What transistor power amplifier to use to drive antenna and amplify signal to reach 50km

Thread Starter

Minchuu

Joined May 13, 2021
14
Hello!

I have a 4vpp signal at 500 kHz that needs to be transmitted at a max distance of 50km. Therefore, it needs to go through a transistor power amplifier that will also drive a 50ohm load (theoretically, the antenna). The bit rate will be 50 kbps. The receiver sensitivity at the end of the 50km distance is -50 dBm.

As a newbie student at this kind of topic, I am at lost how to choose what type of amplifier shall I choose. I apologize if I sound stupid TT^TT, but I cannot seem to connect how to choose the amplifier and what is the gain needed based from the weird specifications given. Also, everything will only be simulated so I don't need good results in real life, only that it works optimally in simulations.

Any guidance on this matter will be very much appreciated. Also, if you guys have any power amplifier analog circuit schematics that you can share that will do the bare minimum of the specifications, it will also be very much appreciated also.

Thank you very much!
 

Thread Starter

Minchuu

Joined May 13, 2021
14
Before you go there, check with your local authorities first. It is likely that you will need a license.
It's just for simulations. Due to the current remote learning due to the pandemic, we sadly won't be doing this in real life so I won't need any permissions to use the frequency.
 

Thread Starter

Minchuu

Joined May 13, 2021
14
That is the weird thing about this project. We are not required to simulate the transmitter and receiver antenna gains, from my understanding. I just need to be able to drive a 50ohm load through the amplifier and that will suffice as having been able to simulate the antenna. To be considered complete, the output I need to show will only be the amplified signal that should theoretically be detected as far as 50km. How will I test if the amplified signal can do this, I am not sure.

The signal that will enter the transistor power amplifier is a BPSK signal. It is a modulation of a carrier signal of 500kHz and the Vmessage which will be in binary.
 

Papabravo

Joined Feb 24, 2006
16,160
What you need is a model of how the signal will propagate from transmitter to receiver. Let us say the signal starts out as 1 Watt of radiated power, which is +30 dBm and you lose 1 dBm per kilometer. What signal level will reach the receiver? If you said -20 dBm that sounds correct. Is that enough for the receiver with a sensitivity -50 dBm? Does that help?
 

Thread Starter

Minchuu

Joined May 13, 2021
14
What you need is a model of how the signal will propagate from transmitter to receiver. Let us say the signal starts out as 1 Watt of radiated power, which is +30 dBm and you lose 1 dBm per kilometer. What signal level will reach the receiver? If you said -20 dBm that sounds correct. Is that enough for the receiver with a sensitivity -50 dBm? Does that help?
It does help, thank you very much :) It led me to the concept of the transmit power, and I calculated around 51 dBm if I set a set a link margin of 5 dB, thus setting the minimum signal power the receiver should receive to be -45 dB. The path loss around 61 dB.

But 51 dBm for a transmit power seems large? Especially since my input signal is only 4vpp. So back to my original question, if you have any suggestions which transistor power amplifier topology should I use and any sample schematics I can simulate it would be very much appreciated.
 

Papabravo

Joined Feb 24, 2006
16,160
It does help, thank you very much :) It led me to the concept of the transmit power, and I calculated around 51 dBm if I set a set a link margin of 5 dB, thus setting the minimum signal power the receiver should receive to be -45 dB. The path loss around 61 dB.

But 51 dBm for a transmit power seems large? Especially since my input signal is only 4vpp. So back to my original question, if you have any suggestions which transistor power amplifier topology should I use and any sample schematics I can simulate it would be very much appreciated.
So 30 dBm corresponds to a radiated power of 1 Watt,
and 51 dBm corresponds to a radiated power of ≈ 125.9 Watts
This is not a ridiculous amount of power for a garden variety transmitter. Remember that the reference power in the dBm calculation is 1 milliwatt!
Glad the response was helpful.
For an efficient RF amplifier you should be looking at a Class C design. This design is characterized by the active device (the transistor or FET) only being in the active region for a small portion of an AC cycle at the frequency of interest. This trades efficiency for increased distortion.
 

Audioguru again

Joined Oct 21, 2019
3,199
A simulation program usually allows severely overloaded parts.
The simulation does not show poor performance or failure when transistors have minimum or maximum specs, only performance when the specs are "typical".
 

Papabravo

Joined Feb 24, 2006
16,160
A simulation program usually allows severely overloaded parts.
The simulation does not show poor performance or failure when transistors have minimum or maximum specs, only performance when the specs are "typical".
It would be cool if the simulator monitored power dissipation and made the overstressed components disappear in an animated puff of smoke and remaining debris. You would then have to clean up your schematic pixel by pixel -- LOL.
 
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