Quick Start Guide for the Wideband Power Meter

There are three main functions of the Wideband Power Meter:

  1. Directly measure the power output of your radio
  2. Analyze the radiated power from a nearby antenna
  3. Taking radio astronomy measurements with a satellite dish and commodity LNB receiver

In this guide, we will be covering the first two use cases.

Perhaps you have a QRP setup and want to test what settings get you maximum efficiency.  Maybe you want to test your maximum transmission power and ensure you’re within regulations.  Or maybe you are troubleshooting and need to know what you’re really outputting. To really know, you’ll need a power meter, and that’s why Enhanced Radio Devices made the Wideband Power Meter, designed to work well with the HamShield right out of the box.  Find the meter here. 

Connect to the meter by plugging into the micro-USB port and then using the terminal program of your choice – a table of good options is available here

Wideband Power Meter measuring output power from HamShield, using our optional Mini-Circuits attenuator, available for sale.

Using Wideband Power Meter, to measure nearby RF fields using an antenna. Attached is a Tri-Band Smiley Antenna. 



 

 Once you’re connected, the meter will be most accurate when it knows what frequency you are measuring.  To choose frequency, use the command “Fx”, where x is the desired frequency in MHz. The power meter has a calibration table programmed into its software.

You can find more commands for the Wideband Power Meter in the manual

What is a Power Measurement?

If you’ve seen decibels (dB) used in measurements, it’s worth knowing that they are actually an exponential ratio, a multiplier used to relate two measurements, and not a unit with a dimension of their own.  Decibels are in this way similar to scientific notation, but written differently:

 

Multiplier

Decibel-Milliwatts (dBm)

Watts

1

0

.001

2

~3

.002

4

~6

.004

8

~9

.008

10

10

.010

16

~12

.016

20

~13.0

.02

30

~14.8

.03

40

~16

.04

100

20

.1

1,000

30

1

1,000,000

60

1,000

.1

-10

.0001

 

When a decibel is used to relate a particular unit, it can be written with that unit.  For example, 1 watt could be written as 1W, or in milliwatts as 1000mW. However, it can more concisely be written as 30dBm or 30dBmW when we are measuring power output.  Since milliwatts are a linear measurement and dBm are an exponential measure, it is correct to refer to mW as “power” dBm as “power level”.  A convenient rule of thumb is that every increase of 3dBm is a doubling of wattage, and so a level increase of +12dBm would be multiplier of 24, or 16 times the power.

For more information on dB and dBm, follow this Wikipedia link.

What can the Wideband Power Meter Measure?

The Wideband Power Meter is designed to measure up to +20dBm (100mW) of input maximum.  Exceeding this power level may damage the meter. To measure higher power levels, an attenuator may be used.  A recommended attenuator for most radio devices can be found here.

 

The meter is designed to accurately measure power at frequencies from 10MHz all the way to 2.7GHz.  Measurement of frequencies below 10MHz suffers large increases in measurement error. To achieve the greatest accuracy, always set the measurement frequency on the meter through your terminal program.  The terminal command is “Fx”, where x is the frequency in units of MHz.

Measuring a Device – The HamShield

Start by connecting to your Wideband Power Meter as a serial port through your preferred terminal program.  If you are not familiar with using a terminal for this purpose, take a look at this guide here from SparkFun.

 

In the example below, the meter happened to show up as usbmodem29.  Make sure to connect at 9600 baud in “no line ending” mode.

After pressing enter to start the connection, our power meter gives some amount of
output from just background noise. This amount should be very small, since our
HamShield is not transmitting and we also have a 20dBm attenuator attached.

In this example, we sent the command F144 in the terminal because we plan to
transmit at 144MHz. Remember to set your power meter for the frequency you
specifically want to measure for maximum accuracy.

Wideband Power Meter protected from HamShield max output by attenuator on SMA jack

⚠️ The HamShield is capable of outputting enough power at high settings (up to
1W) to damage the Wideband Power Meter. To prevent this, we will be using a
20dB attenuator on the SMA cable between the meter and the HamShield.

⚠️ There is always a risk of causing electrical shorts when working with
exposed components. Example photos in this guide show the PCB of the
Wideband Power Meter and HamShield (with Arduino). For most use, it is safest
to enclose these parts to avoid accidental contact with conductive materials that
may damage your equipment or cause electrical shock.

We’ll use the convenient HandyTalkie sample code from the HamShield library. If
you’ve never set up code for transmitting and receiving on your HamShield, take a
quick look at our guide here: Building HandyTalkie

Of course, if you already have the transmitter you wish to measure set up, then just attach the meter to proceed.


To measure our HamShield’s output at different power settings, we will choose a
frequency with the radio.frequency function:

And we will choose our power setting 0 through 15 with the radio.setRfPower function:

 Pressing the Tx button on our HamShield now gives a new result in the terminal
window getting serial output from the power meter. At power setting 0, we get this:

If change the power setting to 12, we get this:

After testing a few different power settings, we want to find an average power level
at each power setting. Key up the HamShield and record ten or so readings from the meter while transmitting. Throw out any outlier from the very beginning of
transmitting that may have contained previous signal, and average the others
together. We added 20dBm to the measurement in our example because we are
using our 20dBm attenuator. The power meter has allowed us to create a graph
plotting the output of the HamShield:

Now, try the same above experiment, but this time, connect an antenna two both the HamShield and Wideband Power Meter. Command the HamShield to transmit and move the Wideband Power Meter's antenna around the other antenna. You will be able to not only measure the nearby field, but nulls of the antenna. Under a controlled setup, you could take readings at each point around a Yagi, and generate your own antenna plots!

Have fun experimenting!