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?
Can I assume the meter's input impedance is 50 Ohms?
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 firstname.lastname@example.org
150 Felker St #D http://www.LearnByDestroying.com