MAG - LOOP AERIAL (Remote Tune)

This is my example of a remote controlled "magnetic-loop" receiving antenna.
It is a convenient union of two of my earlier projects, namely:
Magnetic "LOOP" Aerial and Servo Test Jig
So now the loop aerial could be permanently mounted in an elevated position, say on a rotatable mast,
and also be remotely tuned!
I have used the hula-hoop (75cm diameter) as a means of supporting the coil turns needed for the "loop"
Using a discarded hula-hoop; I ground 2 small slots on the outer-facing surface, approx 10cm apart, each side of the 'double-bung' joiner.
I then fed two 4 pair cables (16 turns) through the hoop. (CAT 5 ethernet cable)
All I would then need to do, is to join the finish and starts of successive turns (in "series-aiding") to give me a 14 turn primary, and 2 turn secondary.
As all cores are colour coded and traced, this is relatively easy.
I have included a 12 position single pole rotary switch.
This achieves both the purpose of a tap-changing coil, and uses the switch connection as termination points for all of those wires!
Coil inductance for various tap switch positions was measured and tabulated using my LC meter : see schematic.
The combination of the selectable taps and the tuning capacitor, should allow the loop to be tuned over all of the MF and much of the HF spectral assignments i.e. 300 - 3000KHz, and 3 - 30MHz

The switch, 500 pF tuning capacitor and all wiring are included in a weatherproof, electrical junction box.
The J.B. is affixed to a length of metal tube,
and also a flat polycarbonate strip to which the (light-weight) hula-hoop is fastened with some cable ties.
The tuning capacitor and tap-changing rotary switch are each coupled to a RC servo motor.
These servos give about 200 deg of shaft rotation, and have 3 connections:
+4.5V, Gnd and a pulse width modulated control signal (see schematic).
The RC servo's are affixed to a light aluminium mounting plate.
The mounting plate sits atop a square cutout in an elevated platform through which the o/p shaft can protrude.
2 screws thru' slightly oversize fixing holes secure the plate upon the platform.
These screws are not fully tightened down.
This allows the servo-motor to "float" which accomodates any lateral displacement occuring
whilst the shaft is rotating.
Necessary because the whole deal might otherwise "jam-up"
The servo o/p shaft is coupled to the 1/4" shafts of the tuning cap and tap change switch,
with brass collared - bakelite coupling tubes (manuf' by Eddystone - found in my junkbox!)
See pictures for details.
Re, the tap changing rotary switch: I had to remove the detent ball from the clicker plate mechanism
to make it easier to turn.
The servo's, although quite "tourquey" still couldn't rotate the switch against such mechanical resistance.
A DPDT (centre off) toggle switch is used to enable either L or C adjustment,
and then switched to the centre off pos'n.
This should avoid any RFI from the control wiring (PWM square wave oscillator) into the receiving loop.
I economize by using a single 555 oscillator (which I'd already constructed)
and switching o/p and timing potentiometer with DPDT toggle.
Otherwise 2 identical ccts could have been used.
The loop-antenna is connected to the control box and receiver by a 4 core and coaxial cable respectivley.
Initially, during testing I found the servo's were "jittery"
This may have been caused by "cross-talk" between the PWM line and the power supply line in the control cable.
I cured this with (+5V DC supply) 10uF decoupling capacitors fitted right at the servo connections. (see diagram)
Also an on-board (+5V) 3 terminal voltage regulator with 4.7uF decoupling capacitors
- in the control box seemed to help.