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FM Transmitter?

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sjaguar13

New Member
On the FM transmitter project, would a bigger antenna and more batteries make the signal go farther? I want an FM transmitter to go about 30 feet. I was looking at those plug and play FM transmitter USB devices. It alledgedly goes 30 feet, but anything after 10 gets a lot of interferece. Would it be better to try to make that go the full 30, or just build the project listed here?
 

Ali Sajjad

New Member
A bigger antenna cannot increase the signal strenght. However if the circuit permits increased supply voltage would increase the power ouput of the antenna. Almost every small FM transmitter is capable of transmitting signals upto 30 feet. And it is not feasible to try to alter those USB FM ports. You would damage the unit. If you know the operating frequency of such a unit then you can use a low gain RF amplifier to boost the output of the transmitting part.

- Ali Sajjad
 

nerosrevenge

New Member
I haven't read the project thread you are refferancing but I have seen kits for FM Transmitters the will transmit up to 100'. I will see if I can find something on the internet and post the link for you.

Steve.
 

nerosrevenge

New Member
Ok Ali here is what I found.

I am not sure of the input type required. I found a kit from Cana kits which will transmit Hi-fi Stereo signal up to 100' in the 88MHz-100MHz range (at least according to the description). The power supply is 6vdc or 12vdc @ 20mA (which could be modified if nessecary I'm sure).

You can find it here:

https://www.escience.ca/circuitT/RENDER/0001/C9/3043/10809.html

It's from Canakits in case you need to source it elsewhere.

Now, I understand you will likely want to build this on your own (I am wanting to building one as well) I will dig around and see if I can find a schematic, perhaps you would do the same and we'll see what we can come up with?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ali Sajjad said:
A bigger antenna cannot increase the signal strenght.

Not 'quite' true! - a bigger, correctly designed, antenna would increase the signal strength considerably - at the expense of making it more directional. Depending on the polarisation of your transmissions, you can increase signal strength while keeping an omni-directional transmission pattern (for vertical polarisation) - by concentrating the signal more horizontally, and less 'up and down'. With horizontal polarisation, you can increase the strength in a particular direction, basically 'focusing' the energy one way. A gain of 10dB (10 times) is easily achievable with fairly modest aerials - obviously the size depends on the frequencies in question, and you have to accurately construct the aerial systems.

You can also use similar aerial systems at the receive end, so 10dB at each end would give 20dB (100 times) improvement in signal strength.
 

panic mode

Well-Known Member
What is it exactly you are looking for?
Schematic I've posted in https://www.electro-tech-online.com/threads/current-sink-source.4618/ was usable up to ca 300' (ca 100m) with no antena or ca 1/4 mile (400m) with 70cm piece of wire as antena. This is in open areas and with FM receiver using antena of course - but with antenas on both transmitter and receiver I was getting ca 100m regardless of location.
 
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sjaguar13

New Member
That looks pretty sweet, too. About how much does that cost to build? I'm looking for a cheap way to broadcast a signal 30 feet. I'm also curious, though, about what makes the signal go farther. I alway thought it was the antenna size. I thought if I had a 45' antenna I would be able to send the signal several miles. Ali said the antenna doesn't matter, I would need to amplify the output. Nerosrevenge said the kit's power supply could be modified if needed. I'm still a little confused.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
sjaguar13 said:
I thought if I had a 45' antenna I would be able to send the signal several miles. quote]

Aerials have to be tuned to the frequency in use, a 45' aerial at VHF frequencies would most probably be useless. Most aerial systems are based on dipoles or quarter wave whips. A yagi aerial uses a dipole with a reflector and various directors to make it more directional. Vertically polarised aerials usually use variations on stacked dipoles.

What actually are you wanting to transmit?, voice, music, data?. And what frequency are you using?, and is it in a particularly difficult environment?.

The normal simple VHF 'bug' transmitters easily achieve more than 30' without trying.
 

nerosrevenge

New Member
I wasn't implying (at least not intentionaly) that modifying the power supply would increase the range of the unit (that's out of my league). I meant for convenience more that anything.

