Measuring Radiation from Mobile

[RadioRon]

Don't cell phones transmit with different power levels depending on the distance and traffic of the nearest base office? Even during each call?

Yes that is correct, they do. In CDMA systems, using voice, the transmitter is on continuously if you are talking, but turns off (or backs off its power a great deal) when there is little audio energy. In fact, you can see these power drops even during slight gaps in your speech. When transmitting, the power is also stepping up or down slight amounts (like 1dB) somewhere between 600 and 1600 times a second to cope with the varying path loss in an attempt to keep your signal at the base station fairly constant in amplitude.

GSM systems carrying voice traffic also step things up and down depending on range, but not with as fine resolution nor as fast as in CDMA. GSM voice, however, is guaranteed to be transmitting with constant on-off pulses at a rate of about 220Hz (I think), following the GSM TDMA method. This is why you might sometimes hear a buzz in a nearby landline phone or audio player when a GSM phone is brought nearby.

 

[HiTech]

I would have thought so, microwave oven detectors are designed for a specific single frequency - and require sending back yearly for calibration checking and alignment (apart for the toy ones, which aren't even calibrated in the first place).

and what's wrong with a $39 detector?
**broken link removed**

 

[Nigel Goodwin]

and what's wrong with a $39 detector?
**broken link removed**

They aren't calibrated and are total rubbish! - we occasionally get people bringing microwave ovens in for repair complaining it's leaking - when we test it (with a yearly calibrated and certified meter) we find no leakage at all. We then find they bought a toy meter!.

Would you buy a multimeter that read anywhere between 1V and 100V when you tested a 9V battery?

 

[HiTech]

I gather that you didn't realize I was joking about those cheapo detectors. Now from your last reply there seems to be a good purpose for those toy meters: $ervice income for you from their faulty indications!

 

[Olihou]

They aren't calibrated and are total rubbish! - we occasionally get people bringing microwave ovens in for repair complaining it's leaking - when we test it (with a yearly calibrated and certified meter) we find no leakage at all. We then find they bought a toy meter!.

Would you buy a multimeter that read anywhere between 1V and 100V when you tested a 9V battery?

Hi Good morning !

With the very good advices I am receiving from the forum, I am quite confident that I will not end up with something as you have described !

I have no knowledge of the $39 detector, but I am surprised to hear that it detects some radiation from the microwave oven, but is tested nil by a calibrated meter. I guess there are at least two possibilities:

1 The detector was not detecting the 2.4 GHz microwave, it might just be responding to the 50/60Hz mains

2 The calibrated meter was actually faulty

As a matter of fact, I have never seen any microwave oven claiming zero leakage, be it just in the uW/cm2 range

 

[Olihou]

Hi RadioRon

I have found this $320, 0.5 MHz to 3 GHz meter -

**broken link removed**

It looks "professional" by the large 5-digit display.

However it claims only a very modest accuracy of +-25%, reflecting indeed the difficulty in the wideband measurements.

Thanks a lot for your wonderful advices, which have enabled me to start this project. I am now searching for good receiver circuits, hopefully with locally available RF diodes.

Do you think a spectral flatness of +-25% for 800 MHz to 2.4 GHz a difficult target for me ?

 

[RadioRon]

Yes, I would say that flat+/-25% is indeed a tough target to reach, for a beginner. There will be several things that contribute to a lack of flat response. One is your receiver circuit, but you have control of that and can try hard to make it broad and flat. The next is your antenna, which will not be flat.

The third problem is that the field you are trying to measure is not supposed to be flat. If you radiate a "flat" amount of power from a radiator that has a "flat" frequency response, then the field strength you measure at a fixed distance will not be flat, but should decline in strength as the frequency goes up. This is in theory anyways.

So, if your meter was perfect and ideal, it should show a declining field strength with an increasing frequency.

Well, don't let that bother you. Just try to build something that is sensitive and if you include a microprocessor to read the voltage and display the result, you can include a correction table within a program that allows you to calibrate your unit and cancel out all the errors. Neat eh?


I'd like to add that +/-25% error in reading a power density is an error of only about +1/-1.25 dB. In the world of antennas and field strength measurement, this is actually pretty good. I think that if you achieve +/-5 dB error you are doing ok.

 

[Nigel Goodwin]

I have no knowledge of the $39 detector, but I am surprised to hear that it detects some radiation from the microwave oven, but is tested nil by a calibrated meter.

