I'd like to take an audio signal from an MP3 player and transmit it on multiple FM frequncies simultaneously (for example 88-95 MHz). I've looked at FM transmitter circuits that do this but it appears they all do it for a specific center frequency. How can I transmit the same signal on multiple frequencies at the same time such that I can play a song and have my handheld FM radio pick it up on 88mhz, 95mhz, etc?

Side Note: I won't be breaking FCC or other rules as I'll be below the FCC part 15 power requirements. It'll be low power and shortrange so interference won't be an issue with anyone or anything. I know it's not best practice but something I'd really like to do. Any advice on how this can be done is appreciated, thanks in advance!

 

The only feasible way I can think to do this is to split the signal and send it to multiple transmitters with center frequencies spaced at certain intervals throughout the FM range, but that's a lot of transmitters...I'm sure there is a more efficient way to do it...


FM is preferred but even if it was on the 200khz band, I'm more curious about how it can be done

 

Why can't you just tune the radio to the frequency of one transmitter ?

Les.

 

Mehtod 1:
You can do this with the help of 555 timer ic and 4017 ic circuit and colpitts oscillator using veractor diode.
Mehtod 2:
For example you need four channel (88, 92,96,100mhz) you need four singal transistor fm transmitter,
Mehtod 3:
You can also use car modulator for each channel.

 

FM transmitters that actually meet FCC Part 15 (U.S. 47CFR §15.239) are fairly rare, but even those that don't are quite cheap.

A center-fed,1/2-wave, linear dipole transmit antenna produces the maximum legal FCC field of 250 µV/m at any distance 3 meters away from it when driven with only 11.43... nW of r-f power (0.000 000 011 43... watts).

Using a separate transmitter/antenna per FM channel, probably one could create a multi-channel FM system for less than $50. But if those transmit systems are physically located together, there could be spurious signals radiated in the r-f spectrum that may cause problems for the operator and/or other radio services, and possibly come to the attention of the FCC.

 

This cannot be done with one transmitter. You will need a separate transmitter for each frequency you want to receive. If you choose freqs adjacent to one another, your receivers will have problems with co-channel interference. As @Broadcast Engr retired stated, your problems will multiply if you co-locate your antennas.

Perhaps if you were to tell us exactly what you are trying to achieve, better options and workable solutions could be determined.

 

"Perhaps if you were to tell us exactly what you are trying to achieve, better options and workable solutions could be determined."

I'd like to build something that I can plug my phone into that will allow me to play a song on my car radio on a range of FM stations.
I thought it would be cool, a good learning experience, and something fun.

Ideal end result: I'd like to be driving down the road and tell my passenger, tune in the radio to any station from 88 to 95mhz (or whatever) and have it be playing my song. From what I've seen it's easy to buy/make a FM transmitter on a single frequncey, but I haven't seen someone transmit on a number of different bands. I'll build a PCB with 100 different transmitters if that's what it takes. Could I just tune to one station? Yes, but that's not really interesting or challenging

 

FM transmitters that actually meet FCC Part 15 (U.S. 47CFR §15.239) are fairly rare, but even those that don't are quite cheap.

Using a separate transmitter/antenna per FM channel, probably one could create a multi-channel FM system for less than $50.

Do you have a suggested circuit for an FM transmitter? I've found a bunch are of circuits online but I have no experience on what might be a good choice, especially if I'm going to be building a few dozen.

One option I was thinking is to have my signal split up and multiply each split by a different carrier signal to get it to the desired frequency. I'm guessing there's a good reason why I can't/shouldn't do this but I don't know the reason. Thanks again.

 

It's a waste of time. Yours and ours. You will never be successful at this. It could be done. But it is really stupid. Go bluetooth. Play on.

 

It's a waste of time. Yours and ours. You will never be successful at this. It could be done. But it is really stupid. Go bluetooth. Play on.

I'm sorry you feel that way. Right now it's a reason for me to dabble ever so slightly in RF and a reason to get out of bed in the morning. All my life everyone has been telling me: that's not possible, you can't do that, you can't become an engineer, you can't build that, and all my life I've been proving them wrong. This is what I'm trying to do, if anyone including you wants to provide insight that would help I'll gladly listen. Thanks for your time.

 

"Perhaps if you were to tell us exactly what you are trying to achieve, better options and workable solutions could be determined."

I'd like to build something that I can plug my phone into that will allow me to play a song on my car radio on a range of FM stations.
I thought it would be cool, a good learning experience, and something fun.

Ideal end result: I'd like to be driving down the road and tell my passenger, tune in the radio to any station from 88 to 95mhz (or whatever) and have it be playing my song. From what I've seen it's easy to buy/make a FM transmitter on a single frequncey, but I haven't seen someone transmit on a number of different bands. I'll build a PCB with 100 different transmitters if that's what it takes. Could I just tune to one station? Yes, but that's not really interesting or challenging

So you would only be listening to one station at a time, right?

