Need a working long range FM Transmitter circuit

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sribasu

Joined May 10, 2017
18
I am a beginner of Electronics and a hobbyist. As a beginner FM transmitters has caught my attention. I've made few of them following the online blogs and project tutorials. They are working. I want to increase the power of the transmitter to transmit the signal to longer distance. I am looking for a 10 KM range. I am not sure of it sounds practical or not, with basic components.

I am currently referring the following circuit.


How can I take input from the output of the above circuit (i.e. the antenna) and amplify the FM signal power to transmit it to a wider range? I've come across VHF amplifier circuits. But most of them are really confusing and uses an obsolete RF Transistor and several inductors without much explanation. For a beginner like me, it's difficult to troubleshoot, when there is less or no explanation.

If someone could please post a link to a detailed circuit or share the circuit here, I'll be grateful. I would also like to know if a high power RF transistor can be replaced by multiple 2N3553 transistors (in parallel) or not.
 
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Alec_t

Joined Sep 17, 2013
12,091
I want to increase the power of the transmitter to transmit the signal to longer distance. I am looking for a 10 KM range.
It will then be easily detectable by law enforcement officers. In most countries there are strict rules about RF transmissions, so that interference with air traffic control, emergency services and other legal broadcasts is avoided. What frequency bands are legally available for non-licensed use in your country, and how will you ensure that your transmitter complies with the regulations?
 

Thread Starter

sribasu

Joined May 10, 2017
18
It will then be easily detectable by law enforcement officers. In most countries there are strict rules about RF transmissions, so that interference with air traffic control, emergency services and other legal broadcasts is avoided. What frequency bands are legally available for non-licensed use in your country, and how will you ensure that your transmitter complies with the regulations?
Thanks for responding. I am willing to make it just for learning purpose and test if the range can really be increased so much with home-brew circuit. I will definitely test it at a frequency where no commercial or government stations are transmitting. I am not at all willing to transmit 6 hrs a day and let others listen to my channel :) That's not my purpose and I know the law.
 

Audioguru

Joined Dec 20, 2007
11,249
A simple 100MHz FM transmitter has harmonics at 200MHz, 300MHz, 400MHz, 500MHz etc. They might block important legal communications which is why the law prevents you from fooling around with a high power radio transmitter. The strength of your high power signal plus its harmonics will wipe out legal radio reception all around you.
 

WBahn

Joined Mar 31, 2012
26,398
Thanks for responding. I am willing to make it just for learning purpose and test if the range can really be increased so much with home-brew circuit. I will definitely test it at a frequency where no commercial or government stations are transmitting. I am not at all willing to transmit 6 hrs a day and let others listen to my channel :) That's not my purpose and I know the law.
Good. So since you know the law, what is it? What are the legal requirements that your transmitter has to satisfy? It's hard to have a discussion about RF power amplifiers without that information in mind.

And how will you determine that no commercial or government stations are transmitting on any of the frequencies that your transmissions might impact? Say you are transmitting from Point A to Point B that is 10 km to the north, how do you know that there isn't a receiver 15 km to the south trying to receive a legal low-power transmitter located even further south that your transmissions are going to jam?

What about noncommercial and nongovernment stations?
 

Thread Starter

sribasu

Joined May 10, 2017
18
I want to test at at approx 75 MHz. I am afraid to see the reason why your country is not producing as much engineers as mine; you guys are so reluctant to discuss on a topic, which is very well and in detail taught in Electronics Engineering curriculum, showing the reasons of law. I am from computer science background, and unfortunately didn't get much chance to learn Electronics in my curriculum. I know, we have a very popular subject in CS which is 'Ethical Hacking'. Nobody feels reluctant to teach it, with the terms that the leaner won't misuse it! I see nothing similar in Electronics here. Thanks for your response by the way. I probably won't get a chance to learn here, and will find a way somewhere else.
 

WBahn

Joined Mar 31, 2012
26,398
I want to test at at approx 75 MHz. I am afraid to see the reason why your country is not producing as much engineers as mine; you guys are so reluctant to discuss on a topic, which is very well and in detail taught in Electronics Engineering curriculum, showing the reasons of law. I am from computer science background, and unfortunately didn't get much chance to learn Electronics in my curriculum. I know, we have a very popular subject in CS which is 'Ethical Hacking'. Nobody feels reluctant to teach it, with the terms that the leaner won't misuse it! I see nothing similar in Electronics here. Thanks for your response by the way. I probably won't get a chance to learn here, and will find a way somewhere else.
And those same people that teach "Ethical Hacking" are VERY reluctant to teach hacking skills to people that make it obvious that they INTEND to misuse it.
 

