Measuring RF power levels with any computer and a USB port!

Measuring RF power levels with any computer and a USB port!

Measuring RF power for QRP radios

The Q-Code "QRP" stands for "reduce power". It represents a growing part of the hobby where amateur radio operators try to use as little power as possible to communicate as far as possible. From $15 HF kits on eBay, HamShields, programmable crystal oscillators to unlicensed Part 15 HF operation, there is something at any price range for anybody.

But how can you accurately measure these small power levels? Sure, there is expensive test equipment available for this. And you might be able to look at a power meter intended for higher wattages and guess where the needle is pegged, but we came up with a better way. And you get a digital read out on just about any computer with a USB port.

Introducing the new Wideband Power Meter

After using a custom designed digital meter for testing our own QRP rig (HamShield), we have decided to take it to full production. We will be offering 100 meters for sale today. If demand is strong, we will continue to manufacture it in larger volumes. 

Above: The production version of Wideband Power Meter 

Using the board is quite simple: You just plug an RF source into the SMA connector and read the power output on your computer. The power levels are sent over USB serial to a terminal program, such as CoolTerm. A program running on the Atmel Mega32U4 collects data from Analog Devices' HMC713 power detector to make it all work. 

While it supports a maximum of 20 dBm (about 100mW), you can add an attenuator (say 20dB) and increase the maximum power you can detect. It would be pretty easy to calculate, since you just add 20 to the resulting dB value. 

We will also be stocking a limited amount of 20dB attenuators from Mini-Circuits, the VAT-30W2+, which can handle up to 2 watts of continuous power (even higher with short pulses). 

You can now test the power output from crystal oscillators, DDS IC's, exciters, and just about any other homebrew QRP transmitter you can come up with.  

Measurement in the HF range

While this power meter works great from 39-2700MHz, allowing highly accurate power measurement across 10 VHF/UHF/SHF amateur radio bands around the world (including several ISM bands, WiFi and Bluetooth), we also offer an "extended range", which offers calibrated HF measurements down to 10MHz.

What does the HF range look like uncalibrated? 

Our logarithmic amplifier is quite linear from 39-45MHz (outside of its advertised range) and then sharply drops off past 10MHz with more than 4dB of error.

We have placed software calibrations to counteract this error and flatten the response, but it gets pretty unreasonable below 10MHz!

While we actively work to find a solution for the folks operating from ELF to the 10MHz range, we do have a rather nifty power meter product that works across many HF bands and unlicensed HiFER bands. Be sure to check it out and tell us what you think!

You can purchase our Wideband Power Meter here.


2 comments

  • Casey

    Yes, you can! I would suggest you do some baseline with a known power level as that is a 75 ohm system. The power meter is 50 ohm, so the measured power will be a little lower than the actual power. I never thought of using this for CATV systems, but you could!

  • Jorge Audino

    Hi! Can I use this device to “tune up” a 35.75MHz 57dBmv output distribucion line, to setup taps accordingly to feed -3 dBmv sensitivity receivers? Thanks!

Leave a comment

Please note, comments must be approved before they are published