An Amplified FSL
By Bill Marsh
The inspiration for this aerial came from the FSL built by Gary DeBock and latterly our own Tony King.
Some years ago I inherited a large number of “Ferrite Slabs” which were kept in the “may be useful” storage bin. After reading Gary’s article I started to experiment with them by bundling them all together to make a core and placing them end to end, with mixed results. After talking to Tony at our Moeraki convention I decided to have another go.
I have been experimenting recently with coaxial loop antennas and had concluded that the sensitivity of the loop is proportional to its capture area. With this in mind I decided to look at building an FSL with my ferrite slabs.
Initially I thought it would be circular like Gary & Tony’s FSL’s. I started looking for something large & circular made of plastic etc. and ended up getting frustrated. Try finding a circular container with parallel sides and additionally just the right size. I happened to be in a kitchen-ware shop one day with my wife and spied a hexagonal shaped gift box and the mind went into overdrive. I had reservations about mounting flat ferrite slabs on a round surface as this could lead to breakage and mounting them on a flat surface was obviously more desirable.
I decided on building an 8 sided shape out of some ply wood that I had lying around as this was closest to circular and the easiest to construct.
The length of each side was determined by the width of my ferrite slabs 75x18x3.5 mm). As I had 56 slabs I decided to make each of the 8 segments, 6 slabs wide i.e. 108 mm. I was a little eager as I should have made it 7 slabs wide. I was possibly mindful of having some spare in case of breakage as they are very fragile. The width of the 8 segments was made wider than the slab length to offer mechanical protection. The slabs were attached to the wooden shape using double side tape. Plastic insulation tape was wound over the slabs for protection. The coil consisting of 12 turns is wound from small gauge litz wire salvaged from an old 465 kHz I.F. transformer winding. Tony has given me some larger gauge litz wire to try, but this will be done after suitable tests are carried out at remote DX site.
I decided to adapt my coaxial loop preamplifier design to work with this FSL. The preamp has two significant advantages in that it has very high input impedance and does not load the loop winding and secondly the output is designed for 50 ohms suitable for inputting into most receivers.
The loop is mechanically tuned at this stage using a single gang 10 – 400 pf tuning capacitor. The blue switch switches in an additional 250 pf capacitor for the low frequency end of the medium wave band. The loop tunes 500 – 1750 kHz. The red switch is for the 9 volt battery powering the preamp.
The preamp provides a stable 15 dB approximate gain over “Inductive coupling”.
Stage 2 will include the provision of remote tuning using tuning diodes, trials with larger gauge litz wire and mounting it on a “Lazy Susan” for rotating.