Re: Working with shorter (and cheaper) ferrites - The Backpack FSL

Kevin Schanilec

Hi Michael:

Thanks for your post. I have thought about it some, and I think the problem is that the coil itself would be very small. An FSL with rings of ferrite would be easy to make (no ferrite winding form is needed), and IMO would be very cool to look at! However, based on the various FSLs I have assembled and torn apart, it is the diameter of the coil of wire, not the ferrite, that is primarily responsible for grabbing the RF energy, and increasing the coil size is the primary factor in a more sensitive FSL.

Another aspect: using rings would put the ferrites, and the gaps between them, parallel to the Litz wire, rather than the wire running perpendicular to the ferrites and gaps as with bars & rods. My sense, based on a limited understanding of electromagnetic theory, is that the latter is actually preferable, although I could be wrong. Anybody know more on this???

There are materials available to check all this out, though. I see that a 100 mm (4 inch) diameter ferrite sleeve, 54 mm (2 inches) long could be assembled from six 100 mm rings from eBay seller Combat Super Store, for $44 shipped. This would lead to a Litz coil of about 4.5 to 5 inches in diameter, depending on the coil/ferrite gap you choose. There would likely be enough room for a MW coil using moderately-thick Litz wire, especially with a large-value varicap such as the 684 pF model from zlowe7. Having built a few FSLs of this size with bars and rods, the performance doesn't match up with FSLs of 8 or 10 inches in diameter, so I wouldn't think that it would be worth the cost. However, I encourage someone with $44 burning a hole in their pocket to give it a shot :-)

Kevin S
Bainbridge Island, WA

--- In ultralightdx@..., "Michael" <michael.setaazul@...> wrote:

Expanding on this idea, has anyone tried using a row of
25mm or larger dia RF ferrite cores simulating a
thick hollow ferrite rod in a budget FSL?

Are diameter and length or is the mass of ferrite inside
the coil more determinant?

A smaller coil diameter might be in part offset by greater
length if more cores were used, pressed firmly together,
and/or by using a trangular array of three rows of cores,
thereby increasing the mass of ferrite.

Pre-empting experimental results, what performance
might theory suggest?

Ferrite cores are, I know not why, more affordable than
a recommended number of rods or bars. However, the
cost-performance ratio might not be better.

Michael UK

----- Original Message ------------------------------------------------------------------------

From:Kevin S
Subject: Working with shorter (and cheaper) ferrites - The Backpack FSL

I've been swamped at work lately, but have managed to do some experimenting with Ferrite Sleeve Loop
(FSL) antennas using shorter ferrites, seeing if smaller and cheaper FSLs are possible. Graham's
initial FSL article indicated that ferrite length in itself is not a critical parameter: it is the
coil of wire that receives the signal, not the ferrite. As long as there is sufficient ferrite in
the vicinity of the coil, providing the "FSL effect" if you will, the signal developed in the coil
should be the same. Some of Steve R.'s experiments earlier this year seemed to confirm this.

To test this, I recently built what I call the Backpack FSL, a summary of which is now posted here
in the Files Section, in the "Ferrite Sleeve Loop Antenna" folder (bottom of the main Files page).
This design uses 65 mm (about 2.5 inch) ex-Soviet NOS ferrite rods, which are of the same #400
ferrite material as most other ex-Soviet ferrite. I have also started to rough out a large FSL
using the 62 mm Soviet bars, sold by at least a couple eBay sellers, which run about three for a
dollar depending on the quantity.

. . .

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