Can some one recommend me a low range FM transmitter design? - Page 1
Forum section: electronics / rf-microwave

author: ezalb98
author_rank: Contributor
timestamp: August 14, 2024, 11:07:37 pm
content: hello, as the title says i want to build from scratch a FM transmitter.I want to build it to transmit music and voice to a nearby(500m or less) friend of mine homeI searched on internet, but i found projects that turned out to be non working or too complex to buildCan someone recommend a simple to build circuit capable of that?thanks to anyone who will help me

author: geggi1
author_rank: Frequent Contributor
timestamp: August 14, 2024, 11:37:56 pm
content: get a kit

author: ezalb98
author_rank: Contributor
timestamp: August 14, 2024, 11:47:39 pm
content: Quote from: geggi1 on August 14, 2024, 11:37:56 pmnah, i prefer building it from scratch, i want a challenging project

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 15, 2024, 01:10:25 am
content: You can build simple ones with just a couple of transistors.But they'll drift in frequency with temperature, if you touch the antenna, move your hand too close, etc.Next up would be a single frequency crystal controlled one. 1-2 more transistors and coils, but stable with frequency.Then you could build a PLL type with crystal reference. Now you're into ICs as well. But no drift and multi frequency.These are pretty cheap to buy and were used back in the day when your car radio didn't have an MP3 player or bluetooth. (Plug MP3 player into device and tune it to a blank FM freq)

author: A.Z.
author_rank: Super Contributor
timestamp: August 15, 2024, 07:11:49 am
content: here's a really barebones onehttps://electronics-diy.com/1watt-fm-transmitter.phpwill probably drift, but you asked for a simple one...and then, here's another onehttps://techlib.com/files/fmxmit.pdf

author: A.Z.
author_rank: Super Contributor
timestamp: August 15, 2024, 07:31:15 am
content: and, willing to go for an AM transmitterhttps://techlib.com/electronics/amxmit.htm

author: Tation
author_rank: Regular Contributor
timestamp: August 15, 2024, 10:36:37 am
content: https://www.arrl.org/arrl-handbook-2023

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 15, 2024, 07:43:06 pm
content: My suggestion (I went more or less this path for music distribution in our property)- get a kit with an si4713 from adafruit or similar. This works fine up to 10m / 30ft and will give you the base to build something more powerful- read this and build a low power amp:https://ludens.cl/Electron/RFamps/RFamps.html(the rest of the electronics topics there are quite good too)

author: LM21
author_rank: Regular Contributor
timestamp: August 15, 2024, 10:08:59 pm
content: There is an IC for that. Some one makes ready assembled module or Arduino shield with it. Search something like arduino FM transmitter shield or something. It is stereo, so better than anything with one transistor.What challenge is there in a 100MHz FM transmitter. You can build one on any stripboard.

author: biastee
author_rank: Regular Contributor
timestamp: August 16, 2024, 07:05:42 am
content: Quote from: Kim Christensen on August 15, 2024, 01:10:25 amThe frequency deviation requirement of FM broadcast, nominal 22.5 kHz (75 kHz peak), makes crystals unusable. Fundamental crystals have very high Q (20,000 to 200,000) and so, are pullable over a couple of kHz only using varicaps; i.e. insufficient deviation for wide FM. On the other hand, ceramic resonators have Q values that are two order magnitude lower, hence, increasing pullability. Using a 3-terminal ceramic filter originally intended for 10.7 MHz IF, the frequency could be pulled sufficiently for FMBC service. The frequency can be changed by swapping ceramic filters with different colour dots. Frequency drift is <= 11 kHz from cold start to 1h later; i.e. well within the 180 kHz channel BWFM exciter stabilized with a 10.7 MHz ceramic filter.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 16, 2024, 08:34:11 am
content: 500 metre range would be illegal in most countries.

author: mikerj
author_rank: Super Contributor
timestamp: August 16, 2024, 09:23:44 am
content: Quote from: wasedadoc on August 16, 2024, 08:34:11 amUnlikely a law would specify range, usually it's output power and/or field strength.

author: Karel
author_rank: Super Contributor
timestamp: August 16, 2024, 10:07:17 am
content: Quote from: pardo-bsso on August 15, 2024, 07:43:06 pmThank you for this excellent information!When I was young I built a lot of FM transmitters and 27MC power amplifiers and I always struggled with paracitic oscillations :'(The page you linked to describes in practical ways how to deal with this.By the way, I can recommend a very good book that hasn't too much mathematics about RF design:https://www.amazon.com/RF-Circuit-Design-Christopher-Bowick/dp/0750685182

author: wasedadoc
author_rank: Super Contributor
timestamp: August 16, 2024, 10:30:40 am
content: Quote from: mikerj on August 16, 2024, 09:23:44 amTrue, but how to get that range without exceeding the power and/or field strength limits?

