Current consumption vs Radiated / Conducted test

[chihhao]

I got a weird test result.
Please share your opinion if you have any idea. Thanks.

I tested a mobile phone on conducted test condition (a RF cable connect
from instrument with mobile phone directly). The current consumption is
about 180mA.

But I loaded antenna on it for radiated test (Air link between instrument
and mobile phone). The current consumption is about 300 mA.

I can't figure out why it got so different current consumption result in
conducted test & radiated test.
The PA output power is fixed by reading parameter table in Flash memory
and can not be changed.

The current consumption reduced in radiated condition if I changed antenna
matching.

Can any one tell me why from your point of view??? Thanks for your help.

 

[vfone]

This problem is related to the PAE of the Power Amplifier under different loads.
An RF PA can have different current consumption, for the same output power, for different load impedances
Output match shall make a trade off between Output Power and Power Added Efficiency.
When you are doing the test conducted, the load is pure restive (almost ideal), but when is radiated the antenna impedance has high reactive components.
Also is possible to see higher PA currents when is testing the antenna behavior under human body presence (de-tuned antenna).

Here is an application about efficiency vs load line
http://amsacta.cib.unibo.it/archive/00000582/01/GM_05_02.pdf