Jan Jozef Oksiuta
Brisbane QG62lk
Australian Amateur Radio Station

DC to light, homebrewing, minimalist antennas and projects, QRSS, QRPp and less
WIA_logo FISTS_logo CWQRP_logo
Home Projects - Ferrite Sleeve Loop

FSL Antenna (Ferrite Sleeve Loop)

More detailed description will appear in the fullness of time. Briefly for now - an FSL is a cross between common ferrite loopstick as used in AM radios and an air-core (multiturn) receiving loop. In their original form they consist of a sleeve several inches in diameter made of ferrite rod, a coil wound on the ferrite sleeve and a tuning capacitor. The whole assembly is intended to be placed close to a broadcast receiver to ensure loose inductive coupling with the radio's internal ferrite loopstick antenna. It was found by other experimenters that an FSL used in this fashion, and tuned to the desired frequency, markedly boosts the reception of weak AM station. It is further claimed that a relatively small FSL would have an equivalent receiving gain as a much larger air-core loop with the same number of turns.

Here are some original links referring to this concept, and many more can be found on the web.


Inspired by the idea I decided to build a similar loop for use with a communications receiver - by adding a direct-coupled "masthead" preamplifier at the loop and connected to the shack via either a coax or a twisted-pair cable. Several loops have been built and they definitely do work well. How well - I cannot say just yet and lots of testing is still in progress.

Below is a brief specification of the largest of my loops, with some images thereafter.

Ferrite sleeve made with 120x ferrite sticks 10x180mm.
Outer diameter of ferrite sleeve: 375 mm
Winding diameter: 425mm
8x windings of five turns each in contra-quadrifilar combination
Wire: 100/44 Litz
All windings switchable in combinations from all-parallel to all-series and in-between
Inductances obtained: 39uH, 130 uH, 560 uH, 2.3 mH
Self-resonance at 2.3 mH: 491 kHz
motor-driven variable cap 270 pF
Additional fixed capacitances: Bank 1: 270, 470, 820, 1500 pF.   Bank 2: 2n7, 4n7, 8n2, 15n. All combinations available.
Built-in pre-amplifier at the coil: Hi-fi audio chip THAT1510.
Additionally, alternative broadband preamplifier can be switched in, using coil only in current amplifier (transimpedance) configuration using LT1028 op-amp.
Solar panel for local battery (no switchmodes).
SIgnal returned via Cat6 UTP
All switching with 16x DPDT latching relays
Simple TTL logic controls the lot through an auxilliary 8-core cable (no noisy microcontrollers).
Total tunable range 20kHz-3.2MHz, but poor performance noted above 2MHz (LF ferrites).

Winding at a glance, with a temporary capacitor/amplifier box
FSL - wndings at a glance

Details of the contra-quadrifilar winding method
Contra/quadrifilar winding detail

Finished winding undergoing initial testing
The completed winding

Relays, capacitor banks and the shack control box

Circuit boards 1

More relays and motorised variable capacitor
Circuit boards 2

Added 2014-08-25

back to top
back to Home


Created with CompoZer and Blue Griffon. Last updated 2014-08-25