author: ezalb98
author_rank: Contributor
timestamp: August 16, 2024, 10:37:53 am
content: Quote from: A.Z. on August 15, 2024, 07:11:49 amWhich one will transmit farthest?what should i do if i want to increase the output power of those circuits?

author: LM21
author_rank: Regular Contributor
timestamp: August 16, 2024, 10:51:11 am
content: Range depends more of your antenna and how it is matched to your circuit than the circuit. Usually the most powerfull transistor and device with most current will get you further, and bring local police to you sooner.

author: ezalb98
author_rank: Contributor
timestamp: August 16, 2024, 12:58:34 pm
content: Quote from: A.Z. on August 15, 2024, 07:11:49 amwhat about this?http://electronics-diy.com/2km-fm-transmitter.php

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 16, 2024, 02:52:13 pm
content: Quote from: Karel on August 16, 2024, 10:07:17 amYou are welcome, Manfred is quite good at explaining.

author: A.Z.
author_rank: Super Contributor
timestamp: August 16, 2024, 04:52:54 pm
content: Quote from: ezalb98 on August 16, 2024, 10:37:53 amI'd go for the one in the PDF, then , willing to increase the range add a decent antenna and a small amplifier stage, a ground plane over the roof is easy to buildor a flowerpothttps://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.html

author: biastee
author_rank: Regular Contributor
timestamp: August 17, 2024, 04:08:20 am
content: Quote from: ezalb98 on August 16, 2024, 12:58:34 pmA long-range transmitter (the linked article claims 2 km) with an unstable frequency will maximize the chance of interfering with your neighbours.Short-range "FM microphones" (read: tens of meter) without frequency stabilization are already painful to use because of the frequency drifts substantially from a cold start.To produce more range / power as in that article, the transistor will conceivably operate even hotter - thus exacerbating the drift.In a nutshell, frequency stabilization is essential for a long range transmitter. Spectral purity, too.

author: A.Z.
author_rank: Super Contributor
timestamp: August 17, 2024, 06:10:05 am
content: willing a stable transmitter one may consider thishttps://electronics-diy.com/BH1417_50mW_PLL_Stereo_FM_Transmitter.phpmore complex but much more stable and with stereo input/encoding, add an amplifier stage, for examplehttps://www.electroschematics.com/1-watt-fm-amplifier/optionally feeding thishttps://www.electroschematics.com/fm-radio-amplifier-25w/or another one, connect a decent antenna (a flowerpot will do) and there you go

author: wasedadoc
author_rank: Super Contributor
timestamp: August 17, 2024, 07:24:21 am
content: Quote from: ezalb98 on August 14, 2024, 11:07:37 pmUnless that is self-made music you are likely to infringe the terms under which you obtained it.

author: tggzzz
author_rank: Super Contributor
timestamp: August 17, 2024, 08:52:03 am
content: Quote from: ezalb98 on August 14, 2024, 11:07:37 pmDoes it have to be legal, or is an illegal transmitter acceptable?Does it have to guarantee not interfering with other users, including those operating different services on "completely" different frequencies? Such other users can become annoyed and cause the authorities to prosecute you.Can you measure the output, and demonstrate that your implementation is functioning correctly?

author: A.Z.
author_rank: Super Contributor
timestamp: August 17, 2024, 02:54:52 pm
content: start herehttp://radioinitiation.chez-alice.fr/but as others wrote, consider the legal aspects, since you may get in trouble, some mW won't be an issue, but going up to some W...

author: janoc
author_rank: Super Contributor
timestamp: August 17, 2024, 06:49:28 pm
content: Just don't.Such transmitter is certainly going to be illegal - in the EU you can operate an AM or FM broadcast band transmitter without a license only up to1050nW (yes, nanowatts!) - i.e. only the various dongles to send music from your phone/satnav to your car radio.Nothing elsecan be legally owned or operated on the broadcast frequencies unless you have a broadcast radio license. Don't get fooled by articles posted by people in the US and such - their rules are a lot looser than in the EU.If you actually manage to build something emitting enough RF to receive it at the other end, you can rest assured that you will have Italian authorities on your doorstep pretty quickly - and the fines are steep! And yes, you will get caught because you will be causing interference to neighbors (and not only, given the general lack of any sort of bandpass filters and similar in many of the posted articles) - and someone will sooner or later report it.E.g. causing interference with airband which is right next to the broadcast FM radio band could well get you not only a hefty fine but even a criminal charge for endangering airplane traffic. That carries a possible prison time with it, depending on how desperate the prosecutor is going to be to make an example out of you. You really really don't want to mess with that.