For example, the unit I am intending to build will be set into a sealed box to be use in my car with my portable cd/mp3 player. So I don't want anything that requires a plug in for power and I want it to be small. I don't really need anything that will transmit more than 3' but it doesn't make sense to me to build a unit with those limitations. If I can do it for more or less the same cost and get 100' or more out of it, why wouldn't I?

I am going now to check out the unit that Panic Mode posted and see if that will work for me.

;)

Bye for now!

Ok, I'm back. I have a question regarding modifying the input of this circuit:

I need the input to be mini jack (1/8"). How complicated would that be? If I could do it in stereo that would be great, if not Mono is fine.

Thanks again for the help everyone!

:D
 

Ali Sajjad

New Member
Not 'quite' true! - a bigger, correctly designed, antenna would increase the signal strength considerably - at the expense of making it more directional. Depending on the polarisation of your transmissions, you can increase signal strength while keeping an omni-directional transmission pattern (for vertical polarisation) - by concentrating the signal more horizontally, and less 'up and down'. >>

Yes Nigel, this correct. But for a home made Fm handy transmitter, i dont think that an antenna with directors can be used. Even a simple yagi would be a an over-engineering. Aerial antennas give best result when their lenght is 1/4 of the wavelengh that is being trnasmitted, neither the larger nor small would give better result.

- Ali Sajjad
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ali Sajjad said:
Yes Nigel, this correct. But for a home made Fm handy transmitter, i dont think that an antenna with directors can be used. Even a simple yagi would be a an over-engineering. Aerial antennas give best result when their lenght is 1/4 of the wavelengh that is being trnasmitted, neither the larger nor small would give better result.

Hi Ali,

Again, not 'quite' correct, a 1/4 whip is actually half a dipole, with the other half 'reflected' in the metal the whip is sat on. It's also the least efficient (correctly tuned) aerial there is (which is why a dipole is used as the reference for aerial gains). Correctly designed aerials, usually of combinations of tuned dipoles, or tuned dipoles and reflectors/directors, far outperform a simple 1/4 wave whip or 1/2 wave dipole.

Other size aerials can also perform better, a 5/8 whip outperforms a 1/4 wave one considerable, but requires a loading coil to make it function correctly - they are often used as mobile aerials on the 2M amateur band.

You can connect an aerial to a small bug if you need to, it's simply a matter of impedance matching.

Anyway, in the event it's not required, now we get more of the story it's only got to work inside a car!. Two bean cans and a piece of string would do it :lol:

But back to his last post, he would need to add a stereo encoder to transmit stereo with it, this would be a great deal more complicted than the transmitter circuit. If you want to, there are loads of circuits for stereo encoders and higher power FM transmitters on the net, all you need to do is google for them.

To input an audio signal, remove R1 and the microphone, and input the signal to C1 and the negative rail (ground connection). You will probably have to attenuate the incoming signal, as it's expecting a microphone level. You could also add a resistor in the emitter of the first transistor to reduce it's gain.
 

nerosrevenge

New Member
Wow, the farther I go the deeper I get! :eek:

I am trying to souce parts for Panic Modes' schematic posted above. He has some of the Caps listed with "nF". I can't find anything but "pF" and "uF". I'm assuming it's a variation to replace several decimal places is that correct? If so how do I convert from either "uF or "pF" to"nF".

Next, as far as attenuating the input is concerned it would be line level from the cd/mp3 player. So what resistor would you recomend?

I think for this one I will remain with mono signal, that whole "K.I.S.S." theory!

Thanks again!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
nerosrevenge said:
Wow, the farther I go the deeper I get! :eek:

I am trying to souce parts for Panic Modes' schematic posted above. He has some of the Caps listed with "nF". I can't find anything but "pF" and "uF". I'm assuming it's a variation to replace several decimal places is that correct? If so how do I convert from either "uF or "pF" to"nF".

1nF is 1000pF, or 0.001uF - 'nano' is the prefix for 10 to the power of -9.

Next, as far as attenuating the input is concerned it would be line level from the cd/mp3 player. So what resistor would you recomend?

I would try about 100/1 or so to start, and see how it works. Use a 100K series resistor and a 1K to ground, to form an attenuator.
 

panic mode

Well-Known Member
Why don't you list some of the capacitors you have on hand and
I will make some recommendations. I am pretty sure you can
find all parts in one old transistor radio (this is where I used
to get parts for my first transmitter).

