RF transmitter module current varies with physical placement

[penneyj]

I was trying to measure the current on the power supply of a 2.4GHz module transmitting at max RF power and I noticed that depending where I physically placed it on the bench and if I was touching the insulated power wires or not, or had my hand physically near the module, the current ranged from 125mA to 220mA.
Does that make sense? How can I take an accurate measurement?

 

[FvM]

Connect an artificial antenna (a 50 ohm resistor).

 

[penneyj]

Re: RF transmitter module current varies with physical place

Connect an artificial antenna (a 50 ohm resistor).

I was about to try that, I have an 0402 50 ohm tight tolerance part I used recently. I noticed when I have the spectrum analyzer plugged into a test port the readings are only varying by 3mA if I try to disturb the board with proximity to things so that's probably the correct measurement, which I'll compare to the 50 ohm test.

Thanks

 

[throwaway18]

Re: RF transmitter module current varies with physical place

It might be RF affecting your meter.

I find that typical cheap digital multimeters give incorrect readings with around 500mW at 30cm at 20-500MHz.

They are usually less sensitive to 2.4GHz but on current the leads can conduct RF straight into the meter.

A cheap analog meter that works on the current range without a battery is usually less sensitive to RF.

 

[penneyj]

Re: RF transmitter module current varies with physical place

I'm using a Keithley source meter to supply the 3.3v and measure the current at the same time since I also need to measure the 10uA sleep currents and required something accurate. I assume that would give good measurements and I'm most likely affecting the antenna network by moving things around and getting different loads genuinely.