - 11 different channels.

Regards Jim.

[golfnut]

Sounds like you do club digs with 10 - 200 other machines knocking you out. The best way is get your own permissions and go alone.


The detectors you wish to detect - if all commercial units will have respectable e-field screens - So it is probably harder to detect the e field, so not a whip and fet .

I would try a six inch ferrite rod, the type in an AM radio. Put about 150t of 0.15mm on it. You could resonate it at a mid frequency - say 10kHz, perfhaps de Q it a little to broadband it with maybe a 1k resistor on parallel.

The hard bit is generating a range of frequencies for the Rx Local oscillator. Discreet frquencies are likely to have gaps and miss machines.


Ideally sweep the L.O. from A to B and repeat. Record the loudest reply frequencies - Sounds like a BIG project suddenly using RF, analogue and digital HW plus software and displays.



You could sweep manually from a pot with a scale on the pot for frequency - some sort of vco.

S

[sinclairuser]

its definitely going to have to be built from scratch, what you need is a replica of the tuner used in the rx of the md to be detected, it will need to be wideband as well at least as wide as freq1- freq11.
frequency counters or scanners are not much use, well none of mine pickup the tx of any of my detectors, only wideband radio noise coming from the electronics, which is no good to you.
what you need is basically a frequency counter, built tuned to scan from freq1- freq11, fixed on a shaft with a tuned receiver coil on the end, this would tell you the frequency of near by detectors.
not an easy task, i have tried with frequency counters using the "whip antenna", but only picked up electronic "mush" from the electronics.
you could try a frequency counter, fitted with a tuned coil as an antenna, as a start and if it detects the right frequencies, then narrow down the receiver in the counter( the right counter can do this with programming), but as i said not easy, the signals are tiny.

[sinclairuser]

just to clarify, the reason i think you should try a frequency counter is 1 relatively cheap and in its own case, 2 freq counters have enough components to detect a particular frequency without all the unneeded bits(demodulators, audio,etc), 3 they are sensitive and capable enough to detect the tiny signals (bare em carrier), tuning some form of antenna and its relative range is going to be the hard bit.

[TH Jim]

Thanks for your input guys. I can detect a magnetic TX field using my frequency meter and an 8mH inductor placed right up against the coil. This is what i use for debugging/testing tec. But, what i would really like is to detect the frequency of said detector at a much greater distance. Using several inches of ferrite rod and a whole load of turns is getting into fluxgate territory, and great for sensitivity, but awful for noise surely?

That's why i was thinking of something like this:
http://www.vlf.it/fritz/pocketrx.html

Regards, Jim.