Building Your Own RF Field Strength Meter
A simple and easy project for a rainy Sunday!
Why would you need one? Sometimes we build electronic projects for fun only, or just for satisfying our technical curiosity; this is what I do sometimes. This project, however, started with a need.
The need originated when a friend called for help to solve an intermittent problem with the UHF transceiver on his boat. The receiving ends reported frequent cuts in his transmission.
I quickly improvised a very simple Field Strength Meter composed of a small antenna, a detector diode, a resistor, capacitor and a Micro Ammeter. Assuming that this simple circuit would be sensitive enough to detect the HF field, providing we keep it close enough to the transmitting antenna. This worked quite well, and the intermittent problem was easily diagnosed and fixed; it was caused by a broken antenna cable, making intermittent contacts. With the meter we were able to see a clear drop of the field strength when moving the cable.
From there, the technical curiosity took over; what if we could improve the circuit, making it more sensitive and, how high in frequency it would still work? Would it work for much higher frequencies such as 2.4 GHz, to check RF fields from Wi-Fi routers and mobile phones?
I gave up (at least temporarily!) to the idea of building a wide range RF amplifier to amplify the antenna signal over that wide range of frequencies. This might be another project for another rainy Sunday (or many more rainy Sundays!). Instead, I preferred trying to amplifying the DC signal after the RF detection and see how far we can go.
And here we go; we have a very simple instrument that can detect the activity of your Wi-Fi router, your mobile phone and about any RF transmitter through a wide range of frequencies.
Fig. 1 Schematic diagram
It is powered by a 9 V battery. The diode is a critical component; it should have a low threshold voltage. The diode used in this circuit is a germanium diode 1N60. A ghost from the past, but it is still available on the market (Escol for example).
Fig 2. Inside the box. Plenty of space for further developments!
The diode and the 1 nF capacitor are soldered directly at the base of the antenna connector. In the next version of this circuit I would put the 100 K resistor there as well.
Another critical point is the antenna. It should be cut to ¼ wavelengths for the frequency range you want to measure. In this project the antenna was cut to around 3cm length (1/4 wavelength at 2.4 GHz). I soldered a 50 Ohm resistor at the base of the antenna for matching the impedance (Well… a 47 Ohm was close enough!).
Fig 3. The antenna for 2.4 GHz
This simple RF Field Strength Meter proved to be much more effective than expected. It will react to your Wi-Fi router and mobile phone transmitting activities. You can also check the leaks of your microwave hoven, but be careful, put the sensitivity button to a very low position as the signal proved to be very strong! I guess this is normal…
Fig 4. Reacting to the mobile phone transmission.
Note that the sensitivity button is at a quite low position, showing that the device it quite sensitive at those frequencies.
This project can be built with components we have in our drawers. Who want to go out buying components on a rainy Sunday? I used a Vu-Meter from a scrapped amplifier for the indicator. The scale is not important as the measurements are comparative anyway. Those Vu-Meters are quite sensitive. The OP Amp can be any one of the common type as we amplify DC only. I used one fond in a drawer…
Possible improvements:
– A power jack so we can power the device with an external power supply when it is convenient, saving the precious battery life.
– A better antenna with a better gain. This device can be used to compare the performances of various antennas.
– A wide range RF amplifier… maybe later.
This article was contributed by Gerald Musy from Penang, Malaysia
Note: If you have any interesting repair article that you want to share to others please do email me HERE
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Waleed Rishmawi
June 1, 2013 at 5:09 pm
very impressive work. thanks for sharing. I am going to try to build one of my own.seems very interesting..love it
Jestine Yong
June 1, 2013 at 5:12 pm
Hi Waleed,
Yes this seems to be a very good test equipment. Simple and easy to build.
Jestine
Adegbite Abdul Lateef
June 1, 2013 at 5:49 pm
Well, It's a good project for us. I like your courage. Keep it up. Thanks Jestine.
Jestine Yong
June 3, 2013 at 12:15 am
HI Adegbite,
You are welcome
Jestine
Tana
June 1, 2013 at 9:20 pm
Rainy day project could turn out to be a project for a flash flood day too.Just joking. That measuring meter is an interesting project. You are really good in thinking out ways to repair electronic gadgets. Improvising useful gadgets on your own is a good gift.
Jestine Yong
June 3, 2013 at 12:15 am
Hi Tana,
Sometimes in this repair line one has to be innovative.
Jestine
Phumlani Manyathela
June 1, 2013 at 9:34 pm
wow,what an good idea,I lov it
Jestine Yong
June 3, 2013 at 12:14 am
Hi Phumlani,
Special credit to Gerald Musy.