Just far too sensitive - it makes no attempt to give any kind of measured reading, just a needle moving with no idea what value it might represent.

Our yearly calibrated professional meter has the legal requirement marked about 2/3 of the way across the scale - and it's VERY rare to even see the needle move when testing. Only exceptions have been a couple of Philips ovens (and they were under peritted limits), basically because the doors opened downwards, with the latch only in one corner.

 

[Olihou]

I see.

Contrary to what I thought, the "toy meter" is probably too sensitive by design then !

I have a similar "toy" with very simple circuit: just a 1N23 RF diode and a 100 uA moving-coil meter, plus a potentiometer for sensitivity control. No battery power is required. The mid-scale reading is 5 mW/cm2, which is the standard limit for microwave ovens (if I remember correctly). For all the ovens I have tried, there is alway some leakage detected, but none seems to exceed 1 mW/cm2.

This meter however is barely responding to the handsets. It is probably just not sensitive enough. Some amplification (RF or DC, hope just DC will do) would be needed, which I now plan to do.

For the $39 meter you have mentioned, I trust it has got an amplifier and therefore a battery is used ?

 

[flatfootskier]

antenna

If you want a reasonably linear antenna across that band, make yourself a horn antenna. Get a C band recieve (3.7 - 4.2 GHz) waveguide transition to N type (ebay?) and some copper or aluminium plate, making a "rectangular funnel" leading back to smoothly meet the edges of the waveguide. I mad a collapsible one a few years back & it was suprsisingly linear within most individual bands. It has a few notches across 500MHz to 5 Ghz, but nothing too steep.
That said, you need to be well away from the subject being measured to get anythin meaningful.

 

[Olihou]

Thanks flatfootskier but sorry I do not feel that I would be able to follow your above instructions and make a good horn antenna. May be I should first study into the subject and would be grateful if you could help me to access to some good tutorials.

My present goal is to have the field strength meter made "flat" from 800 MHz to 2.4 GHz, say to +-5 dB. Further to minimize the loading effect on the existing field, I think I should try to make the antenna as small as possible. I will start experimenting with simple dipoles or loops. I am searching but still not able to get proper (step by step and preferably with theory) procedures in designing these antennas.

 

[Sceadwian]

A dipole is probably your best bet, a simple whip will work. Getting a high degree of linearty across that much of the spectrum is going to require some extreme compensation circuits.

 

[gixxig]

Whao...

I am working on a very similar project at the moment!
I have found a link about Schottky diode detectors, however since I do not have much knowledge about them, I am not sure if it is of any use...

**broken link removed**

go to the above link and you can download the pdf file...
I have received a lot of good advice from your page, thanks a lot!!
I hope there will be more discussions on this topic as your project goes along, as I don't think I can do this on my own at all...

 

[Olihou]

Whao...

What a coincidence !

In the course of selecting a suitable RF detector diode for my Field Strength Meter project, I am puzzled by the saying that some Schottky diode can work zero-biased at rather low signal levels, in mVs. This is very different from the ordinary Si diodes I am familiar with, those requiring hundreds of mVs to be functional.

Thanks gixxig for providing the link. The article does give me more understanding and hence confidence in these GHz detecting diodes. I will check if I can get locally SMS7630 or similar.

Will report when I have achieved some progress.

Cheers !

 

[Olihou]

Hi I am back to report what I have done so far with regard to my attempt to build a radiation detector for mobile phones.

Please refer to the circuit diagram and photo attached.

Due to unavailability of microwave diodes locally (such as 1N5711 or SMS7630 as recommended by friends at this forum ) I have used one found in the shelf. There is no marking on the diode though, I remember it is one for microwave.

As for the antenna I have done it unconventionally. I just bend the 30 mm leads of the diode to form a loop, and solder them at the ends to the 100 Kohm resistors. The two resistors are meant to be RF chokes, though the conventional way of ferrite beads (again not available locally!) might be better ?

In this way, I think I have made a quatrerwave dipole at 2.4 GHz but without the need to solder the diode close to the junctions (the conventional way that make me feel uneasy as my precious diode might thus be destroyed by heat!)

The amplifier circuit is the one modified from a UVB light detector I built not long ago. No provision for zero adjustment is built in. With TLC274P IC1A set to unity gain by S2, IC1C set to X10 gain by VR1, I have found the output DVM reads as high as

0.5 V when the detector is two meter away from a microwave oven ! and

0.4 V when it is 50 cm from the talking mobile phone !

A sensitivity higher than what I have expected !