ex SP2EBP, VK2EBP
Jan Jozef Oksiuta
|Australian Amateur Radio
DC to light, homebrewing,
minimalist antennas and projects, QRSS, QRPp and less
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
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
Built-in pre-amplifier at the coil: Hi-fi audio chip
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
Details of the
contra-quadrifilar winding method
undergoing initial testing
Relays, capacitor banks and the shack control box
More relays and
motorised variable capacitor
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|Created with CompoZer
and Blue Griffon. Last updated 2014-08-25