Good AM Radio Transmitter Circuit?

[UVB-76]

I'm looking for a good AM radio transmitter circuit. (I fix up old radios for fun and a lot of them are AM only, and I don't get any good stations) I found some on the internet but they either use obscure parts, don't look like they would function very well, use high voltage, or use higher power than is legal in the US (100mw). Anyone know of a good one?

[Wallace Gasiewicz]

Look for a RF generator that has modulation capabilities.
Essentially modulate your RF with perhaps a 1K audio. 
Some of the old Heathkit and RCA frequency generators did this and are very inexpensive.
However since you repair transistor radios, you may also wish to have FM capability both in the higher frequency used and in FM modulation.
I just looked on ebay for signal generator and I think the prices are way too high.

[szoftveres]

What do you mean by "don't look like they would function very well"? How does a well functioning transmitter look like, just by looking at the schematics?

[UVB-76]

What do you mean by "don't look like they would function very well"? How does a well functioning transmitter look like, just by looking at the schematics?

Many of them are more "hey it works!" (example: https://ab1dq.com/2021/11/27/what-is-rf-modulation/) than actually for broadcasting to music.

[David Hess]

Unfortunately this one requires an appropriate crystal, but the oscillator section could be replaced.

[Andy Chee]

Here's a low powered AM transmitter based on a MC1496 balanced modulator

https://archive.org/details/silicon-chip/Silicon%20Chip/2018/silicon_chip-2018_03/page/64/mode/2up

[UVB-76]

Unfortunately this one requires an appropriate crystal, but the oscillator section could be replaced.

I looked in my parts bin and I have a 1MHz crystal, which there are no stations on, so I could do this one if I bought the LT1xxxs.
Edit: Unfortunately, I thought it was a 2-pin but it's a 4-pin. The 2-pin ones at low frequencies are pricey!

[antenna]

Take a look at figure 14 on this page
https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_5

[RFDx]

Edit: Unfortunately, I thought it was a 2-pin but it's a 4-pin. The 2-pin ones at low frequencies are pricey!

The 4-pin part could be TTL oscillator.

https://www.ti.com/product/de-de/LMV821-N

See page 23,24 in the datasheet.

[UVB-76]

Take a look at figure 14 on this page
https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_5

Looks good but it does require an IF can...

[antenna]

Looks good but it does require an IF can...

It doesn't need to be a can, but the shielding does help. You could salvage one from an old radio if it is close to the frequency you want, or just make your own.  The can is just a neatly packaged LC tank with a small coupling coil for a primary.  It's just an LC oscillator. I suggested that one because it is almost fool-proof.  The can keeps the frequency fairly stable and the circuit will oscillate even if the compnents are not exact.  That design is very tolerant to changes, so not having the exact same parts is not usually an issue.  You can also take a can apart and rewind it easy enough.  Last time I did that, I took a 455kHz can out of a CB radio, cut out the little capacitor (to use an external one) and played with the windings.  You may even be able to leave the windings alone and lower the capacitance to get the frequency up into the AM broadcast band.

[UVB-76]

Looks good but it does require an IF can...

It doesn't need to be a can, but the shielding does help. You could salvage one from an old radio if it is close to the frequency you want, or just make your own.  The can is just a neatly packaged LC tank with a small coupling coil for a primary.  It's just an LC oscillator. I suggested that one because it is almost fool-proof.  The can keeps the frequency fairly stable and the circuit will oscillate even if the compnents are not exact.  That design is very tolerant to changes, so not having the exact same parts is not usually an issue.  You can also take a can apart and rewind it easy enough.  Last time I did that, I took a 455kHz can out of a CB radio, cut out the little capacitor (to use an external one) and played with the windings.  You may even be able to leave the windings alone and lower the capacitance to get the frequency up into the AM broadcast band.

I don't have any spare radio parts on hand, so I'd have to buy one somewhere. And pretty much all the low frequency bands in my area are covered so I'd have to get a pretty high frequency coil.

[antenna]

Don't take this the wrong way, but how can you fix up old radios and not have any spare radio parts laying around?  Old AM radio parts are sometimes hard to find and a box full of broken radios is something everyone that fixes radios has, from learning how to fix them to getting ones that are not worth the repair.  Are you just recapping them and keeping the ones that work and tossing the remainder?

Just go on ebay and search for medium wave transmitter.  You will find one for under $20USD.

[UVB-76]

I've only been fixing them for about a year, so I haven't got my box full of broken radios yet. So far I've been able to get most things working.

[szoftveres]

So far you've deemed every circuit suggested to you unsuitable for your needs, for various reasons, but haven't specified what your needs are, just gave a vague description, of which 99% of the suggested circuits fall under.

