HG3 plus Controller

HG3 plus Controller

HG3 plus Controller. It is completely redesigned. It controls both the HG3 PRO and HG3 QRO MLA models and the AR1 Rotator. It remotely tunes 7-30 MHz with stepper motor precision and resolution. RapidTune™ automatically scans each band for the lowest SWR and works with most HF radios.

HG3 plus Controller - qro magnetic loop

HG3 QRO – Higher Power and Performance

No Compromises

Retaining all the great features of our HG3 PRO model, the new HG3 QRO high power (1.5 KW) model raises the bar again in magnetic loop antenna (MLA) performance.

It covers 80*-10 meters. Adding the optional second radiator loop (two turns), allows full power operation on 80 meters.

Unrivaled Tuning Capability

Shown at left is the high Q vacuum capacitor with a 45,000-step resolution stepper motor. This delivers an unprecedented 511 Hz tuning resolution and allows the operator to set his/ her band preferences. This is very helpful when making QSOs under non-ideal and crowded band conditions.

HG3 plus Controller
PreciseRF Adds New QRO Model to Its HG-3 Series

A new QRO model, which is capable of handling 1,500 watts of PEP, has been added to preciseRF’s HG-3 line of stepper-tuned mag-loop antennas.

Covering the 80- to 10-meter bands, the HG-3 QRO has also added a new control module to support the finely tuned 45,000-position stepper motor that is accurate and repeatable.

The QRO model joins the other antennas in the HG-3 series, including an Express model (or Standard) that supports a 2,000-position stepper motor, manual tuning, and external resonator; a Pro option that requires a USB key, has a 2,000-position stepper motor, manual tuning, external resonator, auto-tuning, antenna rotation, and includes integrated SWR bridge and ERP functions; and a Lab option, which includes the Pro version plus an 8,000-position stepper motor and comes as a kit.

Retaining all the features of the HG3 PRO model, adding the optional second radiator loop (two turns) allows full power operation on 80 meters. The QRO model also features the AR1 rotator that has a high Q vacuum capacitor with a 45,000-step resolution stepper motor.

Its output shaft is connected to the vacuum capacitor via a 5:1 planetary gear. This provides increased torque, very low backlash, and excellent resolution. This delivers 511 Hz of tuning resolution and allows the operator to set his / her band preferences, which is ideal under crowded band conditions.

The new controller is completely redesigned. It controls both the HG3 PRO and HG3 QRO MLA models and the AR1 rotator. It remotely tunes 7-30 MHz with stepper motor precision and resolution. RapidTune™ automatically scans each band for the lowest SWR and works with most HF radios.

Remote loop tuning, band selection, and the usual setup procedure are accomplished via the front panel controls. The LCD display provides the current tuning and performance status. Also making up the front panel are a tuning knob and an LED indicator for SWR.

On the back of the controller are connections for 12-14-volt DC power input, the CAT6 controller cable to the loop tuner, USB, CAT6 cable connection to control the AR1 Rotator, DB9 controller cable to the loop tuner for the QRO model, RF output for the antenna or to drive an external linear amplifier, and an RF input from the transceiver.

Making up the loops are 120 inches of MLR-600 cable for the radiation loop and a 26-inch copper tube for the induction loop and provides 113 square inches of conductor surface area.

Holding up the loops are an optional aluminum or PVC mast, with the PVC mast comprised of three sections and is intended for portable uses. PreciseRF recommends that masts be kept to 6-10 feet height, as there no gain in performance with greater heights.

HG3 plus Controller CONNECTIONS

The HG3 plus controller is compatible with the EXPRESS, PRO and QRO remotely tuned MLAs. Not all connections may be used. Refer to the figure:

HG3 plus Controller - qro magnetic loop

12-14 VDC inputPower input. Ensure it has adequate voltage and current for the model in use.
TUNERCAT6 controller cable to the loop tuner for the EXPRESS and PRO model.
USBFactory used only for programming.
ROTATORCAT6 cable to control the AR1 Rotator.
QRO tunerDB9 controller cable to the loop tuner for the QRO model.
RF outputRF output for antenna or to drive an external linear amplifier.
RF inputRF input from the transceiver.


