Analog automatic tuner for a “Magnetic" loop antenna.
Containing only hardware, no software.
pa0nhc 20160321 / 20180204 / (20181210)
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COPYRIGHT. The use, copy and modification of all info on this site is only permitted for non-commercial purposes and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

This is an upgraded version of the PE0RIG design, published in Electron October 2014 page 430.


This professional PCBset is available. PCBcatalog

The here described version is one for use in a cabinet placed near the transceiver.
It is optimized for use between 3.5 and 7.2MHz.
If used on higher bands, i advise you to reduce the number of turns on L1, and adapt C17 and C201.
My automatic loop tuner was connected to the 10m circumference loop, using 19 m 50 Ohms coax cable, and a 5-wire motor drive cable.
A version connected directly to the antenna is possible, but not described here.

For best performance, the current sensor and phase detector PCBs are now separated.
The four NPN / PNP TO220 driver transistors are mounted onto the bottom of the die cast aluminum cabinet.
Thier ECB legs can be soldered directly, without wiring, onto the corresponding connections at the phase detector PCB.

Print the PDF files in size "100%" or "Original size".

Phase detector
drilling template
FRONT plate
drilling template
and parts locations
BACK plate
drilling template
and parts locations
PCB + power transistors
component placement
Assembling the
back panel
Assembling the
front panel

Schematic. 20180326 (p20180326-10)


It is important, that a LOOP ANTENNA shows SWR 1 : 1.0,
while it is exactly tuned to radiate maximal magnetic field strength.
This is not automatically true.
It depends on the way the 50 Ohms transmission line is matched to the feed point impedance of the loop.

If not:
when that loop is tuned to VSWR 1:1.0,
it will NOT have good  efficiency !

Low VSWR coďnciding with maximal field strength can be checked by using my :
magnetic field strength indicator

REM : The new matching system of my 40-80m TXloop is changed in 20181101.
It resulted in (VSWR 1:1.0 frequency) = (max. field strength frequency).
AND better performance.

            How it operates.
In the transmission line of a correctly tuned antenna, both RF current and RF voltage have the same frequency, and are IN phase (0deg. phase difference). 

The voltage on IC1 "CAR" is IN phase with the voltage on the transmission line.
Current sensor L1 transforms a part of the RF current in the transmission line, into an RF voltage
As the secondary winding of L1 is nearly UNloaded, the phase shift between the antenna current and L1 output voltage is nearly 90 degr.
The voltages on IC1 "CAR" and "SIG" will now have a phase difference of 90 deg. 
IC1 then generates on both outputs "OUT" and "OUT' " square wave HF currents, each with exactly 50% duty cycle. The existing fairly large HF components are filtered out by C15 and C16. After filtering, the effective DC voltages are exactly equal. There will be no voltage difference between both motor line outputs "M1" and "M2". The motor does not run.
Without RF signal on the antenna feed line, the motor also does not run.

When the antenna is not correctly tuned, the RF current and voltage on the transmission line are OUT of phase. The voltages on "CAR" and "SIG" now do NOT have exactly 90deg phase difference. The result is that the duty cycles of the square wave currents on IC1 "OUT" and "OUT' " changed in OPPOSITE directions. Consequently there is a DC voltage difference between "OUT" and "OUT' ", and "M1" and "M2". The tune motor now runs until it re-tuned the antenna, and the phase difference between both inputs is exactly 90deg. again.

            My improvements to the original PE0RIG design :
1.  The tuner is installed in a screening ALU (eurocard) cabinet.
     It contains a fused noise-free 12V 0.85A power supply.
     The tuner is switched on/off by connecting / removing power to / from the detector PCB (IC1)..
     240V~ remains always on, as the 12V supply also feeds the relay, and the anti-condensation heather, in the tuning box at the bottom of the loop antenna.

