a simple 2.4GHz power meter? - Page 1
Forum section: electronics / general

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 28, 2010, 11:41:45 pm
content: While working on my new Asus RT-N12 wireless router (installing DD-WRT on it and modifying it), I remembered reading about the leads of a common signal diode being about the right length for a 2.4GHz antenna. So I took one (I think it is a 1N34A) and connected it to my oscilloscope, then held it near the antenna. To my surprise, the measured voltage was peaking at almost 2 volts! It was a series of pulses at about 2.2ms in length repeating at just under 10Hz, which seems about right for the SSID broadcasts.Perhaps that is the start of a simple 2.4GHz power meter? Calibration is going to be tricky, but relative measurements could be easily done.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2010, 11:59:00 pm
content: What's the bandwidth of the 'scope?If it's a typical 100MHz 'scope I'd be surprised 2.4GHz displays on it.I couldn't find any information on the speed of the 1N24A, the only thing I saw on the datasheet is it has a rectification efficiency of 55% at 40MHz.http://www.datasheetcatalog.org/datasheet/HoneyTechnology/mXturrw.pdfHave you tried a high speed Schottky diode?Another idea is using two diodes and a couple of capacitors to form a voltage doubler.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 29, 2010, 12:29:08 am
content: The scope is not measuring the 2.4GHz directly, it is measuring the rectified signal across the diode. Funny part is, when I told my friend Alayna Phillips (a RF engineer) about what I did, she thought I somehow had a scope that could measure 2.4GHz!The only Schottky diodes I have are power diodes and they are too capacitive to work.

author: tyblu
author_rank: Frequent Contributor
timestamp: December 29, 2010, 01:38:51 am
content: This is a diode detector:http://www.microwaves101.com/encyclopedia/detectors.cfm. And it is the basis of simple power meters, or SWR meter:http://pe2er.nl/wifiswr/. It is also used for envelope detection in crystal radios.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 29, 2010, 03:05:34 am
content: I did a little more experimentation. A 1N4148 also worked, but not as well. A RK44 did not work at all.Sticking to the 1N34A, I found that by cooling the RF output transistors by blowing cold air over them, the measured voltage increased from 2.09v to 2.68v! Assuming the measured DC voltage is proportional to RF RMS voltage, that is an increase of power by a factor of 1.6! I repeated that several times to make sure it wasn't just measurement error, and the results were the same: the output would increase after cooling and slowly dropped back down as the transistors warmed up.Next, I connected a bright red LED to the diode and I was just barely able to make it glow from the RF power. Finally, I connected the diode to a pair of headphones and made a simple crystal radio for the 2.4GHz band. 802.11 sounds like a helicopter or machine gun from the periodic SSID broadcasts, with bursts of buzzing from data packets.

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2010, 08:47:04 am
content: You could the LED to make a voltage doubler which should make the LED light at lower voltages making it brighter. The trouble with this idea is the LED might not be very good at 2.4GHz, so you could try putting another diode in series with it. You'll also want to reduce the vale of C1 to something more suitable, try 10pF.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 29, 2010, 12:27:44 pm
content: you'll probably find a white LED is more effective as these will produce visible glow at much lower currents, despite the higher forward voltage.

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2010, 02:41:15 pm
content: I've experimented with this too.Yes you're right that white will glow at lower currents than red but as you say it has a higher forward voltage so won't detect weaker signals. In my experience, I've found high brightness green LEDs to be the most sensitive. I built the circuit shown above and was able to light the LED by holding one lead and connecting the other end to an earthed piece of metal such as a radiator. I know the power was coming from the main hum because it didn't work outside at the end of the garden, away from power lines.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 29, 2010, 10:34:13 pm
content: If anyone was wondering what mods I did to the RT-N12, here's what I have done so far:http://www.dd-wrt.com/phpBB2/viewtopic.php?t=85004

author: williefleete
author_rank: Regular Contributor
timestamp: December 31, 2010, 07:25:07 pm
content: maybe adding an RF tank circuit would make it a little more selectable to 2.4Ghz, instead of acting like a broadband EMF rectifieralso the 2.4Ghz is the modulation frequency what the scope will be measuring is the data/signal the 2.4Ghz is modulated with. same deal with a crystal set, the speaker is just reproducing the sound modulated in the HF

author: Time
author_rank: Frequent Contributor
timestamp: January 04, 2011, 05:09:02 pm
content: Quote from: NiHaoMike on December 29, 2010, 10:34:13 pmYou're famous!http://hackaday.com/2010/12/30/adding-a-serial-port-through-an-rj45-connector/

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 04, 2011, 07:17:22 pm
content: Quote from: Time on January 04, 2011, 05:09:02 pmI know. If you're interested, I made a Wiki page on the DD-WRT website for it.http://www.dd-wrt.com/wiki/index.php/Serial_port_on_unused_Ethernet_pins