Calculating the trasmitted power

[elockpicker]

Hi,there are many simple FM transmitters over the internet like

http://www.aaroncake.net/circuits/fmtrans.asp
http://english.cxem.net/radiomic/bug7.php
...

how can I compute the transmitted power (like when they say it's a 2W transmitter)?

Thank you in advance.

 

[pranam77]

Voltage supplied (measured by the voltmeter while tranmitting) multiplied by the Current (Ampheres consumed while transmitting ) = Wattage
In simple V x I = W

Cheers

 

[elockpicker]

Hi,do you mean the maximum instantaneous power ?

 

[biff44]

FM is what we call "constant envelope" modulation. That is, the carrier amplitude is constant, even as the frequency is moved up and down. As such, if a transmitter says it is a "2 Watt Transmitter", it pretty much means what it says. 2 watts RMS come out of the transmitter. If you hooked up a matched coaxial load to the output port, you would measure 2 watts of heat coming out of the load.

 

[davenn]

Voltage supplied (measured by the voltmeter while tranmitting) multiplied by the Current (Ampheres consumed while transmitting ) = Wattage
In simple V x I = W
Cheers

Not quite, pranam77

that will determine the total power use of the transmitter circuit, NOT the RF output.
The RF output will be somewhat less, depending on the efficiency of the devices in the cct, which in all likelyness would only be in the 40-60% range
There will be noticeable power losses in the circuit some of which will be as heat.

So for a transmitter producing 2 Watts of RF carrier output, the total power usage could easily be in the 4-7 Watts.

cheers
Dave
VK2TDN

Added after 3 minutes:

Hi,there are many simple FM transmitters over the internet like
http://www.aaroncake.net/circuits/fmtrans.asp
http://english.cxem.net/radiomic/bug7.php
how can I compute the transmitted power (like when they say it's a 2W transmitter)?
Thank you in advance.

When they say its a 2W transmitter it is a 2W transmitter but as said in my previous post the total power usage of the cct will be higher.

Most of those "simple " transmitters like in the links you provided only produce a few milliWatts. Most likely less than 10mW.

cheers
Dave
VK2TDN

 

[pranam77]

Not quite, pranam77, that will determine the total power use of the transmitter circuit, NOT the RF output.

Yes..you are right. The said measurements shows the power of the transmitter and not the transmission. I should have mentioned the Voltage and current measurements of the "Final RF amplifier section". Thanks

 

[davenn]

[quote="pranam77
Yes..you are right. The said measurements shows the power of the transmitter and not the transmission. I should have mentioned the Voltage and current measurements of the "Final RF amplifier section". Thanks[/quote]

those measurements will still only show the total power usage of the final device, not of the RF output power

for example one of my transmitters is drawing 22A at 13.8V, the final stage is drawing 20A @13.8V
20 x 13.8 = 276W BUT the transmitter is only producing 100W of RF
as mentioned earlier, efficiencies are relatively low in most RF devices
MOST of the rest of the power is being wasted as generated heat and lots of it, hence the HUGE heatsinks on transmitters

its just the way it is

I recently built a 1296MHz power amp using a M57762 power block module its efficiency is only ~ 28 % !!!
1W in produces 20W out ... 13.8V @ 5.5A

thats not cuz of my cct design ... thats the ratings of the device

cheers
Dave
VK2TDN

 

[pranam77]

Ok..then using a Digital RF watt meter is the only way to get precise output wattage measured i suppose. Anyways, the OP has asked just to know how one has to compute output of 2 watts. He may try either ways possible...Cheers