2.  IC1 is fed by a 9v stabilizer. The values of R5, 6 and 7 are accordingly adapted.

3.  R15 prevents static build-up on C4, should the antenna not be grounded for DC.

4.  C2 is enlarged for better decoupling. C3 and R9 also are adapted.

5.  C15 and C16 prevent that the (fairly strong) RF voltages reach IC2.

6.  Current sensor L1 and the current shunt have changed dimensions.
     L1 can be wound on a Amidon T50-6 (or T50-2 or FT50-61) core.

7.  During operation on the 3.5MHz band, Rel1 on the current sensor board now adds a small capacitor C201 in parallel to C4 / C17.
     This compensates for a small difference in set point of C4, when switching down from the 7MHz to the 3.5MHz band.
     REM: Rel1 changes position together with a band switching relay in the tuner box (see "Efficient "MAG" loop transmitting antenna") .

8.  ==>>  D3-6 limit the HF voltages on IC1 inputs to abt. 1400mVpp, to prevent overdriving IC1, and instability of IC1 "OUT" and "OUT' "  <<==

9.  C12, 13, 14 and R9 prevent an oscillation tendency of T1-4, due to the (hi-Q) self inductance of the motor. Without them T1-4 oscillated at HF and LF.

            Details of the current sensor.
The primary of L1 is a (1.7mm dia) straight wire (link), going once through the hole of the Amidon T50-2 or T50-6 iron powder iron ring core.

The secondary winding (0.5mm dia insulated wire), is going 10 times through the hole in the ring core.
The secondary winding direction determents the phase relation between IC1 "SIG' and "CAR".
If the functions of the "ANT" and "TX" connectors seem to be reversed, the phase of the voltage on L1 is 180 degr. wrong. 
Then change the labeling of the BNCs, or rewind L1 with the secondary in opposite direction.

For use on higher frequency bands, adapt L1, C17 and C201.

The primary impedance of the current transformer seems to be "inserted in series" with the impedance of 50 Ohms line, possibly influencing the coax line VSWR. The RF-SHUNT bypasses a part of the transmission line current, thereby lowering the influence of the current transformer circuit to the line impedance, and lowering the output voltage of L1. In the prototype, this shunt was a piece of wire 0.7mm dia and abt. 40mm long, shaped in a U form. The shunt runs around the core of L1 (not through the hole), and is situated at the "Back panel INside" of the PCB, and soldered to the center pins of the BNCs. See PCB silk screen.

My DC motor (transmission 3000 : 1) started and stopped running at 0.6Vdc motor supply voltage.
When the antenna is correctly tuned to VSWR 1 : 1.0, the motor supply voltage therefore could still be 0.6V, and the motor current could be 0.5 A. 
As soon as the antenna is tuned correctly, the tuner can be switched-off, by removing the 12V from the detector PCB, to prevent damage to the collector of the DC motor.

If the tuner is remained on while the antenna is correctly tuned, two of the driver transistors could dissipate continuously (2x) 3.5W. They are therefore cooled by screwing them onto the bottom of the ALU housing. The NPN transistors T2 and T3 are insulated from mass. 

REM: the current sensor PCB layout is changed and now contains a relay.
The wiring of the power transistors is omitted.
The twisted (black) I / Io current sensing wires between back panel and phase detector PCBs should not be twisted to keep stray capacitance as low as possible.
The on/off switch on the front does not switch the mains power anymore, but switches the +12V supply to the phase detector PCB.
The mains voltage is always connected to the transformer and the 12V supply to the antenna relay and anti-condensation heather is always present.

The current sensor is situated on a double sided PCB (left). The PCB is mounted onto one M3 neutral grounding screw, and soldered onto the pins of both BNC receptacles. Current transformer L1 is situated in-between the PCB and the ALU back panel. The screening ground plane at the PCB "Back panel INside" blocks radiation from the antenna circuits towards the rest of the circuits.


For optimal performance, the wire connected to "I" on the back panal PCB should have low stray capacitance.
Do NOT twist it and keep it free from surroundings.

All PCBs have printed wire connection IDs, corresponding with the schematic diagram.

            The back panel.
A 6 pole mini-DIN connector for motor power is installed in the ALU back plane of the tuner.