author: mikerj
author_rank: Super Contributor
timestamp: August 18, 2024, 04:14:20 pm
content: Quote from: wasedadoc on August 16, 2024, 10:30:40 amHigh gain directional antenna on the RX side, coupled with a sensitive receiver.

author: janoc
author_rank: Super Contributor
timestamp: August 18, 2024, 05:29:45 pm
content: Quote from: mikerj on August 18, 2024, 04:14:20 pmAin't happening - you do need an antenna on the transmitting side too. You can't transmit into a dummy load and try to make up for that loss of radiated power by some enormous receiving antenna and uber sensitive receiver - if the signal isn't there to begin with, you can't extract it. There are limits due to thermal noise, etc. of what you can achieve by increasing the gain of the antenna & receiver sensitivity. Moreover, given the OP's questions they are very unlikely to be capable of building any sort of high gain antenna and sensitive receiver to begin with.And the moment you actually start emitting anything on the transmitter side, you have most likely violated the legal limits already - they are really really low! See e.g. FCC Part 15 Rules specifying 250uV/m at 3 meters from the antenna - which roughly corresponds to 10nW of transmitter power. EU rules stipulate the10nW ERP, can't find the reference right now, unfortunately.50nW ERP (i.e. antenna gain included!), with analogue FM modulation:https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32017D1483Obviously, any other legal requirements (e.g. copyright, licensing of the content being broadcast, etc.) may still apply and are a completely separate issue.

author: tggzzz
author_rank: Super Contributor
timestamp: August 18, 2024, 05:43:17 pm
content: Quote from: janoc on August 18, 2024, 05:29:45 pmAnd, as I expect you appreciate, in many cases the regulations state that if you use a 10dB gain tx antenna, you must reduce the tx power by 10dB.

author: janoc
author_rank: Super Contributor
timestamp: August 18, 2024, 06:35:17 pm
content: Quote from: tggzzz on August 18, 2024, 05:43:17 pmYup. Which is why the regulations specify ERP power, not just transmitter power.

author: Bud
author_rank: Super Contributor
timestamp: August 18, 2024, 06:54:59 pm
content: And how do those regulations Measure the ERP?

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 19, 2024, 12:25:20 am
content: Quote from: Bud on August 18, 2024, 06:54:59 pmhttps://en.wikipedia.org/wiki/Effective_radiated_powerUsage by USA and UK lower down the page

author: ezalb98
author_rank: Contributor
timestamp: August 19, 2024, 12:04:54 pm
content: Quote from: biastee on August 17, 2024, 04:08:20 amthx this is very useful!you're right, it will increase the chance of interfering with something else... i'll go with the 1km one linked up there (hoping that it will transmit at least 500m)is there a way to avoid frequency drifting?

author: ezalb98
author_rank: Contributor
timestamp: August 19, 2024, 12:21:57 pm
content: i decided that i will go with this designwill it work? is the circuit alright?thanks to everybody who helped me, i appreciated so much

author: biastee
author_rank: Regular Contributor
timestamp: August 20, 2024, 06:30:42 am
content: Quote from: ezalb98 on August 19, 2024, 12:04:54 pmThe answers are already given in thread replies #9 and #20, viz. 1. stabilize with a ceramic resonator/filter or 2. phase locked loop.Quote from: ezalb98 on August 19, 2024, 12:21:57 pmThis is a particularly nasty design. The amplified audio is superimposed on the base of the oscillator, hence changing the base bias voltage. This fluctuating Vbe produces AM in addition to FM.Now, to teach a person how to fish instead of giving him a fish. A quick & dirty way to judge whether a circuit posted on the internet can work or is alright, is to look for evidence of measurement / characterization in the explanatory text. If there is no evidence that the poster has measured his circuit, even cursorily, then it is likely a junk. Murphy's Law which says that anything can go wrong is particularly applicable to RF electronics. Those who don't measure, will not be able detect the faults. To paraphrase Socrates: The unexamined circuit is not worth building.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 20, 2024, 08:26:44 am
content: Quote from: ezalb98 on August 19, 2024, 12:21:57 pmIt will eventually work, but that is a very crude design, made to transmit voice. You said you want to send music. For voice, that circuit might be good enough to understand the words, but for music, expect poor audio quality.Same circuit can send music, too, but I would expect that transmitter to add (audible) distortions. If high fidelity is not a concern, then yes, the circuit might be alright. It all depends of if you only need to hear some companion music, or if you need to make Hi-Fi auditions.Try it in practice, and see if it's good enough. Might be. It's a relatively small circuit, so even if you don't like the end result, at least you'll get some fun building it without spending much.