Then why not put a selector on your gadget that let's you choose which frequency it transmits on. You can then have as many frequencies as you want. You can buy FM modulators that do exactly that, usually with three to eight stations that are hard coded based on the assumption that, in any given area, at least one of them will be available. These have been around for decades and take any analog audio source as input. I'm sure some exist that interface directly to most smartphones.

You could even set it up so that they are programmable and you then have a control that lets you adjust the frequency to one you like and then set it as one of your one-button stations.

 

This cannot be done with one transmitter. You will need a separate transmitter for each frequency you want to receive.

I don't think this is true, at least in principle. If you had a transmitter with sufficient bandwidth, then you could generate the entire FM broadcast band into a single waveform and transmit it via a single transmitter tuned to the middle of the band.

This CAN be done in the AM broadcast band and lower HF ham bands.

 

Don Lancaster did a great article on this in American radio history 1995 edition on Page 46-47. He suggest using a "Fourier series comb transistor" approach might allow one to isolate harmonics and amplify them. Wicked smart guy, with such advances since 1995 I wonder if that's still the best approach. Just sharing what I've found so far.

http://www.americanradiohistory.com/Archive-Radio-Electronics/90s/1995/EN-1995-09.pdf

 

You need a better reason to get out of bed. Do you like coffee? How bout sex. Do you still wake up with a woody? Lucky bastard.

To understand rf circuits.......you HAVE to study in an ORDERED manner. From the beginning. You HAVE to understand the first concept.......before you can understand the second.

Believe me..........I am an expert on stupid. Much more experience than you at it.

Ask anyone here.

 

I don't think this is true, at least in principle. If you had a transmitter with sufficient bandwidth, then you could generate the entire FM broadcast band into a single waveform and transmit it via a single transmitter tuned to the middle of the band.

This CAN be done in the AM broadcast band and lower HF ham bands.

Well, you can transmit a signal over multiple channels, but your deviation would have to be so high that no receiver could interpet the signal.

Actually, you can't do this in the AM band, because of the modulation scheme. The bandwidth of an AM broadcast signal is twice the maximum audio frequency, which is around 4.5kHz max. The legal BW of an AM broadcast channel is 10kHz. If you had a soprano singing at 15kHz, the sound would only appear on the channels above and below your TX channel - or, the audio information is spread over adjacent channels and no single channel gets enough of the signal to make much sense of the transmission.

 

Well, you can transmit a signal over multiple channels, but your deviation would have to be so high that no receiver could interpet the signal.

Actually, you can't do this in the AM band, because of the modulation scheme. The bandwidth of an AM broadcast signal is twice the maximum audio frequency, which is around 4.5kHz max. The legal BW of an AM broadcast channel is 10kHz. If you had a soprano singing at 15kHz, the sound would only appear on the channels above and below your TX channel - or, the audio information is spread over adjacent channels and no single channel gets enough of the signal to make much sense of the transmission.

So what you are saying is that if someone samples and captures a portion of the RF spectrum centered at, say, 1000 kHz with enough bandwidth to capture, say a 400 kHz wide signal, that it is not possible for them to turn around and retransmit that signal centered at 1000 kHz and have the resulting signal be a reasonably faithful representation of what was in that 400 kHz portion of the spectrum?

Because, if they can, then what is preventing them from constructing that same signal in software and transmitting that, instead?

Does it matter what the signals are that are in that 400 kHz? That there might be ten different stations transmitting 10 kHz wide signals at ten different center frequencies within that band?

Does it matter what modulation scheme any of those stations are using, provided their total signal bandwidth is within the portion of the spectrum that was captured?

A pretty common demonstration for software defined radios is to capture most, if not all, of the AM broadcast band, which is only a bit over 1 MHz wide, as a single data stream, usually IQ sampled at something like 4 MSa/s. This one waveform is then demodulated offline in software by entering a center frequency. The software knows that it is AM modulated and what the channel bandwidth is, but it's straightforward to have those be parameters that are entered and the modulation scheme could be anything, including spread spectrum.

 

Indeed.... a DSPer should be able to generate I/Q modulation sets to multiplex and transmit a wideband multi channel signal.

A reverse SDR RCer. Multiplexing a carrier.

But it would wipe out any nearby FM RCers. What might be better is a short range device made for these type of applications.

Maybe someone will come up with something.

Edit: I would think that there are already DSP algorithms for this.

 

Indeed.... a DSPer should be able to generate I/Q modulation sets to multiplex and transmit a wideband multi channel signal.

A reverse SDR RCer. Multiplexing a carrier.

But it would wipe out any nearby FM RCers. What might be better is a short range device made for these type of applications.

Maybe someone will come up with something.

They have, it's called: http://ecee.colorado.edu/~ecen4242/marko/UWB/UWB/Ultra-Wideband_Technology.pdf

 

They have, it's called: http://ecee.colorado.edu/~ecen4242/marko/UWB/UWB/Ultra-Wideband_Technology.pdf

UWB is usually (I think) referring to impulse-based radio, also known as time-hop spread spectrum although I don't think that term is very widely used.

 

nsaspook.........that hilarious. When I first read this thread I thought of spark gap. Thanks for the link.