Building Hints for the Simple RF Power Meter
8March2015, 1April2015 w7zoi
A few years ago Bob Larkin and I described a simple power meter
in QST. See w7zoi and w7pua, "Simple RF-Power
Measurement," QST, June 2001, pp38-43. The
design is based upon a logarithmic detector integrated circuit
from Analog Devices. The IC has a very wide dynamic
range, high gain, and wide bandwidth, making it ideal for
numerous amateur radio
applications. The IC functions as
a wide bandwidth detector. An input RF voltage
results in a DC output. The important feature is a
DC output proportional to the logarithm of the input
voltage. The IC output changes by 25 mV
for every dB change in input. Log compliance
("linearity") is excellent, within a small fraction of 1 dB.
This instrument has a maximum input power of only 40 or 50
milliwatts. As such, it should not be used to
measure a transmitter output unless a suitable attenuator or
"tap" is included. This is discussed in the
QST paper. The instrument has a 50 Ohm input
impedance that dissipates the applied power. It is not
intended to be an in-line VSWR or power meter.
This is a measurement tool for the experimenter rather than an
accessory for a radio station. (Of course, the
experimenter with a radio station can find mutual applications.)
Recent correspondence with several experimenters has brought
a problem to the forefront. These builders
seem to have been constructing the power meter without benefit
of a clean ground plane. Rather, they have
tried to use a breadboard intended for low frequency
applications. The breadboard is, I suspect, a matrix
of holes with attached islands, allowing parts to be soldered to
the board and attached to other components. We used
breadboards of this sort for experiments in the industry.
These boards are suitable for low frequency (e.g., audio)
applications, but are not ideal for VHF/UHF. Even
when building a power meter for HF use, UHF methods are
recommended, for the AD8307 is capable of operation at
UHF. Sloppy construction may lead to parasitic
Shown below is a photo of the version we built for the QST
article and for Chapter 7 of EMRFD.
Take a look at the original QST article or the description in
EMRFD (page 7.7) to see the schematic. The important
construction detail here is the ground plane. This
is formed with a scrap of circuit board
material. All of the places in the schematic
where a ground connection is shown match with a foil solder
connection in the photo. There are a
couple of 1.5 Meg-ohm resistors on the breadboard that do not
appear in the schematic. These are mechanical
support elements. The high value resistor is
soldered to the ground foil. Other components are then
attached to the ungrounded end. This is the essence
of what we call Ugly Construction.
See KA7EXM and W7ZOI, "The Ugly Weekender," QST, August,
1981. The high resistance value guarantees
that there will be little interaction with circuit
operation. Exact resistance value does not matter so long
as it is high.
This is not a pretty circuit. Components are NOT
necessarily configured in a straight line and none are soldered
to pads. But this is the essence of Ugly.
The circuit still functions well because the grounding is
good. Signals flow in transmission-line-like
paths. Parasitic inductance related to
long component leads is small. Undesired coupling
between circuit elements is small when the impedance to ground
is very low for all grounded elements. (Keep leads
short.) If care is taken to use just enough "standoff"
resistors, the circuit will even be mechanically
robust. Remember that a pretty circuit is no
guarantee of functionality.
We are not saying that other construction methods won't
work. However, methods that ignore the coupling and
stray inductance issues may suffer. Surface mount
circuits, implemented with proper microstrip design, are ideal
for a power meter of this sort. See, for example,
the system described by KA7EXM, "A PIC-based HF/VHF Power
Meter", QEX, May 2005, page 3.
Here are two additional photos showing the input. A
standard BNC connector is used to mount the board flush with the
More recent thoughts:
One recent builder wanted to add a negative power
supply. The negative supply on the LM358 is within
the common-mode input range of the op-amp, so a negative supply
is NOT needed.
One other detail bears repetition: This power meter will have a residual
response. Disconnecting the
input cable to eliminate an input signal will not force
the meter to go to zero. This is not a flaw in
the instrument, but a simple fact. The power meter
is responding to the noise generated in the IC input
That noise is amplified by the many cascaded stages
within the AD8307 until it is detected. This
is no different than the noise we hear in a receiver. Even
if a receiver has the antenna disconnected, there will be some
output noise. Our version of the QST power meter has
a residual indicated power of -75 dBm.
Assuming a 500 MHz bandwidth, this response is consistent with a
12 dB noise figure. * Some builders have
tried to re-design the op-amp circuit to get rid of the residual
response, but we don't recommend this. (We don't do it
with our receivers.)
*The residual input power is kTBF where the terms
are linear rather than dB. k is Boltzman's constant,
T is temperature in Kelvin, B is bandwidth in Hz, and F is noise
factor which is the ratio corresponding to a noise figure in dB.
In logarithmic terms, kT is -174 dBm, B is 87 dB, and F
is 12 dB for the -75 dBm we observe.