author: ezalb98
author_rank: Contributor
timestamp: August 20, 2024, 11:43:32 am
content: Quote from: biastee on August 20, 2024, 06:30:42 amYou're so right, and thanks you for the advice.the only problem here is that i can't find well detailed circuits that are easy to build and satisfy my needsAlso, is just a fun project, i don't really care about audio quality... i just hope is not that bad

author: ezalb98
author_rank: Contributor
timestamp: August 20, 2024, 11:47:25 am
content: Quote from: RoGeorge on August 20, 2024, 08:26:44 amis just a fun project, i only hope that is not that bad, i don't really care about high quality

author: tggzzz
author_rank: Super Contributor
timestamp: August 20, 2024, 12:27:38 pm
content: Quote from: ezalb98 on August 20, 2024, 11:47:25 amDo you care about annoying other people nearby? "Annoying" could include "preventing them from using their radio". In bad cases it might cause government authorities to visit your home.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 20, 2024, 01:17:52 pm
content: The ones that look "too complex" are often the easiest to build and understand successfully because everything isn't doing 3 things at once, usually 2 of them poorly and undocumented, reliant on parasitics, hand selected parts, etc. The truly simple circuits are pretty rare, especially in radio by the time FM came around.Not sure about Italy/Europe, in the US 225-400 (I.E lots of FM broadcast third harmonics) is reserved for military use and they get unhappy when they find things on their band they didn't put there. Civilian airband is also right above FM broadcast. Not a good place to have any sort of unstable or harmonic rich transmitter.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 20, 2024, 03:18:52 pm
content: Quote from: ezalb98 on August 19, 2024, 12:21:57 pmNo & No.I doubt that one transistor oscillator/modulator can output 1 watt. Not that you'd want it to. I remember building similar designs as a kid, but they ran from a single AA cell and barely broadcast from one end of the house to the other. That is the type of thing you want (power wise), because it's not going to get you into trouble with the authorities nor annoy anyone. Great if you want to play around, but not really practical.That diagram also has a major flaw. Can you spot it?***Hint: Notice where the antenna is connected.

author: LM21
author_rank: Regular Contributor
timestamp: August 20, 2024, 08:29:05 pm
content: I am not sure about the schema. Antenna connection and C10 look unfamiliar to me.I have got good results just sending audio to the oscillator base (years ago). One transistor wireless microphones work well, and you can study antennas and their impedances with them.

author: ezalb98
author_rank: Contributor
timestamp: August 20, 2024, 11:11:08 pm
content: Quote from: Kim Christensen on August 20, 2024, 03:18:52 pmby fixing the antenna connection (how?) is it gonna work?i live in a area with very few people, is very unlikely that I'll disturb someone

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 21, 2024, 12:14:02 am
content: Usually, the antenna is tapped near the top of the coil, 1 turn turn from the end. You can also connect the antenna at the collector of T2, but a hand near the antenna will have more influence on frequency drift when you do that. (It'll still drift the same with temperature though)C1 being 470pF doesn't seem right to me. It should be sized for audio coupling. Also, T1's biasing method is heavily dependent on transistor beta. If DC voltage at collector of T1 is less than base of T2 then change the value of R4 so that the DC voltage at T1 collector is a bit more than 1/2 Vcc. (It'll depend on the individual transistors you use)

author: RoGeorge
author_rank: Super Contributor
timestamp: August 21, 2024, 08:19:56 am
content: The schematic seems identical with this one,https://electronics-diy.com/1watt-fm-transmitter.php, where the text says the antenna is optional. In the PCB picture, the antenna is coupled with the coil by a small PCB trace.Did once such an oscillator when I was a kid. The maximum fun was that the air coil + PCB were acting somehow like a microphone, catching all the vibrations and knocking of the circuit ringing like a bell (in the received sound). Plus, it wasn't shielded, so waving the hands around, or touching parts was making all kind of sound effects.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 21, 2024, 01:45:30 pm
content: If it helps anyone this is a related post made by OP:https://www.eevblog.com/forum/projects/need-help-with-a-fm-radio-transmitter-project/

