How to choose a 50 Ohm RF attenuator for radio, bluetooth and wifi power measurements

Question

I have built a power meter with AD8318, a chip which contains a logarithmic amplifier, often used for power measurements.

I have read its datasheet and it is written that the maximum input power is equal to 12 dBm.

I do not know exactly which kind of measures I can do with this limits, but it correspondes to 15.85mW, which I think is quite low for many applications. Precisely, I wanted to use it to measure power from wifi networks, bluetooth, mobile network and radio stations.

So I need some advices about if it is necessary to buy a RF attenuator to put between the antenna and the power meter (with 50 Ohm SMA coaxial cable) and how many dB of attenuation I need for these applications. Since I am not an expert of rf attenuators, if you think I should consider other parameters for choosing an attenuator, tell me!

Answer 1

The typical P_EIRP of standard wifi/ble/zigbee networks is in the range of 0dBm to 10dBm. This is the equivalent output power of an isotropic ratiator.

If you have a distance of (only) 1m in between a wifi/ble/zigbee sender and your receiving meter, the attenuation of air will be around -40dB at 2.4GHz.

Given that you have mostly isotropic radiators with little antenna gain, your receiving power will be about max. -30dBm at 1m distance to the wifi/ble/zigbee sender.

So you can safely do wireless measurements with your power meter without worrying about the power limit. But you should not wire the power meter directly to a wifi/ble/zigbee source without attenuator.