How long the ferrite rod?

Hi,

"Seek and ye shall find", Mathew 7:7 Or read the two "Ferrite Rods Bars Plates" PDFs on this site.

Cheers - Joe P.S For maximum induced volts, use the entire rod.

Hi.

I'm wanting to exchange "air cored" frame aerials for ferrite rods, for the medium waveband (Broadcast band).

The ferrite rods are 10mm dia, and the MW coil for it is 35mm long, I think about 90 turns. Now, how long should the ferrire rods be do you think? The thing is, I've got some fairly long rods (200mm long) and I'm thinking should I cut the rods so they are say 100mm long or even 66.66mm long. The point is, does it matter that much how long the ferite rod is? Could I in fact use a 66.66mm rod and it would be no better or worse than a 100mm rod?

Also, how do you cut a ferrite rod? TIA.

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Actually I bet it's good to have no less than 4" (100mm), so if I do cut I'll not cut shorter. No idea whether there is much advantage to rod being longer.

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I suppose there is a formula somewhere showing output volts against length.

Thanks.

So, I beleive the formula I'm looking for is:

Loop Induced Voltage (Fhe) = 2?NAµeF __________ (in µV) ? Anyway as per:

And as you point out, for maximum induced volts use the entire rod.

If it's powdered iron, you can cut it with a diamond saw. If it's ferrite, you probably can't cut it, but there's a possibility you could score it with a triangular file and snap it, like glass tubing.

Good luck! Rich

But unless you need to shorten it for reasons other than reception quality, there is little reason to shorten it. The only reason radios don't come with foot long ferrite rods is cost and overall unit miniaturization.

Not really. Transformers are made as small and cheap as possible, but making them very much larger than necessary does not add much but weight. Making a rod antenna much larger (besides making it more costly) also provides lots more signal.

I am also pretty sure that gluing a large ferrite bead (like those around power cords for RFI suppression) on each end of the rod, beyond the winding, also gets you some extra signal. No need to have the bead hole filled with the rod. Just a bit of overlap works.

Yeah, I read the other responses saying pretty much the same thing about an ohnosecond or three after I posted. :-)

Thanks! Rich

Back when I was in the USAF, I stole a 4-400A by checking it out of bench stock, and watched a guy build a nominally kilowatt linear out of that and a bunch of surplus crap. This was in Okinawa (early 1970's), and you couldn't walk down the street without passing a couple of surplus dealers. He did it grounded-grid, and for his filament choke, he took about a half-dozen tuning slugs from various coils, transformers, etc - but these were big fat tuning slugs - about 3/4" diameter and about 1" long. He just glued them together. He had found a beautiful silver-plated coil for the pi-net output, and of course, it was covered with tarnish. I thought he should have just wiped a little Brasso on it, but this idiot SANDPAPERED it! AAAAIIIIEEEEeeeee! Later, I came to find out that silver tarnish not only doesn't hurt in that situation, but might actually be beneficial! But SANDPAPER! Well, it was only HF, and I'm sure that wasn't the only ineffeciency with the thing. But he did DX with it. I have to admit, it was kinda cool seeing the plate go bright orange until he got it tuned properly. :-) And it hissed. Somebody told me that that was the ka-dink effect. The electrons hit the plate so hard and fast, that they knock off secondary electrons, which when they hit the glass, go "ka-dink!" ;-)

Cheers! Rich