I was playing around with magnetic loop antennas for QRP and it was bothersome having to tune very carefully the loop after a change of frequency of only a few kHz. So the idea was born to use a simple microprocessor board (off-the-shelf Arduino), a stepper motor and associated drive electronics to “slave” the tuning of the loop to the frequency that the authors FT817 was tuned to.
The Small Transmitting Loop (STL) is only 1 metre in diameter yet it has performance comparable to a full size dipole. The video shows well how it works. There is a brief period of autotuning on all bands where the system calibrates itself by transmitting a short 200mW signal on all bands whilst calculating the best tuning points. Then, even on receive only, the tuner will follow the FT817 frequency as reported from the CAT port and interpolate values for the capacitor position. When the rig is used transmit the ATU will fine tune and learn the precise tuning for that frequency
The main loop material is RG213 Coaxial Cable, around 1 metre in diameter. The feed loop is RG58 soldered according to the picture shown.
Other feed arrangements were tried. A toroidal ferrite core transformer for example with 8 turns on the primary and the loop passing through the core once acting as the secondary.
This worked well on one band but requires different numbers of turns depending on the band so was less practical.
The diameter of the small feed loop approximates to 1/5 that of the main loop and does not seem critical. Positioning does impact on the matching but it is not overly sensitive to position and is easy to adjust. In fact, it really was surprising how easy.
Tuning capacitor is a 500+500pF from an old valve receiver, obtained for £2 on the second hand market. It tested ok up to 700 volts which makes it ideal for 5-10w QRP transmitters.
The capacitor is wired with both sections in series making it 250pF in total. This is done to avoid having a moving contact in series with the connection – resistive losses caused even by only 5milliohm will be significant with this antenna.
Similarly, attempts were made to reduce the resistance by making connection to the SO239s with a tag on each of the four mounting screws – all in parallel. To crimp or to solder? This age old debate was resolved by crimping one connection, soldering another, and both crimping/soldering the final two. Can’t be wrong!
The motor is a stepper type, two coils (bipolar) with a maximum of 300mA for each coil. Bit of a sledgehammer to crack a nut, but it sure works fast and easily. A small toothed transmission belt connects it to the shaft of the capacitor with a reduction of 72 teeth to 18 teeth.
A micro switch (on the right hand side in the photo) is operated by a machine screw lodged in the drive wheel. This indicates to the controller the zero position. Remember that a stepper motor does not have its own means of positional feedback – the microprocessor must keep count of the steps. Incidentally, having a more powerful motor than is strictly needed does help to reduce the risk of missed steps.
An 8P8C connector is used to interface the motor box to the controller. This will at a future date facilitate using the same controller with different antennas. Handy for when the QRO version is completed !
The control box has a large red button used to initiate either a simple retune, or with a long press, a full tune of all bands to set up the memories to be able to do full autotuning on receive only at a later time.
Therer is a control (a rotary encoder) used to manually adjust the tuning capacitor. The unit can be operated in an entirely manual mode without the CAT interface or it can just be used to tweak the setting a little if desired.
The four toggle switches are for On/Off, Fast/Slow of the tuning knob, Auto/Manual tuning mode, and the final switch is not yet utilised. An LED indicates when the motor is being driven (the motor can be heard usually, but if a long cable is used this might not be the case.)
Connections to the control box are 12V power, a 3.5mm stereo jack socket for the CAT serial connection and an 8P8C connector for the motor box. Use of this connector makes it possible to use off-the-shelf Ethernet networking cables as the control cable. Its possible to purchase these for less than the cost of making one yourself.
An LCD panel is used which is handy to show the SWR/ state of tune and the motor position. Since we have the CAT data available to us for the purpose of tuning the antenna, we might as well make use of it to display other rig data, so once tuning is finished, we can see the real time readout of frequency and S-meter.
Inside the control box is an Arduino Uno. Almost all of the IO pins are used for this project. The stepper motor driver was originally a SN754410 H Bridge IC for only £1.00 but it was felt that a more sophisticated PWM driver chip might achieve better results with accuracy being a key concern when the power to the motors coils is closed down. So an allegro A4988 was introduced late in the build. This is a very nice chip but is surface mount only so a populated carrier board with all the required pin-outs was purchased for around £9.00
On this point, its worth mentioning that the Allegro was set to work using microstepping at x16 but this did not prove accurate enough for the mode of operation of this unit. Typically we just want to nudge the motor a single step at a time and it was difficult to do this without it moving backwards or forwards an unwanted amount before making the step. Eventually half-stepping was settled on and provide sufficient resolution (just!) and good accuracy. Power to the coils is removed ten seconds after the most recent step operation was requested.
The Arduino requires 5v so a ready built DC-DC converter was installed in the box and being a fully regulated supply, it was connected directly to the Arduino 5V pin. In software start up I introduced a delay from switch on to when the Arduino starts talking to LCD, switches, serial port, motor etc. Without this delay there were random start up problems.
Performance
Test were carried out with 5w to the Magnetic Loop and 5w to the main station antenna, an 88ft doublet at 35 feet. Calling CQ using alternate antennas then checking the received signal strengths on the Reverse Beacon network indicated performance comparable with the dipole.
| Band | Magnetic Loop comparison against doublet. |
| 40 | -12db |
| 30 | -6db |
| 20 | more or less the same |
| 17 | more or less the same |
| 15 | the same or better ! |
A sched was set up on 40m with an amateur friend who is around 10km away. He reported quite clearly 589 on the doublet and 569 on the mag loop. This is more or less in line with expectations.
Summary
For an antenna that is one metre across, standing 1.5metres above ground this is a formidable result. OJ, you’re going to be 2 s-points down on a doublet/dipole on 40m but if you’re operating from a hotel balcony,, portable in the countryside or just at home where you don’t have any normal antennas allowed, it is not just a compromise antenna, it is a real antenna.