Query regarding the use of multiple FM radio transmitters in the same space.

[boniface95]

I am quite new to the use of Radio Transmitters, so please forgive my relatively Lay terminology and understanding. What i'm looking to do is install several radio transmitters, please see the link here.

I'm working on an istallation in a Museum and I want to use 6 at the same time, all transmitting on different frequencies. The radios will also be in this same space, all tuned to the different respective frequencies.

My question is, will it work? Will I be able to send 6 different streams of audio on 6 different frequencies, all at the same time within the same space?

 

[19wrc333]

Hi,

So you want to put 6 FM transmitters that operate in the frequency of 76 to 108 MHz putting out 7 watt on a small telescopic antenna all within a small space.
It's a bit hard to say if it will or won't work.

The first thing to remark is that I don't think that your installation is legal. You're not allowed to transmit on those frequencies with that kind of power. This should be a few 100 milliwatt at most, not 7000 milliwatt.
Even on that small antenna, people will receive your transmission for a few km's.
The power is too high for such a small venue. The transmitters will also likely interfere with each other.
So no I would not recommend this at all.

 

[boniface95]

Hi,

So you want to put 6 FM transmitters that operate in the frequency of 76 to 108 MHz putting out 7 watt on a small telescopic antenna all within a small space.
It's a bit hard to say if it will or won't work.

The first thing to remark is that I don't think that your installation is legal. You're not allowed to transmit on those frequencies with that kind of power. This should be a few 100 milliwatt at most, not 7000 milliwatt.
Even on that small antenna, people will receive your transmission for a few km's.
The power is too high for such a small venue. The transmitters will also likely interfere with each other.
So no I would not recommend this at all.

Thanks. Yes, after some deeper research i realised i'll need to have a set of transmitters that are less than 1 watt power, so that the signal wouldn't reach outside of the museum walls.

Any recommendations as to where I could get something like this?

 

[19wrc333]

Thanks. Yes, after some deeper research i realised i'll need to have a set of transmitters that are less than 1 watt power, so that the signal wouldn't reach outside of the museum walls.

Any recommendations as to where I could get something like this?

Personally I would try a shop for musical instruments. Musicians nowadays also use a lot of wireless setups because it is so much easier. I'm sure that there's also products that specialise on museums etc.. Eg. for people walking around with a headphone through the museum and get an audio tour that way. But I suspect that will be sold at a high premium. I've seen it offered before second hand on online platforms and they wanted many thousands of euro's for it.

 

[boniface95]

yeah, now i'm looking at something like this , with each transmitter being hooked up to an output of a soundcard. I just wonder if having the transmitters all so close to eachother would work ....?

 

[19wrc333]

yeah, now i'm looking at something like this , with each transmitter being hooked up to an output of a soundcard. I just wonder if having the transmitters all so close to eachother would work ....?

That's pretty much impossible to answer without knowing the details of the transmitters and receivers. This is why I mentioned the music shop as they could probably give you proper advice on what product would suit your needs.
It certainly is possible to get it to work.

 

[brandon7861]

What about carrier current? Could stay legal that way and cover the entire building.

 

[boniface95]

The idea is that the transmission needs to be on FM - the effect of 'hacking' into the already existing frequencies is central to the piece. As far as i'm aware, if the transmission power is lower than 0,5W, it is legal, since this would not interfere outside of the museum walls..

 

[brandon7861]

If blocking out an existing station is central to the museum piece, why 6 of them? That doesn't sound like a demonstration, sounds more like the museum doesn't like whats on those 6 stations.

 

[BC Coyote]

the effect of 'hacking' into the already existing frequencies is central to the piece

Hate to rain on the parade, but that is flat out illegal! Intentionally blocking signals of other radio stations is not permitted, it doesn't matter if it's part of an art instillation or within the walls of a public building. Doesn't matter if it's within part 15 limits, It's still illegal as heck.

Running six transmitters at part 15 power levels within the walls of a building is fine, but you HAVE to operate on clear frequencies.

Also be aware that in the US at least part 15 rules on FM work out to something like 10 nanowatts. That's it. If you can hear the transmitter more than 200 feet away you are over the part 15 power limit. On AM it is something like 100 mW into a 3 meter antenna. Part 15 on AM can cover a city block or two so it has better coverage than part 15 FM does.

 

[AudioShockwav]

It's a violation if you are blocking a licensed FM broadcast station, we can run 100 watts ERP on a LPFM station here in the us but it still requires a license, and at the moment they are not accepting new applications.


73
Jeff

 

[AudioShockwav]

I noticed that the OP is in Germany, I have no idea what the legality is running low power FM.

73
Jeff

 

[SuperLid]

I am quite new to the use of Radio Transmitters, so please forgive my relatively Lay terminology and understanding. What i'm looking to do is install several radio transmitters, please see the link here.

I'm working on an istallation in a Museum and I want to use 6 at the same time, all transmitting on different frequencies. The radios will also be in this same space, all tuned to the different respective frequencies.

My question is, will it work? Will I be able to send 6 different streams of audio on 6 different frequencies, all at the same time within the same space?

This is done all day, every day. Look at the transmitters that are used for real estate sales, drive in movies, halloween and xmas light displays. I have one of these things in my storage that was used for real estate. Figure out the frequencies and power level for license free operation and then you can figure out what you want. Here are some examples.

Real estate radio and talking sign - Decade Transmitters Inc

www.decadetransmitters.com

HOME | Chezradio Procaster AM and FM Transmitters, Toromto, Canada

Chezradio low power AM and FM transmitters. Legal to use without a broadcast license. Applications: gyms, auditoriums, churches, schools, businesses, real estate, arenas and stadiums, drive-in theatres, assisted listening, special events, emergency traffic control, campgrounds and many other uses.

www.chezradio.com

SL

 

[brandon7861]

This is done all day, every day. Look at the transmitters that are used for real estate sales, drive in movies, halloween and xmas light displays. I have one of these things in my storage that was used for real estate. Figure out the frequencies and power level for license free operation and then you can figure out what you want. Here are some examples.

Real estate radio and talking sign - Decade Transmitters Inc

www.decadetransmitters.com

HOME | Chezradio Procaster AM and FM Transmitters, Toromto, Canada

Chezradio low power AM and FM transmitters. Legal to use without a broadcast license. Applications: gyms, auditoriums, churches, schools, businesses, real estate, arenas and stadiums, drive-in theatres, assisted listening, special events, emergency traffic control, campgrounds and many other uses.

www.chezradio.com

SL

The procaster is an interesting option, but I doubt it is strong enough to blank out local stations throughout a whole museum. Might need one (6) of those 15w ebay transmitters. I used to play my friend's music in the jail parking lot, and I did that over the local rock station so it came through the intercom radio system (it had 4 local stations chosen by the jail). Not everyone had walkmans, otherwise I would have picked dead space.

The poster thing probably wont work. They are backscatter devices. You need to be in the presence of a strong local station for it to work. The poster has an antenna resonant to the local stations frequency, and across its feed point is a mosfet that shorts the antenna out (making it anti-resonant) at an audible rate. The local stations signal reflecting off the poster antenna is therefore modulated. You need to be close enough to the poster to read it to hear it though. The only benefit is that its totally legal since you are not transmitting any RF, just reflecting someone else's at an audio rate.