How far does a 50nW (legal!) FM transmitter actually reach - UK Vintage Radio Repair and Restoration Discussion Forum

MrBungle · 5th Mar 2017, 4:16 pm

I'm considering building myself a simple 50nW FM stereo transmitter for the sake of broadcasting an mp3 stream from a Raspberry Pi Zero W as there is literally nothing on commercial radio of interest now for me.

Plan is to use a USB DAC IC, an NJM2035 to encode and then a MAX2606 as the transmitter with a suitable LPF and an attenuator to keep the ERP legal. I have the necessary equipment and expertise to test that this is compliant.

However I'd like to understand with the respect of propagation if this is even feasible as a transmitter or not to reach approximately 10m. Propagation is not my strongest area.

My WiFi strength is similar at around -43dBm at the same distance and this is usable with a good antenna.

paulsherwin · 5th Mar 2017, 4:23 pm

You cannot build a legal transmitter no matter how feeble the power. They need to be CE marked to be legal.

The range of a 50nW TX will depend on the environment and aerial used, but it should cover 10m if there are no screening walls in the way. It seems a bit pointless to keep to this limit though. It would make more sense to increase the ERP until you have the range you need, while not radiating beyond your boundary.

G6Tanuki · 5th Mar 2017, 5:09 pm

As Paul says, homebrewing these is *technically* a no-no unless you're intending to submit your finished model to an approved test-house for "RED" certification.

http://eur-lex.europa.eu/legal-conte...002&from=EN%20

Though I suspect quite a few of us have a "modulated test-oscillator" in our workshops.

Given that you can buy legal versions of these transmitters for under a tenner, why bother building your own? I've got one here that's built into a car cigarette-lighter plug along with a 12V-to-5V USB charger. Couldn't have sourced the parts for the amount it cost me.

paulsherwin · 5th Mar 2017, 5:15 pm

I have a car MP3 player which accepts SD cards or USB disks and also has a line input. It outputs on any FM frequency and sits in the cigarette lighter socket. It even has a basic remote control. I think it was about £3 including postage from China.

It was CE marked, though that should obviously be taken with a pinch of salt.

MrBungle · 5th Mar 2017, 7:11 pm

Thanks for the info all, much appreciated. Will find a CE marked one then and naively assume it's legitimate

A bit sad really as the CE mark means literally nothing in the scale of things as the various dubious character spacings over the years have indicated...

I was only going to build one because I have some bits lying around and some time to burn at the moment while I'm waiting for some other parts to be delivered.

5th Mar 2017, 7:24 pm

As Paul said, they are very cheap on ebay etc. add a bit more wire to the aerial and they go a surprisingly long way.

MrBungle · 5th Mar 2017, 7:49 pm

I have ordered one from China now for a whopping £2.70. I'm never failed to be impressed by the price of stuff coming from that part of the world. I'll take it to bits and integrate it with this. It has a tuning selector up/down controls and a display and a stereo input so is spot on.

Got the USB DAC working early evening. Used a PCM2707 DAC (from a canned HiFi project I started a year ago) on a tiny SMD matrix board from RS/Roth Elektronik. Required 6 SMD capacitors, 8 SMD resistors, a crystal and a voltage reg and I soldered it directly to the USB pins on the Pi Zero with kynar. Both are screwed down to a piece of plywood as a "master breadboard". Worked first time for the first time ever. Nearly fell off my chair! My eyes hurt now though. This was my first SMD attempt ever past a few minor things.

stevehertz · 5th Mar 2017, 8:07 pm

Quote:

Originally Posted by paulsherwin

I have a car MP3 player which accepts SD cards or USB disks and also has a line input. It outputs on any FM frequency and sits in the cigarette lighter socket. It even has a basic remote control. I think it was about £3 including postage from China.

It was CE marked, though that should obviously be taken with a pinch of salt.

You want it CE marked? no ploblem, we put CE sticker on it!

ionburn · 5th Mar 2017, 9:15 pm

Personally I would take CE marks with a pinch of salt as they do not indicate what directive something complies with. In the EU an item has to have a CE mark in order to be sold (as far as I am aware). Really something has to have a test house mark (or further info) to enable scrutiny.

I don't remember details but I think if something is custom made for a particular purpose it does not require formal CE marking, at least for internal use. In the NHS we made many items of equipment but never had issues with CE marking (although the requirements for were investigated, but it was long ago so I don't remember details).