Remote loop tuning, band selection and the usual setup procedure are accomplished via the front panel controls. The LCD provide the current tuning and performance status at a glance. See the figure below:

HG3 plus Controller - qro magnetic loop

Power switchThe power switch turns the controller on/off. Most operators use their shack’s 13.5V power supply. For the EXPRESS and PRO models, a common 9 VDC 200 mA will work well.

The QRO model requires more power because of the vacuum capacitor. It requires 12.5 to 14.5 volts at a maximum current of 1,500 mA when tuning and 200mA when at idle.

Function keysThe F1 to F4 keys are soft keys which change functions based on the required operation.
Tuning knobThe encoder knob is used to adjust the tuning for best SWR. The mechanical detents do not necessarily represent the encoded pulses.

Pushing the knob in toggles it between normal and fine sensitivity. The FINE LED indicates its status.

SWRThe ten segment, green, yellow, and red segment bar
bar graphgraph is not calibrated. It provides only a relative indication of the SWR during tuning when used with power levels from 1 to 5 Watts. Any level in the green segment range is acceptable.


User Manual

The HG3 Stepper Mag Loop Antenna (MLA) is made up of the tuner, rotator and controller. The tuner and its components are housed in an enclosure attached to the antenna mast. The components are the tuning capacitor, the stepper motor driver and the stepper motor, which turns the tuning capacitor.

An DB-9 cable connects it to the controller. The AR1 Rotator is in a PVC housing. It contains the pulse width-controlled motor, limit switches and rotational logic. The antenna is supported by a polyformaldehyde (thermoplastic) thrust bearing.

A CAT6 /DB-9 cable connects it to the controller. The controller provides the necessary user interface and control voltages for the driver and the pulse-width signal for the antenna rotator. The controller firmware is written in C++ and provides the necessary functionality and logic for MLA operation.


Refer to the “HG-3 DIGITAL CONTROLLER” (two pages) and “HD-3 Front Panel” schematics. U2 provides +9VDC for the stepper motors. U3 provides +5VDC for the logic and controller. U4 is an ATmega328P Nano V3 micro controller (controller).


S5 is a rotary encoder which sends rotation, fine and coarse commands to the controller. S1 and S2 serve dual purposes, sending F1 key, CCW and F2 key, and CW commands to the controller. D1 and D2 provide knob fine/course and motor status indication. LCD1 is a serial data 20×4 display.

It provides the various user messages and prompts. The controller’s digital output pins D8, D9, D10 and D11 serve as the control signal for the stepper motor driver. The driver is in the tuner enclosure. These control signals are comprised of square waves, phased according to the stepper’s motor driver logic. They can be either full steps or micro steps.

Q1 samples D8 and D11 phases. With a driver signal is present, Q1 turns on and subsequently, turns on LED D2, the motor busy signal. J5 is the CAT6 output for the tuner. Refer to the “HG-QRO tuner” schematic.

The tuner receives the controller signal via the DB-9 cable and applies it to L298N stepper motor driver. The outputs from this driver are routed to J1 and J2. They power the stepper motor with the required phased square-waves. The HG3 is compatible with NMEA17 1.6 deg 5:1 reduction stepper motor providing 45K steps of resolution. U1LP2950/TO is the 5V regulator driving the vacuum capacitor.

D1 -D8 provide protection for LM298 when high EMF is generated by the stepper motor. Optical sensors detect the index and failsafe positions. Q1 turns on and drives D9 to indicate when the Index position has been reached. Q2 turns on and drives D10 to indicate a fails-safe condition.

The bar graph is comprised of the LM3914, along with a 10-segment bar graph, provide a relative indication of SWR, based on the voltages from the U28B output. R5 sets the bar graph threshold. J2 is a USB jack. It sets the available options.