Jestine
Harry
June 2, 2013 at 12:05 am
Hello,
About the microwave reading: it is absolutely NOT normal for a microwave to leak energy, from any part of the cabinet! However some do leak around the door & through the glass in the door: people should never stand in front of nor look into the microwave, but briefly, while it is operating, as doing so can damage sensitive parts of the Human Body, especially in children!
Jestine Yong
June 3, 2013 at 12:13 am
HI Harry,
You are right.
Jestine
Leung Wing Kai
June 4, 2013 at 11:06 am
I agree that not standing close to any microwave oven. It's hard to determine if there is any leaky. The RF meter is just good to detect any leaky.
gpin
June 2, 2013 at 4:03 pm
thanks,i like its good job.
Jestine Yong
June 3, 2013 at 12:11 am
Hi Gpin,
You are welcome and the credit should go to Gerald Musy for his kindness in sharing this information.
Jestine
Princeindi
June 3, 2013 at 12:54 am
Actually I got a problem with a wax warmer. The automatic cut off did not work but it heated the wax. The problem is that everytime the sefuse 151 degrees 2A250V get cut off.
Do you have any circuit diagram for that or any explanation how it works.
Thank you.
Jestine Yong
June 3, 2013 at 12:07 pm
Hi Princeindi,
Sorry I do not carry any schematic diagram. In your case, there must be some device/components that controls the heat so that it will not overheat and cut off the set.
Jestine
mustabz
June 4, 2013 at 9:31 pm
why not try thermostst out of a flat iron? that old type bi-mettallic strip would do yoou wonders.
Jestine Yong
June 5, 2013 at 8:51 am
Hi Mustabz,
Could you elaborate?
Jestine
endalemaw
June 3, 2013 at 7:02 am
Dear jestine I do appreciate all the thing that u have done. But sir I don't have access to ur pay books there is no any other options in Ethiopia to pay on credit cards. I just wanted ur books but I don't know what to do
Jestine Yong
June 3, 2013 at 12:10 pm
Hi Endalemaw,
I have just sent an email to you.
Jestine
rad
June 3, 2013 at 1:45 pm
your book is very useful. how i can buy or download?
do you have online class?
Jestine Yong
June 3, 2013 at 5:46 pm
Hi Rad,
I have just sent an email to you.
Jestine
Leung Wing Kai
June 4, 2013 at 11:03 am
I'd experience in repairing those transceiver before. We usually used the SWR meter to measure the RF power as well as the matching coefficient (VSWR) of the antenna since it will greatly affect the EM generated.It's mostly the connection problem at the antenna rather than the transceiver.
But the RF meter is good for determining the RF strength from the antenna. It's a good tool other than SWR meter as well.
Jestine Yong
June 4, 2013 at 7:43 pm
HI Leung,
Thanks for the good comments. I believe many readers will benefit from those who give good comments.
Jestine
mustabz
June 4, 2013 at 9:33 pm
Very ingenious! i should say am glad you could share such brilliant ideas. well done.
Elven Cabillo
June 5, 2013 at 12:11 am
Hi!,
Thank you very much for sharing ideas, I'm sure many readers like me will benefits this.
Philip odhiambo
June 5, 2013 at 12:46 pm
Hi Jestine,
This is so great. I'll try it out to help me in rf problems.
Thanks.
julius
June 9, 2013 at 2:57 pm
is there any way i can meet an LCD SCREEN SPECIALIST that can carry out all LCD SCREEN fault repairs from ripple pattern,freeze pixels,dead pixels,vertical lines & other related screen faults from "26"to "56" thanks
Jestine Yong
June 9, 2013 at 3:57 pm
HI Juluis,
Sorry no info on such specialist.
Jestine
john (Deaf Guy)
June 19, 2013 at 12:47 am
Wow! I never thought about RF Field Strength Meter were useful. Very interesting I like to build one.
John (Deaf Guy)
June 22, 2013 at 3:24 pm
How RF Field Strength Meter works? Does it need probe to test other device with RF Field Strength Meter?
Jestine Yong
June 22, 2013 at 3:33 pm
HI John,
Check out this video:
http://www.youtube.com/watch?v=8TPifxryeas
Jestine
ivan ws
January 9, 2014 at 2:48 pm
can I change the 1n60 with 1n34? that's what I have right know in hand. thanks.
Darryl Widman
April 21, 2014 at 2:13 pm
Ivan,
I would say that the 1N34 would do an equally good job for you. Remember, even if it does not work quite as well as the 1N60, you can turn up the knob to compensate for the difference.
T Achivida
January 18, 2016 at 10:38 pm
That's a good project and helpful indeed, Jestine.
By the way is it possible that you can send me a copy of a CIGNUS cg818 servicing manual, if maybe you can have access to them. I have trouble with my cignus base radio in the power side as it goes off immediately after I switch it on.
I tried looking for it in the net and it will s not availabke yet.
Thank you and more power.