It is very robust and stable circuit. It will work with just
about any parts and quite big changes of all values. Just try to
keep everything small and short if possible - at least in the
osc. circuit (T2, L, C6 and C7).

If all you care is 30feet range, don't bother with antenas.
Everything will work like charm. If you want to add antena,
use small capacitor such as 1pF to connect to T2 collector
and the other should go to small telescopic antena or piece
of wire ca 70cm long.

If you want to put jack as audio input (maybe to transmit
music from your walkman or discman to your car radio)
I would recomend to use small potentiometer in front of C1
(5 or 10k will do fine).
Actually if you want to use headphones signal (which is already
few hundred mV) to modulate the transmitter, you could
probably remove all parts which are on the left side of C3
(audio preamplifier).
Existing preamp circuit is VERY sensitive. Even with crapy
microphones or tiny speaker abused as microphone, this is
sensitive enough to listen to faint whisper or clock ticks
accross room (and believe me, I am not talking about
Coocoo clock) .
 

nerosrevenge

New Member
Let just start by saying you are all a great bunch of chaps! It's been awhile since I have found a forum as helpful as this one. Thanks for that!

I would try about 100/1 or so to start, and see how it works. Use a 100K series resistor and a 1K to ground, to form an attenuator.

I understand the 100k Resistor, but can you please clarify what "1k to ground" means? I just add a 1k resistor to the ground somewhere in the circuit? Thanks for the Capacitor value equivalents! :)

If you want to put jack as audio input (maybe to transmit
music from your walkman or discman to your car radio)
I would recomend to use small potentiometer in front of C1
(5 or 10k will do fine).
Actually if you want to use headphones signal (which is already
few hundred mV) to modulate the transmitter, you could
probably remove all parts which are on the left side of C3
(audio preamplifier).

I had intended to use a 1/8" audio jack as input. As far as I understand it the issue with signal attenuation is the fact that the line level output on the cd/mp3 player will overload the circuit (to what effect I am still unclear on) is that correct? If so using the headphone output (which is regulated by the onboard volume control of the cd/mp3 player) will alleviate the problem? Correct?

I'd rather not use a potentiometer on the Transmitter because I don't want to have to worry about the posibility of continuous adjustment issues on the unit (beyond frequency of course) for volume. It makes more sense to me to have the unit transmit at whatever level is necessary for the car radio to receive it undistorted and adjust listening volume on the radio. Based on that I think I would rather use the suggested circuit modifications to achieve this.

As far as parts are concerned I don't really have anything laying around I can salvage parts from (at least I don't think I do :)) I had just intended to pick up the parts new and go from there.

This may sound "newbie" but I am quite excited at the prospect of actually being able to do this!

Thanks again for all the help and advice.

Steve.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
nerosrevenge said:
I understand the 100k Resistor, but can you please clarify what "1k to ground" means? I just add a 1k resistor to the ground somewhere in the circuit?

It's a simple resistive attenuator, the circuit looks like this:
 

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nerosrevenge

New Member
Ok. That's what I thought it was. Seems like I have what I need to build at this point. I'll post pics and such soon!

Thanks again for all the help.
Steve.
 

nerosrevenge

New Member
Ok I just got back from my local electronics dealer. I was able to find all of the parts I needed with the exception of the following:

BC109C Transistor I was able to get BC107b.
4.7pF Capacitor. No luck got a 4pF and a 5pF. Will either do?

Also I want to add an on/off toggle switch and an LED to indicate switch position. Green being on. I assume I will have to use a resistor ahead of the LED as a 9v battery will likely pop it. Here's the problem, I don't know the rating of the LED. I am pretty sure I need that to calculate the resistor I require correct? Any suggestions (short of going back to the store and looking it up among the several hundred LED's they have)? :shock:

I also got a proto board to make sure it all works.

I do have another question though. How do I tune the transmiting frequency of the circuit?

Steve.
 

panic mode

Well-Known Member
No problem 8)

You can use BC107B. It is standard NPN transistor that will
work just fine. The letter B after number indicates gain.
A is medium, B is high and C is very high.

As for osc. feedback capacitor, you can use anything from 2 to 8pF.
It will work just fine.
 
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