U1A and U1B comprise two noninverting operational amplifiers which serve as buffers and signal conditioners of the SWR bridge. Their outputs serve as control signals for the analog input, A1 and A2, of the controller. The bridge is optimized for low level SWR levels to allow for auto-tuning (1-10W maximum). At higher power levels U1A and U1B will saturate, and erroneous readings will be displayed.


HG3 plus ControllerThe optional antenna rotator AR1 circuit is implemented as follows: U5 is the pulse width control module. It powers the AR1 rotator. It is enabled by the controller D7 relay logic. Q4 turns on and activates relay K1. Pins 8 and 9 of U5 are the ground return current sources for the pulse width control module through R22.

This serves as a current sensor. Q3 and Q2 are configured as a differential amplifier. Normally, Q2 is on and Q3 is off. When U5 powers the rotator motor, the voltage across R22 increases. It, in turn, saturates Q3 and turns off Q2. Simultaneously, Q3 collector goes low and sends a control signal to A6 of the controller.

This indicates that the rotator is turning, sending a message “Turning” to the display. To detect whether either the rotator’s CCW or CW limit switches have been tripped, a quad 2-Input NAND gate with open-collector outputs is used (U6). U6A, U6B and U6D compare the state of Q2 and the CW and CCW switches.

When either the CCW or CW switches are depressed and Q2 is on (low), this logic indicates the limits have been reached. Subsequently, the inputs of pin 9 and 10 of U6C toggle high and its output goes low. This sends a rotor logic signal to pin A7 of the controller. As a result, the controller sends a message “Limit Reached Reverse Direction”. J6 is the CAT6 output for the rotator.


Refer to the “HG-3 DIGITAL CONTROLLER page 2” schematic. The optional SWR bridge employs the RF transformer-based topology and uses two RF transformers. SO-239 RF-IN is the RF in from the radio transmitter and SO-239 RF-OU output to the antenna.

The transformer’s primary, L1, senses the main line current between the input and the load. A second transformer, L2, senses the voltage on the main line relative to ground. The coupling coefficient is at a nominal -30dB level. Under ideal conditions, when the SWR (Standing Wave Ratio) is 1.0:1, the forward voltage is maximum across R1 D1 and the reflected voltage is zero across R2 D2 (pun not intended).

D1 and D2 rectify the RF voltage. C3, C1, C4 and C2 filter the resultant RF to a DC voltage proportional to the forward and reflected power. J2 provides the output for U28 and U28B for further conditioning.


The HD3 MLS uses an Arduino nano micro-controller. The firmware is written in C++ and can be updated by the end user or by the factory. Check preciseRF.com for more info. We cannot provide telephone product support to help end users upgrades their firmware.

Please contact preciseRF to make shipping arrangement prior to sending your unit to us. CAUTION! DAMAGE RESULTING FROM USER FIRMWARE UPDATE IS NOT COVERED BY THE WARRANTY.


• System Design

Roger Stenbock W1RMS

• Firmware:

Travis Cannon, Roger Stenbock W1RMS

• Industrial Design: Roger Stenbock W1RMS.

• Tuner Design:

Rob Kirkpatrick KI6HNA

• Rotator Design: Robert Kirkpatrick KI6HNA.

• Manufacturing:

Audrie Crane Model Shop and Fabrication: Harold Crane

• Word Smithing:

Florene Stenbock


The PreciseRF HG3 Stepper Mag Loop was created by retired Tektronix engineer, Roger M. Stenbock (W1RMS). He has a life-long passion for electronics. At Tektronix, he worked on a number of 7000 series oscilloscopes and was on the development team for the 7A22 differential amplifier.

He was a design engineer for the 2200 series oscilloscopes FG501, FG502, FG503 and FG504 function generators and PG 501 pulse generator. He holds four US Patents covering oscilloscope trigger circuits and on-line flight planning software. Besides his ham radio activities, he enjoys working in his electronic lab, motorcycling and glider flying.

Roger M. Stenbock W1RMS

MLA-30+ (plus) 0.5-30MHz Active Receive Antenna Low NoiseK-180WLA Active Loop Broadband Receiving Antenna
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