QRP Silent Antenna Tuning Unit (ATU)
compact antenna matching unit and SWR measurement system allows an
operator to match an antenna to a QRP transceiver "silently" i.e.
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.
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
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.
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.
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
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
matched. The transceiver's receiver is simply tuned to the desired
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
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
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
ATU Matching Network
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
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
The full schematic of the silent ATU is shown below.
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.
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.
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.
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.
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