Reducing output power in PMR two-way radios
They do not have an output power setting in the firmware, but I need to reduce their power output, so that they can only cover a small area (inside house/garden) and also for health concern.
What I did, was to remove their helical antennas from inside (classic antenna for PMRs) and replace them with a straight piece of wire, 2cm long.
This modification, gave a smaller transmit coverage, than my LPD (10mW) devices. Of course, such a modification would ruin the RX signal as well, but practically I found it not to be affected too much.
Since I do not have any means of measuring the actual antenna radiation power, I compared the modified PMRs to another pair of PMRs that have an output power of 10mW.
I did TX and RX tests with all the combinations, unmodified PMRs, modified and mixed together. What I mean by mixed together is the TX to be the modified PMR and the RX to be the unmodified one (so as to test the modified one in an unmodified RX antenna).
From my experiments, I concluded that the coverage with the modified PMRs is a bit less than the unmodified ones.
Is it safe to conclude that the output power of the modified PMRs is below the output power of the modified ones?
If you are concerend about the radiation from the radio than you are better off having the receiver as sensitive as you can, and reducing the transmitted power. Rather than use a small antenna, put the helical back and reduce hte TX power. You can do this by removing hthe supply from the PA; there may be enough leakage to cover the range you want. If you need more power restrict the supply current to the PA by a suitable resitor. Either way you will get better battery life by reducing the TX than by restricting the antenna.
The problem is that I do not have the schematic of this two-way radio and I cannot find it anywhere.
If I had it, I could figure out how to do it, indeed a final PA reducing in supply.
For better comparison you have to measure the Radiated Transmitted Power of the two transmitters (modified and unmodified) , using the same receiver+antenna.
In this way the receiver would not affect the measurements, and can give you information about which TX has more transmitted power.
Yes I did this sort of thing.
I held the same receiver (unmodified) and my friend transmitted using a modified and an unmodified transmitter (with low power).
The unmodified transmitter in low power gave better coverage than the modified one.
From this experiment I concluded that the modified transmitter has the same or lower power than the unmodified one with it's low power.
Is there anything else I need to consider?