Need help....Coil formula for ferrite rod

I don't understand why you want to tune the antenna. Lightning is an impulse effect and is best captured with a broad band antenna. If the antenna is untuned, the number of turns is not at all critical. you can pass the signal through an audio amplifier and listen to the lightning.

By the way, there are some nice, negative resistance opamp low frequency receiver circuits available on the ULFELF, and VLF discussion group files sections at Yahoo groups. Both these groups deal with this sort of signal, as well as lots of other low frequency energy sources.

By the way, windings over the middle third of the rod make best use of flux passing through the rod. The outer thirds are essentially flux concentrators (flux arriving from all around the rod), but all the collected flux passes through the center section. It is very handy to find a small spool that slides over the rod and just wind it about full. Hobby and craft stores often sell small lots of colorful wire on such small spools.

Future plan is to make a identical pair of (orthogonal) cross tuned rods and a whip for the 180 degree shift for direction detection, Just getting started

Perhaps this page will have some information you find useful:

I did a search on Yahoo, found a project link to just what I wanted, but as usuall another blank page. 100Khz to 450Khz. I looked at the Url, may take me a day to figure out the formula. Would have thought someone would have written a small program with fill in the blanks. I may have to look for a longer ferrite rod, 12-15 inch. Thanks for the URL

Most antenna rods are optimized for medium wave band, not below .5 MHz. You can assemble a cheap, low frequency rod, by gluing together low frequency ferrite beads made for EMI suppression. Go to Digikey, search for [bead], then select [Ferrite Cores for Cables & Wiring, EMI(108 items)] and then, under Series, select LFB. One of my favorites is the 5/8" diameter LFB159079-000.

If you lap the ends with a bit of silicon carbide sand paper on a sheet of glass, you will get a tiny improvement in effective permeability. I have done lots of testing with an 11 inch rod made of these beads, but the URL I pointed you towards indicates that the rod should keep improving up to around 20 inches long, where diminishing returns sets in.

If I had the urge to build something along these lines, I'd work on those lightning "about to strike" boxes. I never saw one myself, but I understand there are devices that measure the potential of the air to indicate a strike is likely.

The problem I see with building just one sensor for lightning strikes is you really can't do a decent job detecting the distance. Now multiple sensors and triangulation would be a different story. In fact, it would be....

See...

I did design work for Lightning Location and Protection (LLP), Tucson, in the early '80's.

...Jim Thompson

A big aircoil might be better anyway - think a metre square. Those are used as boosters for radios with ferrite rods.

I've seen the article--it's in the Amateur Scientist CD-ROM compiled by Shawn Carlson.

Not being home I can't give the publication date, but do recall they recommended steel reinforcing bar -- "rebar" -- for the core, and something like 100,000 turns of fine wire, using this apparatus to detect very low frequency changes in the Earth's magnetic field, as well as responding to lightning, IIRC. The upgraded core material (preferred, but more expensive) was mu-metal, with suggested suppliers listed.

HTH, James Arthur

In message , dated Thu, 14 Sep 2006, snipped-for-privacy@yahoo.com writes

Oh, that is utter rubbish! It's like the more extreme views of audiophools.

If you have a magnetic core in the form of a rod, long compared with its diameter, its own permeability only has a minor effect on its inductance, because of the huge 'air-gap' in the magnetic circuit. And asking for 1000000 turns is just making things needlessly difficult (and expensive, of course).

Checked out the beads, $20.00 for two 11 inch stacked rods. What size coil did you wrap around the middle of the stacked beads? What kind of output did you get from the beads? Did you use an amp after the output? BIG question. What do you think Boltek is using that is so small in there receiver?

Checked out the beads, $20.00 for two 11 inch stacked rods. What size coil did you wrap around the middle of the stacked beads? What kind of output did you get from the beads? Did you use an amp after the output? BIG question. What do you think Boltek is using that is so small in there receiver?

For rule-of-thumb, the rod permeability is of the order length/diameter. Since this rarely exceeds 20 or so, then this controls the permeability, since the material permeability is of the order 200 or so for most ferrite rods used for antennas.

So just use your air-core formula, and multiply by l/d for an initial guess on the inductance.

Lots of variations from a few turns to hundreds. Some of the test data is presented in the files section of the ferriterodantennaexperimenters Yahoo group.

I just tested rod and coil combinations for Q with an antique Q meter.

Don't know, but I think they are over charging for it.

Back several years ago, Scientific American Amateur Scientist presented a design for just what you're looking for. IIRC they recommended strip laminations, quite a bit longer than the normal ferrite rod. They gave an address for a company that sells specially treated material with top grade magnetic characteristics. They gave full details of the windings and electronics, alrhough like a lot of their efforts at the latter, it's not too hard to improve on it.

Your city library should have back issues. It was one of the last amateur scientist articles, that may help you track it down.

Of course. I believe the rod was 1-meter-ish long, but don't recall the thickness.

It may have "only" been 50,000 turns, but not 1,000,000! Either way, a lot of wire, and quite a bit of stray capacitance, choking off the usable bandwidth.

The "low noise" amplifier for this pickup was something like an OP-07, inverting, with a 1k input and 100k feedback resistor. Oh well ! (I'll post the particulars when I get home to access it.)

Even so, its builders reported having all sorts of fun with it, which is what matters most, yes?

Best, James Arthur

Back several years ago, Scientific American Amateur Scientist presented a design for just what you're looking for. IIRC they recommended strip laminations, quite a bit longer than the normal ferrite rod. They gave an address for a company that sells specially treated material with top grade magnetic characteristics. They gave full details of the windings and electronics, alrhough like a lot of their efforts at the latter, it's not too hard to improve on it.

Your city library should have back issues. It was one of the last amateur scientist articles, that may help you track it down.