These circuits are simple enough to understand and are universally available on the internet and one chooses or designs one that suits their specific needs.

It's unclear what you're looking to get out of this forum topic, it would probably help if you would be more specific, i.e. how many transistors, what type, what other parts you can afford/willing to work with, etc. Until then I wonder if anyone ever can help you out.

[BrokenYugo]

Looks good but it does require an IF can...

It doesn't need to be a can, but the shielding does help. You could salvage one from an old radio if it is close to the frequency you want, or just make your own.  The can is just a neatly packaged LC tank with a small coupling coil for a primary.  It's just an LC oscillator. I suggested that one because it is almost fool-proof.  The can keeps the frequency fairly stable and the circuit will oscillate even if the compnents are not exact.  That design is very tolerant to changes, so not having the exact same parts is not usually an issue.  You can also take a can apart and rewind it easy enough.  Last time I did that, I took a 455kHz can out of a CB radio, cut out the little capacitor (to use an external one) and played with the windings.  You may even be able to leave the windings alone and lower the capacitance to get the frequency up into the AM broadcast band.

Yeah, you don't need the IF can. In fact I'd consider it easier to build with a DIY LC section than altering a 455kHz can to go up into the broadcast band. Could be as simple as a pair of windings on an old pill bottle, use a variable cap to tune it or build it fixed with a fixed mica or NP0 cap, shield if necessary. Could also just use the bar antenna, coil, and tuning cap from a transistor radio, that's built to resonate across the AM band, has a feedback winding in it already, and doubles as an antenna. I think you can still buy that stuff, or barring that I think the $10 Chinese superhet transistor radio kits are still a thing.

Using the harmonics of a 455kHz ceramic resonator you can find in any old TV remote collecting dust also comes to mind.

Don't forget you can be pretty freeform/sloppy down at 1MHz and not run into problems. Making stuff behave at broadcast band frequencies requires very little RF wizardry, mix n match, have fun with it.

My largest concern would be tweaking the modulation for max bandwidth and audio quality. IIRC the bigger, nicer, AM sets were designed with stations in mind that had a bit more bandwidth and fidelity than you see now.
 

[Karel]

See attached pdf for the schematic.

Here are the "specs":

Code: [Select]

Low-power Mid-Wave Amplitude Modulator
======================================

- powersupply:         12V DC
- current:             50 mA
- frequency:           1 MHz, fixed, crystal-based
- output power:        approx. 10 mW
- output impedance:    50 Ohm
- input sensitivity:   approx. 250 mV for 100% modulation
- input impedance:     39 KOhm
- harmonics:           -50 dBc


[LM21]

Looks good but it does require an IF can...

Don't forget you can be pretty freeform/sloppy down at 1MHz and not run into problems. Making stuff behave at broadcast band frequencies requires very little RF wizardry, mix n match, have fun with it.

That depends on what semiconductors you use. If you use a very fast transistor, you need to build your transmitter as a very high frequency circuit. Use something  like an audio transistor. Somethings like BC547 or BC337 are fast but perhaps not too fast.

[A.Z.]

I'm looking for a good AM radio transmitter circuit. (I fix up old radios for fun and a lot of them are AM only, and I don't get any good stations) I found some on the internet but they either use obscure parts, don't look like they would function very well, use high voltage, or use higher power than is legal in the US (100mw). Anyone know of a good one?

check this one

http://techlib.com/electronics/amxmit.htm

and this one too

http://techlib.com/electronics/amxmit.htm#Unfair%20Radio%20Transmitter

[seedkey]

How about the corsair am transmitter?
Works good.
You can leave the irf510 out of it and still get a watt or 2.
Circuit board design is out there too.

[antenna]

You can leave the irf510 out of it and still get a watt or 2.

I think only on the original circuit can you keep the final out.  In the second version, the modulation was moved to the final.  Both schematics are on freeradiotx.blogspot.com

[seedkey]

Yes, you are correct.  So use the first version. Pcb files are available.  Better to make your own modulator anyway.

[vk3yedotcom]

Cheap, simple & may be good enough.

[RoGeorge]

I looked in my parts bin and I have a 1MHz crystal, which there are no stations on
...
Edit: Unfortunately, I thought it was a 2-pin but it's a 4-pin.

That's a good start. 
Post a picture of it, just to make sure what it is.

Usually the 4 pin cans have inside a full oscillator (often with a TTL output), not just the quartz.  Something like the ones in this datasheet https://eu.mouser.com/datasheet/2/122/ecs-100x-4889.pdf (notice the pinout). 

Next step is to power it (usually at 5Vcc), and see if it outputs 1MHz.

Once you have a running oscillator, next step is to choose a circuit to modulate in amplitude (AM), then an RF amplifier, then an output filter/adapter for the antenna.

