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Version:

October 30, 2014:
Revised: v1.0

QRP Silent Antenna Tuning Unit (ATU)

This compact antenna matching unit and SWR measurement system allows an operator to match an antenna to a QRP transceiver "silently" i.e. without transmitting.

Introduction

Several months ago, I spent some time building a Lydford 40m QRP SSB transceiver kit. You can read about that experience here.

Having built the kit and got the transceiver operating into a 50 ohm load, it was time to operate the transceiver into a real antenna. But I had no suitable antenna tuner. I couldn't buy one where I currently live in the Middle East, so it was back to the bench again to build something from available parts.

An antenna tuner or ATU allows the impedance of an antenna to be adjusted to suit the output impedance of a transceiver. Antenna impedances can vary from a few ohms or less up to several thousand ohms, and usually include a mix of reactive (capacitive or inductive) impedance as well as real (resistive) impedance. Modern transceivers tend to operate best into a 50 ohm resistive (non-reactive) load so the adjustable matching network inside an ATU is used to convert the reactive antenna into an impedance reasonably close to the ideal resistive 50 ohm load.

The ATU's matching network components must frequently handle high RF voltages and (sometimes) currents during matching. This demands the use of suitably rated components. With QRP transceivers, these requirements are reduced, and more commonly available parts are therefore able to handle the task. Such is the case in this design where the Lydford transceiver output is less than 10W.

This ATU also includes a simple RF power meter to allow the output power of the Lydford transmitter to be verified.

Silent Tuning

Almost all modern ATU designs fall into two categories; Manual or automatic tuning. Manual ATUs are tuned by hand by the operator, adjusted for the best possible match with the transmitter's output impedance of (usually) 50 ohms. The accuracy of the matching is usually measured using a VSWR (Voltage Standing Wave Ratio) meter, with the operator adjusting the ATU to achieve the lowest possible VSWR. A VSWR of less than 2:1 is satisfactory. A VSWR of 1.5:1 or 1.2:1 or less is regarded as excellent. Alternately, a VSWR of, say, 4:1 or 5:1 would be cause for concern, and might cause damage to the output stage in the transmitter.

An automatic ATU adjusts the ATU for the operator, usually with relays and a microprocessor. A similar VSWR measurement is carried out simultaneously by the microprocessor to determine the optimal matching adjustment. The approach used in these microprocessors to establish a match varies between manufacturers, but most appear to use a trial and error system, testing coarse variations of matching settings to find a near-match, and then fine tuning to give the best match. Some store the results for later use, speeding up subsequent tuning. More sophistocated designs measure antenna impendance, calculate the required matching setting and fine tune settings for an optimal result.

Both manual and automatic ATU tuning requires the transmitter to be operating, initially at low power and then at full output power for a final 'fine adjustment' of the antenna matching network in the ATU. This means that the operator's transmitter is operating for the time required to adjust the ATU, often up to 15 seconds or more. Since this can cause interference to other users, it is highly desirable to minimise this tuning time and/or the output power during the adjustment.

"Silent tuning" is a method used to minimise any transmission during the tuning process.
Ideally, transmissions should be avoided until the operator is actually ready to use the transceiver on the chosen frequency. Some "silent tuning" ATUs attempt to do this simply by minimising the tuning time. Others use an attenuator to temporarily reduce the transmitted power during tuning, often by up to 30dB. Howver, truly "silent tuning" ATUs avoid generating any output at all until the ATU is correctly configured at or near the optimal settings and the operator is ready to use the transmitter.

Military users are (obviously) one group very keen on this type of ATU. A typical modern mil-spec ATU can tune to the antenna in as little as 5mS. Some do this by using a prior tuning cycle which identifies all possible matching settings across the HF tuning range. Since this cycle can take up to 45 minutes and employs full power, it's hard to see how this could be considered as particularly "silent". In any case, such ATUs are often very expensive, typically around $US5,000 each.