Thread Starter

sribasu

Joined May 10, 2017
18
Are the transmission and reception points fixed?
Yes they are. I will transmit and receive from a fixed points probably. My native is at country side and have miles spread farming fields. I will try to test it there, away from the noise of city and without interfering much of the citizens. And I won't test it from as far as 10 KMs. I will test it from 2-3 KM away for example. From my very little experience I found that the transmitter circuits on internet which claimed to transmit 50 meters, actually transmitted no more than 8-10 meters. So, I just added the buffer of few kilometers here.

My aim is simple. So far I have been making single stage transmitters, without a power amplifier. I am looking for a working circuit with a power amplifier stage. And want to toggle the power and see if the distance covered matched the equations which are available. It's just for study purpose. I am not sure why everyone is so much worried about the law.

Perhaps it's my lack of knowledge or experience. The so called 'powerful' circuits which I found so far on internet are not covering more than 50 meters i.e. my own neighbourhood. Law enforcement officers are not roaming around in every hood of my country, to find if an unauthorized RF is being transmitted or not. In my country, the first rights to citizen is Food, Clothes and Shelter; it's not uninterrupted internet or RF connectivity!!
 
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Thread Starter

sribasu

Joined May 10, 2017
18
And those same people that teach "Ethical Hacking" are VERY reluctant to teach hacking skills to people that make it obvious that they INTEND to misuse it.
Sorry sir, if my intention seemed that I'm going to exploit it. I clearly explained my intention... If you please care to read once...
 

Thread Starter

sribasu

Joined May 10, 2017
18
Perhaps there are either of these 2 problems:
  1. I am too childish to ask this 'absolutely NO' question on a public forum. There is a thin barrier between 'learning on this topic' and 'experimenting on the topic'. The several circuits on internet, which despite claiming 10 KMs doesn't cover more than 100 meters are still permitted to be used in law. And if it really covers 10 KMs, it's not expected to be available on the web. You guys are expert in the subject, hence worried about the law. I am really sorry if that's the case. Please tell me if there is a way to learn and experiment it from an authorized entity, without buying a personal band from government. I know it's too expensive for a hobbyist to afford. Going back to college and enroll for a BE in Electronics is not my option either, I take care of a family and won't make a 'Hobby project on Long Range RF transmitter', if it takes as much as getting admitted to a full time Engineering curriculum :)
  2. You guys are expert and never want a hobbyist to make such a circuit. It perhaps poses a threat to your ethics to give a circuit to a hobbyist who is by default considered a 'Hacker' of electronics and doesn't have right intentions on whatever he does in Electronics.
 
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nerdegutta

Joined Dec 15, 2009
2,681
Hi.

Keep in mind that the signals are not flat, and not going in one direction. Some of them will radiate upwards, and might interfere with airplane communication.

Just out of curiosity; What are you planning of making? A wireless microphone, with an extremely long range? :)
 

Thread Starter

sribasu

Joined May 10, 2017
18
Hi.

Keep in mind that the signals are not flat, and not going in one direction. Some of them will radiate upwards, and might interfere with airplane communication.

Just out of curiosity; What are you planning of making? A wireless microphone, with an extremely long range? :)
Thanks for your response Sir. I understand it transmits to every direction. Is there a way to prevent that? Can a unidirectional Antenna prevent it?

I want to transmit audio signal. It can be either a MP3 line or a condensor microphone. To test the transmission, I have to probably open and customize a radio set if it's 75 Mhz.
 

recklessrog

Joined May 23, 2013
985
There is a phenomenon called "ducting" in certain conditions, a low power fm signal can travel several hundred miles. BUT, what is your receiver and antenna? for instance, 100watts of power fed into an unmatched piece of wire 2m from the ground may not be heard 1mile away even with a good communications receiver and antenna, but 100 mW into a beam antenna 50ft from the ground may be picked up 20 miles away with a normal broadcast band radio. So many undefined variables. It is not only down to transmitter power.
 
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dendad

Joined Feb 20, 2016
3,840
Building and operating such an unlicensed transmitter will be illegal and so it should be.
If you want to experiment with such, get in touch with your local amateur radio organisation and they will help you learn enough to get your own licence to build your transmitter on the ham bands. Just do not pick a frequency and make a transmitter as your questions show you do not yet have the expertise to do same. A transmitter, especially a badly designed one will transmitt on many frequencies at the same time and you could well endanger lives by doing so irresponsibly.
As you can tell by most of the replies, this is something you are not to do.
Ham radio is a great hobby and is the way to legally play around with transmitters. It has given me years of enjoyment but I had to pass exams to get it. The training is not hard and most ham radio people are happy to help or point you in the right direction.
But, do not just go and make a transmitter. That is a NoNo!
 
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