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The best way to boost an RF transmitter? Answered

Hi all,

Here I have a small and simple FM transmitter based on a simple transistor oscillator, however, due its simplicity, is weak for my needs: I need to transmit the signal to a distance of about 33 feet.

Notice that I will use the transmitter in a rural location and around the building there are no other houses, so I will avoid the risk to cause interferences to other people.

Well, so I was thinking to implement an amplification stage, after the transmitter, and I ended up with the following schematic:


On the base of Q1 I have a female jack where i get the audio signal from an MP3 player.

However I have some doubts about resistors values for the amp stage: these values seems too much great to me:
I get the Transistor values (BFR36) from the datasheet: http://www15.zippyshare.com/d/nmSyQm6e/625003/BFR36.pdf

and I calculated the values here http://www.vk2zay.net/calculators/transistors/bjtBiasing.php (choosing "desing given emitter voltage")
The power supply, as shown in the schematic will be equal to 12 V DC.

But as I've said I'm not very convinced of resistors values for the amp stage, which seems too much greats. On the other hand, due to the fact that the amp stage is an emitter follower, it will just amplify the current and not the voltage: would be able to give a decent boost on the signal output? What I can expect?



Ok I haven't looked at the schematics but your options are fairly simple:

1. Put a better antenna on the transmitter and or the receiver.

2. 33 feet in open ground isn't very far there are many transmitter receiver sets that offer much further depending on what your sending.

3. 33 feet why not use a wire?

4. Increase the output power of the transmitter

5. Increase the sensitivity of the receiver.

6 Increase the height above ground of either TX RX or Both.

Hi rickharris, thank you for your reply.

1 ,2 and 3: I use this transmitter inside a building, which is placed in a rural location: I've tried to implement a longer antenna, but the rf signal also have to cross some walls inside the building.

The option 4, for me would be the better also for didactic reason :-)

If you will have some time to take a look to my schematic, you will see that the transmitter is a simple/common transistor oscillator: the rf signal is taken from its collector: I was thinking to implement an emitter follower amp, but the emitter follower will amplify the current, not the voltage: by a reading on internet (also on Instructables) someone claims that an emitter follower is enough to boost the rf signal, and someone else disagree about, claiming that a common emitter is better.

I don't know if the emitter follower will be ok for my circuit and I don't know if the values for resistors around the Q2 (the amp transistor) are of the proper values (eg I know that I can also get rid of R5 and R6, the voltage divider and that R4 would be enough).

Emitter followers act like buffers, they have high input impedance, low output impedance, and function a bit like power steering in a car. It doesn't amplify voltage, and won't amplify current unless there is a output load to drive. In other words, your antenna needs to be lower impedance so that the low voltage can drive it with power. So I think there is an impedance mismatch. Impedance mismatch is kinda like driving a bulldozer on the highway or trying to drive up a steep hill in high gear on a stick shift, or trying to power a 100W 120v light with a car battery.


Common emitters are not the best in RF applications because of this thing called the "miller effect." Basically, because common emitter amplifiers invert the signal, the capacitance between the base and collector has huge voltage changes on it over time compared to just the voltage change over time of the base. Because there is a bit of capacitance between all the junctions, that makes them less ideal for high frequency stuff. W2EAW on youtube has a good video on that.


Using a longer antenna is not better. If using a simple linearly polarized dipole antenna, calculate whatever convenient multiple of the wavelength of the frequency you are transmitting. Half wavelength and quarter wavelength are common. Also, if using a dipole antenna, make sure that both antennas are aligned so that both are in the same polarization. (aka, both are vertical, or both are horizontal, or whatever other angle, as long as both are at the same angle.)

well saidMax , NOTE akopelumenuscu I said a better antenna, that isn't the same as longer, It could be directional, or better matched to the frequency/ transmitter or receiver your using.

Resolving things like this from a pure theory position is very hard, unfortunately RF theory and practice are not always in the same place so regardless you may just have to try several options.

OP replied to you saying that he used a longer antenna, so I thought it was worth mentioning that that is not necessarily the case. Theory and practice are totally different things, I get that! Trying to understand all the workings of something from theory is like impossibly hard! :P

rickharris and -max-,

First of all I want to say "thank you" again to both of you, for the time and also the patience which I received in these nice and understandable replies!

I am totally aware of the fact that working with RF electronics could be hard and sometime unpredictable: I am so aware that in fact (also for the didactic purposes) I am disposed to do these experiments, but I'd wish to avoid wasting of time, eg: for me would be bothersome made a circuit which doesn't work at all. I will prefer to obtain a circuit which oddly works.

I want to step back for a moment, -max-, when you told me that the common emitter configuration could introduce the miller effect: I heard of the fact that this "miller effect" could reduce the bandwidth and lowering the frequency: this is obviously bad for high frequencies. But.. If the common emitter isn't good for HF, why there are a lot of circuits which rely on the common emitter? Look for example these schematics:



I think the miller effect is more of a problem at really high frequencies, like in the 100's of MHz to the GHz range. Your little FM bug may not suffer from it that greatly. Like I said though, I don't know much about FM bugs, just that I tried to build one on a breadboard when I was much younger and it did not work. Breadboards have capacitance between all the internal contacts and are not suitable for RF applications higher than 1MHz.

Ill say. How did you learn your electronics?

I'm completely self-taught, hence why I am sometimes a bit off the mark lol :)

Max: your reply was very clear and exhaustive, thank you!

However, if a longer antenna, emitter follower and common emitter aren't functional for my needs, what I can try to do to boost my circuit? I heard that for this kind of oscillator based transmitters, the best choice would be a class C amplifier:


So I attempted to implement something like this:

In this way:

Keep in mind that I don't aim to achieve a sofisticated nor an extremely perfect circuit: for me, since this is a project just for didactic and fun purposes, for me will be enough to achieve a functional circuit, also with some imperfection.

Or I just have to give up, since there is not any way to boost the transmitter?

Kind regards.

I am not an RF expert, and don't have any experiance with these FM bugs. So someone else may be able to better answer your questions. Check out W2AEW on youtube, he is very knowledgeable in RF electronics.


I notice in the schematic, what is the point of the 2 capacitors in series between the 2 amplification stages? Seems like that is leading to some weak coupling. Also, check the datasheet for the transistors you are using, and see if they have any reasonable gain at the carrier frequency you are trying to transmit at. aka see if the gain bandwidth product is good enough.


You might just consider using a different/better circuit. The FM bugs are really not that good. I would just increase the voltage to the transmitter and see what happens. If you double the transmitting power, you can increase range by 50%. To double the range, you will need to 4X the transmitting power.