SKY65116: lower drain when in antenna

Hello everyone,
I'm testing this amplifier and I noticed a lower current drain when I connect the output of it to an antenna instead of the spectrum analyzer/load.

The antenna I'm using is a simple whip / mobile antenna, but it is sold for the frequency I'm using it (433mhz)

Can you please tell me the cause/reason of this behaviour ?

Maybe I'm asking something obvious, please excuse my inexperience.


Thanks,
Giacomo.

If the output power is maximum for lower current (maximum efficiency), means that the output of your amplifier is not optimized for 50 ohms load (which most probably the SA has), but is optimized for whatever impedance the whip antenna has.
Notice that the output impedance of the amplifier could be optimized for output power, for efficiency, for linearity, for gain, or for all of them, making a trade-off.

Assuming that the SA is 50 Ohms (VSWR=1), your antenna has very likely bad VSWR.

RF stages without fixed bias circuit show change in current draw when changing the load impedance (this is not uncommon).

(less likely) Other reason can be parasitic oscillation. The SA shows a wide band 50 Ohms load, but a whip antenna certainly not.

Are you using the antenna within its intended environment (for example sufficient ground plane as you use a mobile antenna).

Steps to take:
Check VSWR of antenna in same electromagnetic environment (use a dummy case/ PCB of same size of actual case/PCB) and use appropiate ferrite (or other) choking device to avoid common mode current at the feed cable that goes from the test instrument to the antenna.
Check spectrum of 433 MHz TX when directly coupled to SA.
Check spectrum of 433 MHz TX when using the antenna. use some short piece of wire or small loop to measure the relative spectrum. Check carefully for new frequency components (as that indicated parasitic oscillation).

For transmitters its OK. Falling Current and rising Power. It depends from load. Critical load has max output pow and falling current ( changing current envelop and as result average value ) It is very simplify, real load and behavior more complicate.

Thank you everyone for your replies, finally I've discovered that the behaviour was caused by the lack of proper rf isolation of the modulator/driver/amplifier/antenna, i've installed it into a metal case with multiple compartments and I'm actually getting the same current drain if i connect the antenna or the SA.

Giacomo.

Much better when current drops. It chows that output stage is alive and we have right matching. Of course it depends internal or non and so on .