20 dB Attenuator for GSM Field Meter

I have a digital field strength meter covering the 800MHz to 2.5GHz range To it is connected, via SMA cable, a log periodic antenna. The available reading only goes to 1999 uW/m2. I would like to place a

20dB attentuator in-line with the antenna to increase this by a factor of 100.

Can anyone please suggest what circuit or components to use? Is something like this all I need?

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Can I assume the meter's input impedance is 50 Ohms?

Richard Jones

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If only it was that simple! at 800 to 2.5Ghz layout is going to be critical if you want a flat response over that range.

At the first online supplier I checked 3GHz attenuators seem to start at around $40 for upto 10dB, 20dB is twice as much.

If it's not it'll probably written somewhere near the input TV uses 75 ohms, but most other things are 50 ohms.

?? 100% natural 

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Reply to
Jasen Betts

How is the actual measurement instrument protected against direct radiation penetration ?

Sure, you can insert various attenuators along the line, but if the signal source will penetrate directly in to the equipment from the air, you are in trouble.

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Could you test for that by terminating the input with a shielded 50R and checking the reading is zero (or very low wrt the normally measured values)?

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Does this digital FSM have a maker and model number? Out of curiosity, what are you measuring?

Ummm... something like this perhaps? About $450. That's the defacto standard meter used by smartmeter hunters, electrosensitive people, and RF paranoids. Next time, buy something from Narda, which does it right.

2 mw/square meter is about 117 dBuV/m field intensity full scale on the meter. At 850MHz, the FCC MPE (maximum permissible exposure) limit for the general population is 0.57 mWatts/cm2 or about 153 dBuV/m for a 30 minute exposure. Looks like you'll need a 40dB attenuator in order to measure at the FCC MPE limit. These should help with the units conversions:

Note that if you're calculating full time (24x7) exposure, such as what you would get sitting under a cell site, the 30 minutes exposure doesn't work. You need to lower the exposure limit by 48 times, or


You apparently have no experience with RF construction. Building your own attentuator is not easy. The problems are finding 1% or better resistors, proper in/out isolation, frequency response, and access to test equipment.

Well, calibration won't be much of a problem since the above Gigahertz Solutions meter is only +/-6dB accurate.


Google for 20dB SMA attentuator. There are plenty available at wildly varying prices. etc. Plenty more on eBay under "20 dB attenuator SMA". Watch out for burned out attenuators, where someone has transmitted into them and ruined the calibration.

Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

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SMA are 50 ohms, hence yes.

Use a Pi-section attenuator, theoretical values are 61.11 ohms, 247.5 ohms, 61.11 ohms.

Good luck with getting it to stay resistive up to 2.5GHz.

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Reply to
Fred Abse


Minicircuits DC-6GHz SMA Attenuatos, regardless of the AttenFactor are around 12$/Piece..!

have a look:


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Lund-Nielsen, Jorgen

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A really nice one!

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