The 4 pins oscillators came in many frequencies, for example I have a couple of them on 4.9120MHz (they were used to generate the standard baud rates for RS232 serial communication), which might seem inappropriate, but if you add a frequency divider after it, you can get very close to the frequency of an MW channel.  For example dividing by 6 gets 819.2kHz, which is very close to the ideal 819kHz (one of the MW AM channels in EU).

In EU the MW AM channels are spaced at 9kHz, while in USA I think it's at 10kHz.  The exact frequency of the transmitter is not important for an analog radio, but for digital receivers it might be important.  The digital ones can only be tuned in steps, at the exact frequency of each MW channel.  They do not have continuous frequency swipe while seeking for a radio station, they can only jump in steps of 9kHz (or 10kHz, depending on the destined continent for that receiver).

If you look for more than just the schematics, this book explains both the practical examples and the basic building blocks:
https://ia902501.us.archive.org/12/items/circuit-analysis-theory-and-practice-pdfdrive/Build%20Your%20Own%20Low-Power%20Transmitters_%20Projects%20for%20the%20Electronics%20Experimenter%20%28%20PDFDrive%20%29.pdf

[mojoe]

Others have suggested quite a few circuits, so I won't.

When I was into restoring old radios, I used to broadcast OTR (Old Time Radio) programs around the house (I have quite the collection of shows). I wanted something with good audio quality, that didn't drift in frequency, as many simple transmitters do. I bought the SSTRAN AM transmitter kit, which has excellent audio, and uses a PLL. Sadly, you can't buy the kit any more. If you look around, you may find a second hand, assembled unit.

I had a spare PC running a free radio automation program called ZaraRadio. I created playlists, so that certain radio programs would play at a particular time, like a real radio station. Of course, I could always queue up whatever show that i wanted to listen to.

I used Virtual Audio Cable to route the audio through some processing software on the PC (a little compression, a little EQ), then out to the transmitter. It sounded great.

It was a fun project, and it was fun to listen to authentic old radio shows on an old radio with glowing tubes Now I just load up my phone with the MP3 files and listen.

FYI for those in the US - transmitting on the AM/MW band is covered by FCC Part 15.219. The two most important points are: (a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts. (b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.

Speaking of OTR, if anyone is interested, the best and most complete collections of shows is put out by the Old Time Radio Researchers (OTRR). When I was into collecting shows, we mailed CD's to each other. Now, you can find it on the Internet Archive. You will find other shows, posted by others, but they usually are a mixed bag. The stuff put out by OTRR is top notch.

Footnote to all of this - Years ago, when I had all of the above running, I was contacted by an FCC field agent. It seems that a local station turned me in. After a lengthy phone conversation, and some emailed documentation, the agent told me that I was compliant with the rules, and that he would close the case. He also complemented me on my setup.

[RoGeorge]

Here in EU/Romania, I remember as explicitly forbidden to intentionally transmit inside any broadcasting bands, no matter the power (but I might be wrong).  Though, there are ready made FM transmitters in the broadcasting 88-108MHz band, very low power, and they can be bought legally (mostly used for inside your car, to use the car's FM radio when there is no line input to connect your own player).

Just out of curiosity, those OTR records preserve the commercials, too, or just the shows?

[armandine2]

from Open University T327

[mojoe]

Just out of curiosity, those OTR records preserve the commercials, too, or just the shows?

Many have the commercials.

Interesting tidbit - for many years, the Jack Benny program was sponsored by Jello. During WW2, the sponsor changed to Grape Nuts cereal, due to the civilian sugar shortage.

And the most famous/infamous radio broadcast of all time was "War of the Worlds", dramatized by Orson Wells. I often listen to that program around Halloween. I think that there must have been only one surviving transcription disk of that broadcast. I have listened to several recordings of that broadcast, from different sources. All of them sound the same, including the part near the beginning, where the disk skips.

[MathWizard]

Cheap, simple & may be good enough.

I know there's TX's with 2-3 BJT's. I'm making an all BJT TX, and I'm already up to 7-8 BJT's, before any final output stage, or the audio input section.

I better make 1 of these 2-3BJT versions again, and see what they look like on a scope. If mine can't do as good as 1 of them, I hope an RF textbook will explain why.

[vorosj]

I constructed an AM signal source designed for old nostalgic tube radios.
It's based on the code found at: https://hackaday.com/2022/05/24/the-stm32-makes-for-a-cheap-diy-usb-soundcard/

I made modifications to the code, introducing additional lines to generate the carrier signal using a timer. The device resembles a USB soundcard. The modulation achieves around 90%, resulting in sound quality superior to typical AM broadcasts due to the broader bandwidth. The output involves a large coil for a near-field connection, resulting in negligible radiated energy.
Some pictures: