Walkie talkie output power

[BarNash]

Hi all

How the output power of a simple walkie talkie is measured ?

I have to make sure that it does not transmits more then 1/w watt .
It is a 3 transistors circuit 49mhz, 9 volts operated walkie talkie

Thanks
Elico

 

[Dan Mills]

There may well be a very easy way, measure the DC input from the battery on transmit, if it is less then 1W then the transmitted power will certainly be less then a watt....
Failing that, use a RF current and voltage probe to measure the values at the feedpoint and do the sums, not hard.

I would be very, very surprised if you were getting a watt from a three transistor circuit, a few tens of mW or so feels much more likely.

Note that the third harmonic of 49MHz is @148MHz so you will also want to make sure your output filter is up to snuff.

Regards, Dan.

 

[BradtheRad]

If you wish to measure broadcast power at a short distance, there are simple rf detector circuits. Your multimeter will provide relative readings. I'm not sure how you would calibrate the scale, however.

There is also the grid-dip meter.

You'll find many schematics via an internet search.

Articles:

http://www.miketooley.info/g8ckt/simple_rf_clip_meter.pdf

http://www.rfcafe.com/references/po...ple-rf-meter-oct-1958-popular-electronics.htm

 

[jiripolivka]

Measuring the real power output from a walkie-talkie transmitter is not easy. The circuit typically is an open PCB. Measuring the power requires the signal source to have a (coaxial) antenna connector, and the output impedance well defined.
The easiest way would be a comparison- use a RF detector connected to a tuned dipole at 49 MHz (or a shorther whip), with the output voltage indicated on a DVM or a mA meter.
Take a standard signal generator with a 50-Ohm output, and use a power meter to set several output power levels, below and above the expected power from thewalkie-talkie.

Then connect a whip antenna like that on your walkie-talkie, and connect it to signal generator output. Take several readings on the RF detector as above, for several power levels as measured from the signal generator.
Put your walkie-talkie in place of the signal generator and its antenna, and repeat the RF detector test. As you calibrated the RF detector for various power levels, you can define the walkie-talkie power.
Take care of equal polarization of all antennas, and try to use obstacle-free environment.

 

[biff44]

if we are talking about an FM modulated signal, it is easier, as that is constant envelope.

You can transmit, and a fixed distance away measure the received power with a spectrum analyzer.

then replace the walkie talkie with a lab signal source and a whip antenna, and increase the power until the received power is the same. THAT is roughly the transmit power.

the only question is does the test whip antenna look like the walkie talkie antenna close enough.

 

[BarNash]

if we are talking about an FM modulated signal, it is easier, as that is constant envelope.

You can transmit, and a fixed distance away measure the received power with a spectrum analyzer.

then replace the walkie talkie with a lab signal source and a whip antenna, and increase the power until the received power is the same. THAT is roughly the transmit power.

the only question is does the test whip antenna look like the walkie talkie antenna close enough.

Many thanks
Elico

 

[chuckey]

Work out what the equivalent resistance and capacitance is of your short aerial. Then make up the components with the resistor connected to earth and the capacitor in series with the aerial terminal in place of the dipole. Measure the voltage across the resistor and calculate the power.
Frank

 

[BarNash]

"Work out what the equivalent resistance and capacitance is of your short aerial." How exactly this is done ?
Elico