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Author Topic: FM Transmission  (Read 1159 times)

Henry Cohen

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Re: FM Transmission
« Reply #10 on: May 21, 2020, 04:46:34 pm »

I am far from being an expert, but as I understand the Part 15 exceptions that allow extremely low power FM broadcast band transmissions, it's not the transmit power that is the limiting factor, but the received power on a signal strength meter that is limited. That signal strength is going to be the result of both the output power of the transmitter and the antenna design. So a 1/4 watt transmitter with a particular antenna might work, but a different antenna might exceed the limitations.

Not the received power, but rather field strength. Specifically, for operations in 88-108 MHz, maximum permissible field strength is 250uV/meter @ 3 meters. This is about -50dBm EIRP. See 15.239(b).


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. . .I want to do what is legal, not hope that I don't get caught.

This is hopefully the attitude of all professionals involved with providing products and services.
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Henry Cohen

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Jonathan Johnson

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Re: FM Transmission
« Reply #11 on: May 21, 2020, 04:56:24 pm »

Not the received power, but rather field strength. Specifically, for operations in 88-108 MHz, maximum permissible field strength is 250uV/meter @ 3 meters. This is about -50dBm EIRP. See 15.239(b).

Thanks, Henry. I'm glad to see an expert weigh in.

I've updated my post to say "field strength" rather than "received power." As a non-expert, I don't quite understand the difference in the two terms.
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David Sturzenbecher

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Re: FM Transmission
« Reply #12 on: May 21, 2020, 05:52:54 pm »


However, AM transmission may be an option. The rules for low power transmission in the AM broadcast radio band are different, apparently based on transmitter output power rather than radiated power, so it's easier to achieve compliance.

One well-known brand of AM transmitter that claims to allow legal, unlicensed broadcast is the InfOspot (formerly "Talking House") AM transmitter. I just ordered one of these for my church, because I don't want to take chances -- I want to do what is legal, not hope that I don't get caught. Maybe I'm still taking a chance, but I know the brand has been around for years and the product does have an FCC ID.

http://www.theradiosource.com/products/infospot.htm

Any idea how much that setup costs?
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Henry Cohen

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Re: FM Transmission
« Reply #13 on: May 21, 2020, 05:57:26 pm »

Thanks, Henry. I'm glad to see an expert weigh in.

I've updated my post to say "field strength" rather than "received power." As a non-expert, I don't quite understand the difference in the two terms.

Received power is in the eye, or rather the front end, of the beholding receiver. IOW, the power level at the first detector stage in the receiver is highly dependent on the antenna type and tuning, the connection and coax between the antenna and the receiver frame, any filtering and pre-amp stages, and the calibration of the detector. It's full of variables. Whereas the field strength is the actual physical energy present caused by the electric field as a vector; volts per meter. All properly calibrated field strength meters will indicate the same value regardless of make and model (accuracy notwithstanding). Since power is a function of energy and current, and there's no current until a circuit is established, only the energy can exist in the free field. (Though I suppose the excitation of air and water molecules might constitute a circuit with heat resulting.)
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Henry Cohen

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Jason Glass

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Re: FM Transmission
« Reply #14 on: May 21, 2020, 06:36:02 pm »

I am far from being an expert, but as I understand the Part 15 exceptions that allow extremely low power FM broadcast band transmissions, it's not the transmit power that is the limiting factor, but the field strength as measured by a signal strength meter that is limited. That field strength is going to be the result of both the output power of the transmitter and the antenna design. So a sub-watt transmitter with a particular antenna might work, but a different antenna might exceed the limitations.

Further, the transmitting device is required to have an FCC ID. Most of those transmitters from Asian manufacturers and available all over the web don't have a valid FCC ID. You probably don't have access to a calibrated RF signal strength meter to ensure you are in compliance.

However, AM transmission may be an option. The rules for low power transmission in the AM broadcast radio band are different, apparently based on transmitter output power rather than radiated power, so it's easier to achieve compliance.

One well-known brand of AM transmitter that claims to allow legal, unlicensed broadcast is the InfOspot (formerly "Talking House") AM transmitter. I just ordered one of these for my church, because I don't want to take chances -- I want to do what is legal, not hope that I don't get caught. Maybe I'm still taking a chance, but I know the brand has been around for years and the product does have an FCC ID.

http://www.theradiosource.com/products/infospot.htm

100% nailed it.

This is what it means to be a good citizen and a good steward of your flock.  You are bound to give your best effort, not your most convenient.

Further worth mentioning, the rules mandate that the antenna that is supplied with the transmitter be used, that that antenna be permanently attached or connected through a unique connector, directly to the transmitter housing.  Monkeying with transmission line cables to remote antennas is strictly prohibited for all unlicensed use.

Part 73 LPFM license use is a completely different matter, but I would venture a guess that not a single church in the United States right now has a proper STA that was granted under the auspices of their existing part 73 broadcast license.
« Last Edit: May 22, 2020, 07:52:12 am by Jason Glass »
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Jonathan Johnson

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Re: FM Transmission
« Reply #15 on: May 22, 2020, 01:24:34 am »

Any idea how much that setup costs?

With a basic wire antenna, it was around $130 plus tax and shipping. There is also an external remote-mount antenna option (mfgr claims it is alao FCC approved) that supposedly extends the range, but I don't know what the price of that is.

I haven't received it yet, so can't supply a review.
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MattLeonard

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Re: FM Transmission
« Reply #16 on: June 21, 2020, 02:40:58 pm »

I'm not advocating for anything here - just sharing my experience.

I do a lot of mobile/off-grid sound reinforcement for demonstrations/marches/protests. With Covid - "car caravans" have become a big thing - basically parades of cars, allowing people to have physical distancing. I've seen some groups try to have a shared program by asking drivers to tune into a Zoom (or similar platform) webstream / conference call - which can allow for music, speeches, MC's, instructions etc to go throughout a large crowd. Most modern cars can have a bluetooth connection into a car stereo. Not amazing audio quality - but usable.

While not legal (as others have shared) - low-power FM transmitters are readily available on Ebay or online sites. There are a few people in my city with 30-watt units who have been creating a similar option for car caravans. Depending heavily on antenna choice and location, this can cover 4-5 several square blocks in a dense downtown area of high rises, or a maybe few miles if up on a good hill or with clear line of sight.

In addition, this has been used for distributed sound systems throughout large crowds. Many large demonstrations don't have significant production budgets (much less a permit where a proper production setup could even happen!), so I'll often provide a main PA for a stage,  while folks take a feed for an FM broadcast, and strategically place speakers (often mounted on trucks, cars, bicycles etc) throughout the crowd to significantly boost audience coverage.

Yes, all sorts of coherence/timing issues arise - but it has been a successful low-cost option using commonly-found community resources (i.e. a JBL Eon on a bike trailer or strapped to a rooftop car rack) and a very workable option.
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Daniel Levi

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Re: FM Transmission
« Reply #17 on: June 21, 2020, 03:59:37 pm »

The thing you need to be aware with cheapo Chinese wireless equipment is not only is it completely illegal but you have no idea what other spurious emissions it is kicking out, if you can't afford to do it "by the book" then you are hardly going to be able to afford to have the equipment to make sure it is to spec. And equipment that is not to spec can interfere with users even in different wave bands, cause issues for emergency service radio communications and it will not end well.

Plus if the FCC catch you then you will really be in the turd.
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Jason Glass

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Re: FM Transmission
« Reply #18 on: June 21, 2020, 05:52:20 pm »

I'm not advocating for anything here - just sharing my experience.

I do a lot of mobile/off-grid sound reinforcement for demonstrations/marches/protests. With Covid - "car caravans" have become a big thing - basically parades of cars, allowing people to have physical distancing. I've seen some groups try to have a shared program by asking drivers to tune into a Zoom (or similar platform) webstream / conference call - which can allow for music, speeches, MC's, instructions etc to go throughout a large crowd. Most modern cars can have a bluetooth connection into a car stereo. Not amazing audio quality - but usable.

While not legal (as others have shared) - low-power FM transmitters are readily available on Ebay or online sites. There are a few people in my city with 30-watt units who have been creating a similar option for car caravans. Depending heavily on antenna choice and location, this can cover 4-5 several square blocks in a dense downtown area of high rises, or a maybe few miles if up on a good hill or with clear line of sight.

