c97. Notes to discuss

Low-frequency tuned circuits....
Radcom TT (G3VA) nr 12/2003 pp71-72

Experimental saturable-core inductor using a ferrite bead and two electromagnetic relays, developped by VP8AM
In january 2003 Michael Smallwood, VP8AM, reported experiments on tunable torids based on the structures illustrated
at the website of Applied Microwave&Wireless Magazine
VP8AM has recently sent further information on experiemntal saturable core inductors using a structure akin to that
proposed by G3JIR, but using a ferrite bead and two Omron G2R-2A 6VDC relays (see photograph). He writes
"The Omron relays are ideal as the pole pieces are easily exposed, and they are commonly available. New ones
can be purchased from Farnell, and RS Components sell the 5VDC and 12V coil types. The ones I use have burnt
out contacts and came out of Liebert PowerSure 400 UPS. Using a ferrite EMI-suppression bead for the coil is not
ideal, but they are good enough to experiment on.
Inductance at zero current was measured as 26.1ÁH with Thandar TC200 3.5digit LCR-meter.
Applying current through the relay colils changed the test inductor as follows:

Current Inductance Current Inductance
10mA (1.4V) 25.3ÁH 60mA (8.3V) 4.0ÁH
20mA (2.8v) 22.4ÁH 70mA (9.8V) 3.7ÁH
30mA (4.2V) 12.8ÁH 80mA (11.3V) 3.6ÁH
40mA (5.6V) 6.0ÁH 90mA (12.9V) 3.5ÁH
50mA (6.9V) 4.5ÁH 100mA (14.5V) 3.5ÁH

Fig.6 How G3IJR (G3JIR?) used an electromagnetic relay to vary the permeability of an FT50-10 toroid for MF and HF
applications (TT 11/2003)

An alternative is the turns-cancelation tuning method described in ART6 (G3VA), pg 148-149.
I tried it in a 70MHz octave sweep-oscillator and it was later mentioned in TT

The original 0.48 ..... 1.13MHz sweep-oscillator designed by Ken Johnson (Wireless World, February 1965 -
then, still called wobbulator).
L1 = 50T on 3/4" dia former,
L2 = 45T.

Fig.5. High-voltage voltage varied capacitor using bank of 12 mosfets (a) capacitor,( b) Variation of capacitance with bias voltage.
(Variable capacitor using 2x APT10059 high voltage mosfets)