My motor cable is a cheap cat 5 cable. 
        Connections are:
Two motor wires at pins 3 and 4, orange and green. They are connected to the pahse detectot PCB {M1} and  {M2}
One band switch wire at pin5, brown. Connected to the band switch on the front panel.
One +12V anti condensation heather wire at pin6, blue. Connected to the +12V at the power supply elco.
Four "grounding" wires in parallel, white at pins 1 and 2, Connected to the -12V at the power supply elco.

RF which could appear onto the motor lines, is blocked inside the cabinet by a cheap small Fair-Rite MIX31 ferrite core.
The bundle thin wires is wound several times through the hole  of the core. 
An extra core is installed outside, with 3 turns CAT5 motor cable.
Both also prevent possible digital noise from the phase detector running towards the antenna.

            On the front of the cabinet are:
One on/off switch, switching the +12V power of the phase detector PCB.

One band switch with yellow LED, indicating 3.5MHz operation, switching (1) Rl1 on the current sensor board, and (2) the band switch relay in the matching box of the 40m / 80m loop antenna.

Two push buttons, each with a red motor LED, to force the motor to run fast in a direction of choice.

The two red motor LEDs indicate the voltage on both motor wires in respect to mass. 
-   If both are glowing equal, the tuner is active, but the motor stands still and the antenna is tuned. 
-   If one red motor LED darkens, the motor runs fast.
-   If both red motor LEDs are dark, the tuner is switched-off.

D1s and D2s function as a voltage threshold, making the difference in brightness of the red LEDs more obvious.

IMPORTANT : if in the command cable to the antenna a short circuit should appear, the power supply can (will) be overloaded.
A correct value FUSE must be installed to protect against fire.

The simple, but adequate noise free internal power supply can also supply the current for the band switch relay and the 4.5W anti-condensation heather in the antenna box.
The tuner is switched to "inactive" (not to "off") by interrupting the supply voltage to the phase detector PCB. 

240V~ and +12Vdc are therefore always present, in order to be able to supply +12Vdc to the 5W anti-condensation heather, and the band switch relay in the antenna tuning box.

A common mode choke blocks RF at the mains cable. A 100mA slow fuse protects the transformer against overload. Four 100n C's over the bridge rectifier connections prevent RF rattle and IMD. The flat-type transformer, the elco and the common mode choke are simply glued upside-down onto the bottom of the cabinet. I used Pattex 100% active glue ("water resistant"). The vent on the elco should be left free.

All other components are screwed onto the bottom.T1-4 and the bridge rectifier with a little cooling paste. The two NPN transistors T2 / 3 are insulated by a plastic washer and a Mica plate. The ECB legs of T1-4 can be soldered directly onto the corresponding connections at the phase detector PCB, without wiring.

            Adjustment / setup.
The RF voltages presented to inputs "SIG" and "CAR" of IC1
MUST be minimal 300mVpp and maximal 1400mVpp.
Only then stable operation of the tuner and adjustment of C4 is possible.

The output voltage of L1 must be adjusted ONCE on 3.5 MHz, by changing the length of the shunt wire parallel to the primary of L1. A longer shunt wire means more output and vice-versa. See the PCB silk screen.

1. On 7.1 MHz feed 5W into a dummy load. This should generate 800mVpp on input "I", measured with an RFoscilloscope.

2. Drill a 6.5mm trimming hole above C4 in the top of the cabinet, and close the cabinet.

3. On 7.1 MHZ, feeding 15W into a good dummy load, adjust trimmer C4 for less than +-10 mVdc DIFFERENCE between both motor wires "M1" and "M2".

4. Without re-adjusting trimmer C4, on 3.65 MHZ, feed 25W into a dummy load. The voltage difference between both motor wires should be less than +-10 mV. If this is more, adapt the value of C201 (nominal ca. 10pF).

5. On 7.1 MHz and 3.65 MHz, varying the RF power between 5W and 100W (or transmitting SSB), the voltage difference between "M1" and "M2" should  not be more than + or - 30mVdc. The tuning motor should not move.