A point to think about, as the CE mark is a EU thing, what will it be replaced by?

paulsherwin · 5th Mar 2017, 9:24 pm

EU rules are enshrined in UK legislation - the EU has no lawmaking powers of its own. However, once the UK leaves the EU then UK politicians and courts will be able to repeal or amend the legislation, which they can't do at the moment.

No discussion of Brexit here please!

CambridgeWorks · 5th Mar 2017, 10:00 pm

Have you the instrument to measure 50nW accurately, assuming you did make one? Just a thought, not wanting to be sarcastic at all.

CambridgeWorks · 5th Mar 2017, 10:01 pm

SORRY. Just saw this "I have the necessary equipment and expertise to test that this is compliant.". Put my brain into gear before typing!!

MrBungle · 5th Mar 2017, 10:07 pm

For ref, I've got an HP 436A power meter which is good to -70dBm.

Wendymott · 5th Mar 2017, 10:34 pm

My understanding of the "CE" mark is that.... IF you intend to sell commercially any electronic product, whether sending or receiving, it must comply with certain tests, such as EMC, Safety, RoSH and if battery powered, then more regarding the battery construction, "IF Internally" fitted. However for home use, CE does not become necessary.
To get CE Certification, Test Houses such as Alphatec,Airbus Defence... etc etc which are registered by UKAS. All test houses charge an arm and a leg...
Alternatively you can "self Certificate", which means, you can do the tests yourself and hope nothing goes wrong.

kalee20 · 6th Mar 2017, 12:48 am

Quote:

Originally Posted by robinshack

Have you the instrument to measure 50nW accurately, assuming you did make one? Just a thought, not wanting to be sarcastic at all.

I think you'd do this by measuring the amplitude of oscillation at the aerial, and appeal to maths to calculate how much power is actually radiated, knowing the geometry of the aerial.

50nW, collected for 20 years, would be enough energy to light a LED for about a second.

MotorBikeLes · 6th Mar 2017, 12:34 pm

I don't do any "local transmissions" myself, but on one occasion 20+ years ago, I used my NordMende SC384 stereo coder signal generator to do the job. Very much pre-CE, I would happily use that with a telescopic rod soldered into the (unusual) output socket. That is how I use it in the workshop for general testing, though I have not used it for a proper FM/Stereo alignment for maybe 30 years.
Les.

Jon_G4XJC · 7th Mar 2017, 12:18 am

Ummm, my answer would have been, If your have a clear frequency & sensitive receiver ## Some considerable distance ##

I had a little one, badged as Realistic ( internal AA battery ) CE certified (no external connections exception flying lead for audio in) " transmitter ' positioned in kitchen ( none elevated site ) Which could be received up to 1/4 mile away (fully quieting stereo) Received in a built up area, on my car stereo.

My guess was that the flying lead was radiating. because repositioning it, effected the coverage area '-)/

De Jon ..

Mach One · 8th Mar 2017, 11:27 am

The ones that I have had in recent years have all had a very small range (a matter of feet) when used on internal batteries but once I plug them into a power supply of sorts (either mains or in-car) the range extends all around the house and into the garden or in the car can be heard quite clearly in other cars. Any power output that is quoted seems to be quite nominal.

HamishBoxer · 8th Mar 2017, 12:17 pm

I used to think but never tried putting an aerial on the o/put of an RF modulator VCR.Not sure how far around or if the house it would travel.

GMB · 8th Mar 2017, 3:40 pm

If you want to see how far the "legal" ones transmit, just tune your car radio to the bottom-most frequency in the FM band and drive along a motorway.

I bought my wife a thing to transmit from an iPod to a nearby FM radio - for car use. By default they tune to the lowest frequency and as that spot is usually empty I though it a good choice. Sadly I had bought a fully legal one, but it seems other cars also use these devices - but clearly don't have the same low output!

By the way, the bigger snag turned out to be very low deviation so it didn't work so well for that reason.

5th Mar 2017, 4:16 pm	#1
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	How far does a 50nW (legal!) FM transmitter actually reach I'm considering building myself a simple 50nW FM stereo transmitter for the sake of broadcasting an mp3 stream from a Raspberry Pi Zero W as there is literally nothing on commercial radio of interest now for me. Plan is to use a USB DAC IC, an NJM2035 to encode and then a MAX2606 as the transmitter with a suitable LPF and an attenuator to keep the ERP legal. I have the necessary equipment and expertise to test that this is compliant. However I'd like to understand with the respect of propagation if this is even feasible as a transmitter or not to reach approximately 10m. Propagation is not my strongest area. My WiFi strength is similar at around -43dBm at the same distance and this is usable with a good antenna.

5th Mar 2017, 4:23 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: How far does a 50nW (legal!) FM transmitter actually reach You cannot build a legal transmitter no matter how feeble the power. They need to be CE marked to be legal. The range of a 50nW TX will depend on the environment and aerial used, but it should cover 10m if there are no screening walls in the way. It seems a bit pointless to keep to this limit though. It would make more sense to increase the ERP until you have the range you need, while not radiating beyond your boundary.

5th Mar 2017, 5:09 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: How far does a 50nW (legal!) FM transmitter actually reach As Paul says, homebrewing these is technically a no-no unless you're intending to submit your finished model to an approved test-house for "RED" certification. http://eur-lex.europa.eu/legal-conte...002&from=EN%20 Though I suspect quite a few of us have a "modulated test-oscillator" in our workshops. Given that you can buy legal versions of these transmitters for under a tenner, why bother building your own? I've got one here that's built into a car cigarette-lighter plug along with a 12V-to-5V USB charger. Couldn't have sourced the parts for the amount it cost me.

5th Mar 2017, 5:15 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: How far does a 50nW (legal!) FM transmitter actually reach I have a car MP3 player which accepts SD cards or USB disks and also has a line input. It outputs on any FM frequency and sits in the cigarette lighter socket. It even has a basic remote control. I think it was about £3 including postage from China. It was CE marked, though that should obviously be taken with a pinch of salt.

5th Mar 2017, 7:11 pm	#5
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: How far does a 50nW (legal!) FM transmitter actually reach Thanks for the info all, much appreciated. Will find a CE marked one then and naively assume it's legitimate A bit sad really as the CE mark means literally nothing in the scale of things as the various dubious character spacings over the years have indicated... I was only going to build one because I have some bits lying around and some time to burn at the moment while I'm waiting for some other parts to be delivered.

5th Mar 2017, 7:24 pm	#6
Guest Guest Posts: n/a	Re: How far does a 50nW (legal!) FM transmitter actually reach As Paul said, they are very cheap on ebay etc. add a bit more wire to the aerial and they go a surprisingly long way.

5th Mar 2017, 7:49 pm	#7
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: How far does a 50nW (legal!) FM transmitter actually reach I have ordered one from China now for a whopping £2.70. I'm never failed to be impressed by the price of stuff coming from that part of the world. I'll take it to bits and integrate it with this. It has a tuning selector up/down controls and a display and a stereo input so is spot on. Got the USB DAC working early evening. Used a PCM2707 DAC (from a canned HiFi project I started a year ago) on a tiny SMD matrix board from RS/Roth Elektronik. Required 6 SMD capacitors, 8 SMD resistors, a crystal and a voltage reg and I soldered it directly to the USB pins on the Pi Zero with kynar. Both are screwed down to a piece of plywood as a "master breadboard". Worked first time for the first time ever. Nearly fell off my chair! My eyes hurt now though. This was my first SMD attempt ever past a few minor things.

5th Mar 2017, 9:15 pm	#9
ionburn Heptode Join Date: Dec 2014 Location: Lincoln, Lincolnshire, UK. Posts: 583	Re: How far does a 50nW (legal!) FM transmitter actually reach Personally I would take CE marks with a pinch of salt as they do not indicate what directive something complies with. In the EU an item has to have a CE mark in order to be sold (as far as I am aware). Really something has to have a test house mark (or further info) to enable scrutiny. I don't remember details but I think if something is custom made for a particular purpose it does not require formal CE marking, at least for internal use. In the NHS we made many items of equipment but never had issues with CE marking (although the requirements for were investigated, but it was long ago so I don't remember details). A point to think about, as the CE mark is a EU thing, what will it be replaced by?

5th Mar 2017, 9:24 pm	#10
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: How far does a 50nW (legal!) FM transmitter actually reach EU rules are enshrined in UK legislation - the EU has no lawmaking powers of its own. However, once the UK leaves the EU then UK politicians and courts will be able to repeal or amend the legislation, which they can't do at the moment. No discussion of Brexit here please!