author: A.Z.
author_rank: Super Contributor
timestamp: August 21, 2024, 03:14:48 pm
content: Quote from: ezalb98 on August 20, 2024, 11:11:08 pmGive up with that circuit, use the first one here, insteadhttps://www.eevblog.com/forum/rf-microwave/can-some-one-recommend-me-a-low-range-fm-transmitter-design/msg5607733/#msg5607733the above uses a single IC(B1417)to implement all the needed stages, including the PLL and gives a 50 mW output, then, to extend the range, start by adding a decent antenna placed outside (not a piece of wire) and, if really needed, an additional power stage (all those circuits are at the above link), regarding the antenna, you may easily calculate and build this onehttps://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.htmluse the calculator at the above link, entering 98 as the center frequency, build it and place it over the roof or a terrace or either on a balcony; the trasmitter is PLL controlled, just add a decent lowpass filter at its output to avoid problems, remember that above the "commercial FM" band there's the airband and, at least in Italy, below the band there are some Police comms, so if by chance you'll be disturbing either of them, you'll be in HUGE trouble, this isn't a joke

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2024, 04:36:03 pm
content: The problem with the circuits posted so far is they're affected by the supply voltage, loading and lack pre-emphasis.Audioguru designed an FM transmitter which includes pre-emphasis, has a voltage regulator and an additional RF amplifier to isolate the output from the oscillator.

author: ezalb98
author_rank: Contributor
timestamp: August 21, 2024, 05:40:20 pm
content: Quote from: Zero999 on August 21, 2024, 04:36:03 pmwow this look's good!Has it been tested? were can i find a documentation? how far will it transmit?

author: ezalb98
author_rank: Contributor
timestamp: August 21, 2024, 05:44:23 pm
content: Quote from: A.Z. on August 21, 2024, 03:14:48 pmthanks but this seems a bit hard to build, but the big problem is that i dont have all thoses components. (It's difficult for me to get them, cause of were i live)

author: A.Z.
author_rank: Super Contributor
timestamp: August 21, 2024, 06:44:08 pm
content: the other circuits, w/o a PLL or a resonator will DRIFT, now, if you're going to use it for some short time, AFTER PREHEATING, that won't be an issue, otherwise...Anyhow, you seem to be deaf to correct advices, I'm giving up, best luck with your FM xmtr

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 21, 2024, 07:23:58 pm
content: Quote from: ezalb98 on August 21, 2024, 05:40:20 pmYou can findhis post here.Sounds like he built it, tested it, and it worked fine. He hasn't been active since 2019.

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2024, 06:31:11 pm
content: Quote from: Kim Christensen on August 21, 2024, 07:23:58 pmYes. it's not my design. I would consider taking the signal from a tap on the inductor in this case, to reduce the output power level.Audioguru lost his logon details and started posting from another account, but even then he's not been here since 2020.https://www.eevblog.com/forum/profile/?u=554352He's still quite active on electro-tech-online, but I was banned from there over 10 years ago.https://www.electro-tech-online.com/members/audioguru.16360/

author: ezalb98
author_rank: Contributor
timestamp: August 22, 2024, 10:22:18 pm
content: Quote from: Kim Christensen on August 21, 2024, 07:23:58 pmthanks!

author: mikerj
author_rank: Super Contributor
timestamp: August 25, 2024, 11:41:57 am
content: Quote from: janoc on August 18, 2024, 05:29:45 pmWho said anything about using a dummy load on the TX side?

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 28, 2024, 11:38:00 am
content: The Talking Electronics website has a pretty good section on small FM transmitters.I've had good luck with them. Eg:You might get 500m down the street or outside.But if you're trying to transmit to an average radio that's indoors the range will likely be less.

author: Juri 69
author_rank: Newbie
timestamp: September 09, 2024, 06:59:46 pm
content: There are many similar circuits on this site, and practical designs are posted. But I warn you - this is not entirely legal!!! You can assemble the circuits - work without going on air on the equivalent load! Study the circuitry and, most importantly, the design features of high-frequency microwave circuits!https://vrtp.ru/index.php?act=categories&CODE=article&article=2369

author: wasedadoc
author_rank: Super Contributor
timestamp: September 09, 2024, 07:12:43 pm
content: Quote from: Juri 69 on September 09, 2024, 06:59:46 pm1. I surmise that you have not read and understood all previous replies.2. I would hesitate to class 100MHz as high frequency microwave.