Electronics > RF, Microwave, Ham Radio

How can I improve this cheap AM transmitter?

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MaximRecoil:


It's very low-powered; it only has a range of about a foot. I use it to listen to old radio shows, which are mostly talking, on an AM radio. I feed it an audio signal from my PC's line-level speaker output jack and power it with a 6 VDC, 2-amp "wall wart" type AC adapter power supply.

With the transmitter's volume/gain pot turned up all the way, and with my PC's Windows volume control slider at 50%, it has a good signal-to-noise ratio (I can barely hear any static when it's at a comfortable listening volume), and talking sounds okay, but when music plays there's noticeable distortion. If I turn the PC's volume control down to 25% (and turn up the AM radio's volume control to compensate), there's less distortion, but a worse signal-to-noise ratio. If I also turn the transmitter's volume/gain pot down to about 50%, distortion pretty much disappears, but the signal-to-noise ratio gets really bad.

As far as I can tell, the transmitter only has 3 active components, i.e., the 3 transistors (they are all S9018, at locations BG1, BG2, and BG3). The lower PCB is just for the optional batteries which I'm not currently using; it doesn't do anything else). Are there better transistors that I could use and/or anything else I could do to improve the amount of low-noise volume I can get before it starts to audibly distort?

Kim Christensen:
You may be able to compress the audio using the application on your PC. That way you could have the volume set to an optimal level for all situations.
I assume you've oriented the radio relative to the transmitter for max signal? (turned the loopsticks for max coupling)
Do you have a schematic for this circuit?

MaximRecoil:

--- Quote from: Kim Christensen on March 30, 2023, 12:08:46 am ---You may be able to compress the audio using the application on your PC. That way you could have the volume set to an optimal level for all situations.
--- End quote ---

What do you mean by "compress"? If you mean file size compression, the stuff I was just listening to is already highly compressed (~16 kbps Opus). If you mean compress the dynamic range, since this stuff was recorded from radio broadcasts to begin with, it already has a compressed dynamic range.


--- Quote ---I assume you've oriented the radio relative to the transmitter for max signal? (turned the loopsticks for max coupling)
--- End quote ---

Yeah, reception isn't a problem. One of my radios has a signal strength meter, and it shows pretty much maximum strength. Like I said, I can get it to a good listening volume and barely hear any noise/static at all; the problem is the distortion when music plays (the distortion isn't really noticeable when it's just people talking though). It's the same type of distortion you get when you turn anything up beyond its clean amplification capabilities, which is what makes me wonder if different transistors would help.


--- Quote ---Do you have a schematic for this circuit?
--- End quote ---

Surprisingly, I do (along with a PCB layout diagram). I didn't think I did, else I would have included it in my original post:

Kim Christensen:

--- Quote ---mean compress the dynamic range
--- End quote ---
Yes, that's what I meant.


--- Quote ---It's very low-powered; it only has a range of about a foot.
--- End quote ---

--- Quote ---Yeah, reception isn't a problem. One of my radios has a signal strength meter, and it shows pretty much maximum strength.
--- End quote ---

These two statements are very contradictory to me... Sounds like it doesn't output much power at all, even for something like this. I did notice that the PCB has a connection for an external antenna. You might want to connect a longish wire to the "monitor" connection. Loudness of AM reception is very dependent upon received signal strength, so you may just be overdriving it to compensate.


--- Quote ---Like I said, I can get it to a good listening volume and barely hear any noise/static at all; the problem is the distortion when music plays (the distortion isn't really noticeable when it's just people talking though). It's the same type of distortion you get when you turn anything up beyond its clean amplification capabilities, which is what makes me wonder if different transistors would help.
--- End quote ---

It's a very simple circuit. The fidelity of the design won't be that great. Was this a kit or did it come pre-assembled?

MaximRecoil:

--- Quote from: Kim Christensen on March 30, 2023, 03:56:17 am ---These two statements are very contradictory to me... Sounds like it doesn't output much power at all, even for something like this.
--- End quote ---

When it's within range the reception is great, which is why there's hardly any static at all; much stronger reception than any actual AM radio station here. I have it sitting just a few inches from the radio's internal ferrite rod antenna (or external one in the case of my radio that has the signal strength meter).


--- Quote ---I did notice that the PCB has a connection for an external antenna. You might want to connect a longish wire to the "monitor" connection. Loudness of AM reception is very dependent upon received signal strength, so you may just be overdriving it to compensate.
--- End quote ---

I didn't know it had a connection for an external antenna. I wonder why it's called "monitor." I'll definitely try that though.


--- Quote ---It's a very simple circuit. The fidelity of the design won't be that great. Was this a kit or did it come pre-assembled?

--- End quote ---

The sound is pretty good for spoken word, and even with some types of music there's no noticeable distortion. I first noticed the distortion with some music that had prominent bass in it; the bass notes were very noticeably distorted. Then I noticed it again while listening to a recording of an old Art Bell radio broadcast, during his "bumper music" ("Right Back Where We Started From" by Maxine Nightingale), and it wasn't bass that was distorting this time, but rather, the higher pitched instrumentals at the start of the song.

It was a kit that I assembled. I used all the parts that came with it except for the off-brand electrolytic capacitors. I had some new Nichicon capacitors of the right values here already so I used those instead.

Aren't the transistors amplifying the signal? If so, could they be replaced with more capable transistors? They wouldn't need to be all that much more powerful.

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