# How long the ferrite rod?

Discussion in 'Electronic Basics' started by Richard, May 15, 2005.

1. ### RichardGuest

Hi.

I'm wanting to exchange "air cored" frame aerials for ferrite rods, for the

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.

2. ### RichardGuest

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.

3. ### John PopelishGuest

The longer the rod, the bigger volume of space it couples into, and
the more energy it might intercept from the waves passing through that
volume. This generality applies to any length much shorter than 1/4
wavelength.

4. ### RichardGuest

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

6. ### RichardGuest

Thanks.

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

Loop Induced Voltage (Fhe) = 2Ï€NAÂµeF
__________ (in ÂµV)
Î»
Anyway as per:

http://www.cwsbytemark.com/cws_catalog/section2/2_31_rods_bars_plates2.pdf

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

8. ### Rich GriseGuest

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

10. ### John PopelishGuest

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.

11. ### John FieldsGuest

---
Except, perhaps, for the central void.
---

12. ### John PopelishGuest

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.

13. ### John PopelishGuest

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.

14. ### Rich GriseGuest

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

Thanks!
Rich

15. ### Rich GriseGuest

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