How does battery voltage affect signal strength in a transmitter?

[safdar]

Hi guys!
What is the link between the signal strength and the battery voltage?
Now I explain it in the following:
Consider a sending device that sends signals for example an anchor node. It sends signals of particular strength and it uses its battery power. As the time goes on and the battery becomes weaker and weaker the signal strength shall also be decreased. So what is the relationship between the signal strength and the voltage of the battery?

 

[berkeman]

Hi guys!
What is the link between the signal strength and the battery voltage?
Now I explain it in the following:
Consider a sending device that sends signals for example an anchor node. It sends signals of particular strength and it uses its battery power. As the time goes on and the battery becomes weaker and weaker the signal strength shall also be decreased. So what is the relationship between the signal strength and the voltage of the battery?

It depends on the situation, but usually the transmit amp will run directly off of the battery voltage to get max possible output voltage swing to the antenna. The output amp will be able to drive the antenna input voltage close to the rails (Vbatt and GND), but not quite. So as the battery voltage droops, the output voltage swing will droop by a comparable amount.

 

[waht]

Also, on many RSSIs, the output can be proportional to the log of the input.

 

[safdar]

Also, on many RSSIs, the output can be proportional to the log of the input.

Does it mean, RSSI is directly proportional to the logrithm of battery voltage?
Any source that can provide further information?

 

[Bob S]

If a buck-mode switching dc-dc regulator is used to reduce the battery voltage to a constant voltage for the transmitter, the received signal strength will remain constant, independent of battery voltage, for a longer time.
Bob S

 

[berkeman]

If a buck-mode switching dc-dc regulator is used to reduce the battery voltage to a constant voltage for the transmitter, the received signal strength will remain constant, independent of battery voltage, for a longer time.
Bob S

Good point Bob. And for even more battery life/range, use a buck/boost (National Semi calls it Sepic) DC-DC. That's pretty common in many 2xAA battery applications, IIRC.