Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Reducing output power in PMR two-way radios

Status
Not open for further replies.

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,846
Helped
13
Reputation
26
Reaction score
10
Trophy points
1,318
Location
Greece
Activity points
34,872
Hi, I have two PMR two-way radios that transmit on 500mW.
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?
 

G4BCH

Full Member level 6
Joined
May 10, 2006
Messages
374
Helped
125
Reputation
250
Reaction score
104
Trophy points
1,323
Activity points
4,685
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.
 

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,846
Helped
13
Reputation
26
Reaction score
10
Trophy points
1,318
Location
Greece
Activity points
34,872
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.
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,371
Helped
1,556
Reputation
3,115
Reaction score
1,167
Trophy points
1,393
Activity points
34,047
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.
 

neazoi

Advanced Member level 5
Joined
Jan 5, 2008
Messages
3,846
Helped
13
Reputation
26
Reaction score
10
Trophy points
1,318
Location
Greece
Activity points
34,872
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?
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top