5th Mar 2017, 10:00 pm	#11
CambridgeWorks Nonode Join Date: Jun 2010 Location: Spalding, Peterborough, Cambridgeshire, UK. Posts: 2,858	Re: How far does a 50nW (legal!) FM transmitter actually reach Have you the instrument to measure 50nW accurately, assuming you did make one? Just a thought, not wanting to be sarcastic at all. __________________ Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd

5th Mar 2017, 10:01 pm	#12
CambridgeWorks Nonode Join Date: Jun 2010 Location: Spalding, Peterborough, Cambridgeshire, UK. Posts: 2,858	Re: How far does a 50nW (legal!) FM transmitter actually reach SORRY. Just saw this "I have the necessary equipment and expertise to test that this is compliant.". Put my brain into gear before typing!! __________________ Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd

5th Mar 2017, 10:07 pm	#13
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: How far does a 50nW (legal!) FM transmitter actually reach For ref, I've got an HP 436A power meter which is good to -70dBm.

5th Mar 2017, 10:34 pm	#14
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Re: How far does a 50nW (legal!) FM transmitter actually reach My understanding of the "CE" mark is that.... IF you intend to sell commercially any electronic product, whether sending or receiving, it must comply with certain tests, such as EMC, Safety, RoSH and if battery powered, then more regarding the battery construction, "IF Internally" fitted. However for home use, CE does not become necessary. To get CE Certification, Test Houses such as Alphatec,Airbus Defence... etc etc which are registered by UKAS. All test houses charge an arm and a leg... Alternatively you can "self Certificate", which means, you can do the tests yourself and hope nothing goes wrong. __________________ Should get out more. Regards Wendy G8BZY

6th Mar 2017, 12:34 pm	#16
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: How far does a 50nW (legal!) FM transmitter actually reach I don't do any "local transmissions" myself, but on one occasion 20+ years ago, I used my NordMende SC384 stereo coder signal generator to do the job. Very much pre-CE, I would happily use that with a telescopic rod soldered into the (unusual) output socket. That is how I use it in the workshop for general testing, though I have not used it for a proper FM/Stereo alignment for maybe 30 years. Les.

7th Mar 2017, 12:18 am	#17
Jon_G4XJC Retired Dormant Member Join Date: Feb 2017 Location: Cheltenham, Gloucestershire, UK. Posts: 9	Re: How far does a 50nW (legal!) FM transmitter actually reach Ummm, my answer would have been, If your have a clear frequency & sensitive receiver ## Some considerable distance ## I had a little one, badged as Realistic ( internal AA battery ) CE certified (no external connections exception flying lead for audio in) " transmitter ' positioned in kitchen ( none elevated site ) Which could be received up to 1/4 mile away (fully quieting stereo) Received in a built up area, on my car stereo. My guess was that the flying lead was radiating. because repositioning it, effected the coverage area '-)/ De Jon ..

8th Mar 2017, 11:27 am	#18
Mach One Heptode Join Date: Sep 2005 Location: Shoreham-by-Sea, West Sussex, UK. Posts: 611	Re: How far does a 50nW (legal!) FM transmitter actually reach The ones that I have had in recent years have all had a very small range (a matter of feet) when used on internal batteries but once I plug them into a power supply of sorts (either mains or in-car) the range extends all around the house and into the garden or in the car can be heard quite clearly in other cars. Any power output that is quoted seems to be quite nominal.

8th Mar 2017, 12:17 pm	#19
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,931	Re: How far does a 50nW (legal!) FM transmitter actually reach I used to think but never tried putting an aerial on the o/put of an RF modulator VCR.Not sure how far around or if the house it would travel. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

8th Mar 2017, 3:40 pm	#20
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,093	Re: How far does a 50nW (legal!) FM transmitter actually reach If you want to see how far the "legal" ones transmit, just tune your car radio to the bottom-most frequency in the FM band and drive along a motorway. I bought my wife a thing to transmit from an iPod to a nearby FM radio - for car use. By default they tune to the lowest frequency and as that spot is usually empty I though it a good choice. Sadly I had bought a fully legal one, but it seems other cars also use these devices - but clearly don't have the same low output! By the way, the bigger snag turned out to be very low deviation so it didn't work so well for that reason.