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

Kevin Schanilec

Hi all:

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.

With both of these "short ferrite" designs, in all my testing I have found that shorter ferrites are the performance equivalent of the longer/bigger ferrites when compared with my 12" FSL with 160 mm (6.5 inch) ferrites, *WITH* a couple provisos. The first is that the shorter ferrites obviously will only accommodate so many turns of wire in the coil, perhaps 30 or so turns depending on the wire diameter: not an issue with MW, but potentially so on LW.

The other proviso is that the shorter ferrites do not put out as strong a magnetic field with which a radio can passively couple. With a reasonably sensitive Ultralight or other portable radio, the resulting SNR and audio are the same as with the FSL with much longer ferrites, although in many cases you may have to hold the radio somewhat closer to the Backpack FSL. On the other hand, if the radio is not very sensitive at the desired frequency, such as the stock Tecsun PL-380 on much of the LW band, then the longer ferrites' coupling field can make a noticeable difference. However, every radio I have does just fine on MW, and even a basic "stealth" modification to the PL-380 mostly corrects the LW shortcomings.

So, this may open up a couple new avenues for FSL design. The article referenced above has further design, construction and testing details.

Good DX to you - Kevin S
Bainbridge Island, WA

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