Very few 'silent tuning' or 'quiet tuning' ATUs have been described for amateur radio use, either in magazines or on the Internet. Notable examples include the well-known PICatune, which uses the attenuator approach, and a similar design in QEX in 2002. Commercially available silent tuning ATU products for amateur radio are similarly few in number, but include the MFJ-929
ATU made by MFJ Enterprises in the US. In this case, the ATU's microprocessor measures the antenna impedance, calculates the required matching settings, and then sets the matching network accordingly.

Similar methods have been used in several manual tuned ATU designs over the past two or three decades. However, neither was suitable for me. The component count and complexity of an impedance measuring system put me off that approach while the switched attenuator approach was not absolutely "silent" in my mind.

Noise Bridges

In my ATU design, I have used a much simpler 'noise bridge' approach. This is a legacy device which was commonly used many years ago to measure antenna impedance. It uses a simple four-port bridge, a basic schematic for which is shown below.

RF noise bridge basic functional schematic

An RF signal drives one port of the bridge formed by transformer T1. In the case of the noise bridge, this RF signal is a wideband RF noise generator. The antenna or impedance to be measured is connected to a second port (labelled "Input" on this diagram), and a parallel or series RC combination
(VR1/VC1) is connected to the third port of the bridge. These are adjusted to minimise the RF signal measured in the detector. In the noise bridge, the detector was usually an HF receiver. When the noise or signal was nulled, the unknown impedance of the antenna could then be directly read from the R-C dials.

Note: C2 is approximately half the value of the maximum value of VC1. This allows the bridge to measure the inductive reactive component of an unknown impedance.

An excellent example of such a noise bridge can be found in the February 1977 issue of 'Ham Radio' magazine. It's long since ceased publication, but this article can be found on a variety of Internet sites.

A variation of this basic noise bridge is used in this ATU. In this case, the variable RC combination is replaced by a 50 ohm resistor (Actually, a pair of 100 ohm resistors in parallel to give the right value) since this is the desired output impedance from the ATU when it is matched. The transceiver's receiver is simply tuned to the desired operating frequency. The wideband noise generated by the noise bridge is used as usual as the RF source, and the ATU adjusted to give a null in the noise heard in the receiver. This is the optimal ATU setting where the impedance seen by the noise bridge - the antenna's impedance as modified by the L/C matching components in the ATU - equals that of the 50 ohm resistor.

Since the transmitter is not used at all, this approach results in 100% 'silent tuning'.

Of course, this silent tuning method has some drawbacks. First, the user has to listen for a reduction in noise from the receiver while adjusting the ATU in order to detect matching. While the noise drop when matched is often quite clear, sometimes the noise dip with some antennas can be a bit more difficult to hear. The other potential issue is that the noise bridge requires a battery. Manually tuned ATUs usually have the advantage of requiring no additional power supply, but this ATU obviously needs a battery for the noise bridge. A standard 9V battery is used, and it's built into the ATU. It's quite light, and should last for at least two years of normal daily operation.

ATU Matching Network

The matching network used in this ATU design is a basic L-match. With the component values suggested, it allows an extensive range of antenna impedances to be matched to 50 ohms. The basic L-match has been used for many years, although it has become more popular in recent years for automatic ATUs and for QRP use. 

I did some extensive analysis of antenna tuner designs before settling on the L-match. This showed that a  pi-match or T-match system would be better, but both require three adjustable components. I prefer to use just two, if only to save space. While the matching range was reduced as a result, this tuner design has matched any antenna I have tried to date.

A standard 180pF polyvaricon variable capacitor is used for the adjustable capacitor in the network. DPDT switches are used to select a combination of fixed toroid inductors for the network's variable inductor. Inductor values have been chosen to provide up to 15.5uH. This approach is both simple and robust.

A DPDT switch (SW3) is used to connect the matching network's variable capacitor to either the input or output of the switched series inductor network. With the capacitor on the antenna side of the tuner, high impedance loads are able to be matched. When the capacitor is switched to the transceiver side of the inductor, low impedances can be matched. Most antenna loads presenting a VSWR of 10:1 or less can be matched with this design from 3 - 30MHz.