In addition, this has been used for distributed sound systems throughout large crowds. Many large demonstrations don't have significant production budgets (much less a permit where a proper production setup could even happen!), so I'll often provide a main PA for a stage,  while folks take a feed for an FM broadcast, and strategically place speakers (often mounted on trucks, cars, bicycles etc) throughout the crowd to significantly boost audience coverage.

Yes, all sorts of coherence/timing issues arise - but it has been a successful low-cost option using commonly-found community resources (i.e. a JBL Eon on a bike trailer or strapped to a rooftop car rack) and a very workable option.

Matt,

Well, yes you are advocating for illegal and unethical operation when you call them "...a very workable option."  They are not.  Not cool.

As a legitimate business I must attempt to compete, operating 100% legally, with the requisite knowledge and skills to do so, with everyone who you're telling that no knowledge and no care is necessary, and that operating at orders of magnitude more than legal limits, is "a very workable option."  ORDERS OF MAGNITUDE.  You're advocating 30 Watts when 11.8 microwatts is the legal limit.  You must do the math to get the uW limit from a field strength of 250uV/m @ 3m, but that's the requisite knowledge that I mentioned.  Speaking of requisite skills and knowledge, 4-5 blocks is absurd.  A properly deployed 30W FM station on a hill can cover tens of miles of radius.

I'll let you know what that means in real hardship terms right now: I've spent a lifetime studying and working to become knowledgeable and valued in our industry.  Since 100% legal FM TX equipment has been backordered for months, my business has been out of business since March 12, with functional transmitting equipment sitting on my shelves that clients aren't eligible to receive licenses or STAs for, while competition has been doing exactly what you advocate to turn a profit.  For tens of thousands of clients and events.  They will continue to maintain their clients after my legal equipment finally arrives, because their clients are happy with their performance and don't wish to be instructed or to be hassled by the ridiculously low legal limits and the performance issues caused by them.  For example they don't want to know that their $149.00, 7 Watt eBay/Amazon imported transmitter, with a "legal" FCC ID number, is totally illegal in multiple specifications and that the ID number was obtained through fraudulent means and filings.  And when I tell them that, they never call me again.  But their station stays up and running.

This is the House of Worship forum, making your advice even more upsetting.

Not. Cool. At. All.

Caleb Dueck

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Re: FM Transmission
« Reply #19 on: June 21, 2020, 07:51:27 pm »

Matt,

Well, yes you are advocating for illegal and unethical operation when you call them "...a very workable option."  They are not.  Not cool.

As a legitimate business I must attempt to compete, operating 100% legally, with the requisite knowledge and skills to do so, with everyone who you're telling that no knowledge and no care is necessary, and that operating at orders of magnitude more than legal limits, is "a very workable option."  ORDERS OF MAGNITUDE.  You're advocating 30 Watts when 11.8 microwatts is the legal limit.  You must do the math to get the uW limit from a field strength of 250uV/m @ 3m, but that's the requisite knowledge that I mentioned.  Speaking of requisite skills and knowledge, 4-5 blocks is absurd.  A properly deployed 30W FM station on a hill can cover tens of miles of radius.

I'll let you know what that means in real hardship terms right now: I've spent a lifetime studying and working to become knowledgeable and valued in our industry.  Since 100% legal FM TX equipment has been backordered for months, my business has been out of business since March 12, with functional transmitting equipment sitting on my shelves that clients aren't eligible to receive licenses or STAs for, while competition has been doing exactly what you advocate to turn a profit.  For tens of thousands of clients and events.  They will continue to maintain their clients after my legal equipment finally arrives, because their clients are happy with their performance and don't wish to be instructed or to be hassled by the ridiculously low legal limits and the performance issues caused by them.  For example they don't want to know that their $149.00, 7 Watt eBay/Amazon imported transmitter, with a "legal" FCC ID number, is totally illegal in multiple specifications and that the ID number was obtained through fraudulent means and filings.  And when I tell them that, they never call me again.  But their station stays up and running.

This is the House of Worship forum, making your advice even more upsetting.

Not. Cool. At. All.

+100. 

Churches have enough negative stereotypes already, we need to weed out the bad apples that are harming the rest of us. 
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Re: FM Transmission
« Reply #19 on: June 21, 2020, 07:51:27 pm »


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