On the transceiver-facing side of the ATU, there is a small circuit comprising Q1, zener diode D2 and associated passive components. Q1 amplifies the wideband RF noise generated by the zener diode and feeds this RF energy into the bridge transformer (T1). Typical legacy noise bridges used a similar approach, but used two or three additional amplifier stages. Typical QRP transceivers are quite sensitive on receive, and I found that a single noise generator amplifier stage was more than adequate for successful silent tuning.

If you find you have too little noise with the single amplifier stage, try another zener diode, or one with a slightly different voltage. The noise generating ability of zeners does vary.

R7 (47 ohms) is the reference resistor for the noise bridge. It is not exactly 50 ohms, but it's the nearest standard value resistor and certainly close enough for this application.

Switch SW2 allows the noise bridge to be inserted in circuit to tune the ATU. It is a 3 pole changeover switch to also allow the 9V supply to power the noise bridge. Once ATU adjustment is complete, SW2 is selected to "Operate" to remove the noise bridge from circuit prior to operating the transceiver. You MUST NOT operate the transmitter while the noise bridge is in circuit (i.e. SW2 in the "Tune/Rx" position). The transmitter output power will likely destroy Q1 and associated parts quite quickly. Not a good thing.

RF Power Meter

A simple RF power meter and low power 50 ohm dummy load was also included in the ATU, but it can be omitted for those not requiring this feature. It uses a basic diode detector (D1) and a low cost meter. I set it for a FSD of 6W in my ATU, and it provides a simple method to measure a QRP transmitter's output power in the field.

The 50 ohm dummy load is made from six resistors in parallel; Three 270 ohm and three 330 ohm 0.5W resistors. This combination of parts provided both a good 50 ohm match and power handling capability suitable for short term use with a 10W QRP SSB or CW transceiver. Sustained RF power measurements over a longer time, say for alignment of a 10W transmitter, will require the use of 2W rated resistors for R1a-R1f.

The full schematic of the silent ATU is shown below.
 
QRP silent ATU schematic


Construction

The ATU was built from scraps of single-sided PCB. A cover was then made from thin tin-plate although light gauge aluminium would be equally suitable. The 9V battery fits inside the ATU, and the overall unit is quite compact. In fact, it's possibly a little light, especially when coax and antenna cables are connected so you may prefer to make it slightly larger, and completely from heavier gauge aluminium or painted steel.

The inside view of my ATU before the front panel artwork was added is shown below.

top side view
The RF power meter is designed to measure anywhere from 2W to 10W at full scale. This setting is adjusted using the preset resistor (VR1) which can be seen in the lower left corner of the picture below.

The front panel artwork was drawn up on my PC and printed on a colour laser printer. It is then covered by self-adhesive transparent plastic film which I bought on a roll from a stationary store. I cut out the holes in the artwork with a very sharp craft knife. This takes a little time to do but I think it gives a nice finish to the project.


Operation

Connect the antenna and transceiver to the ATU. Select the dummy load/RF power meter setting on SW1 and briefly check that the transceiver's transmitter is operating correctly.

Tune the transceiver to the desired operating frequency, and turn on the RF noise bridge using SW2.

Adjust the switches and variable capacitor for the lowest noise level in the transceiver's receiver.

Turn SW2 off to switch the RF noise bridge out of circuit.

Now operate the transceiver.

A VSWR meter can be connected between the ATU and the transceiver to verify that a good match has been achieved with the ATU. After a while, you will find you can dispense with using the VSWR meter as your confidence with tuning and using the ATU grows.



The front panel layout of my ATU is shown below.



Conclusion

I hope you enjoy using this ATU as much as I do. I have found it really easy to use, and once I discovered the switch setting for my antenna on each band, I find it really quick and easy to go straight back to the settings neach time. I can do a quick check with the noise bridge and then begin operating.

I really like the idea of keeping unwanted RF energy off the ham band until I'm all tuned up and ready to go, and I hope you enjoy using this silent tuning ATU too.

Enjoy!
   



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