I have been interested in antique radios for many years, but have focused on Gramophones lately. With the Gramophone I can buy 78's on ebay and listen to the old music from the 20's-40's. Not my era, but I really like these tunes.

I am a EE with 44 years design experience, mostly in digital. I semi-retired at 50 and have been doing stereo equipment design ever since. I have done minimal tube design (one transmitter in college), but I have some nice NOS tubes in my audio system. I have modified those amps to improve the SQ (sound quality).

The problem with me pulling the trigger on a nice antique radio has been the broadcast programming that I can pick up from the airwaves. I can of course use my computer to drive an AM transmitter to get the music and programming I want to hear, but this requires a lot of baby-sitting, ties-up my computer and it only gives me one channel to demonstrate to my friends and to enjoy myself.

This lead me to the idea of building a multi-channel transmitter with different programming on each channel. Long playlists that will literally pay for a week or a month before they repeat. When Using Mp3 tracks with modern playback electronics, you can easily store 3K tracks on USB for each channel. I am building this black box now. It is simple because it has a power inlet, a F-connector output and 2-5 USB ports for thumb-drives on it. Each thumb-drive is for a different transmitter and therefore a different tunable frequency. You power it on and there are instantly several channels to be tuned on the antique radio, each with different genres, like radio shows, historical news broadcasts and vintage music. I have a lot of content ready to go, I just need to gather the parts and build one to test it out. The F-connector output will have a 75 ohm coax cable like RG-59 and then a 75 to 300 ohm balun to the radio's antenna input, so it's not really broadcasting. It will have decent signal strength, but not overload the radio. Only direct cabled to the radio. Once I have this working and proven, I will get myself a nice tabletop antique radio. I have several high-bandwidth scopes, including a 7GHz one, so I can do lots of optimizing. I don't really have an RF background, primarily digital, but I know enough to do this right I think.

I am also considering donating a couple of these to the various antique radio museums, maybe a version with several outputs so that several radios can be driven simultaneously. It would make the patrons experience much nicer if the radios also had vintage programming on them.

No doubt that you can make it work, but be aware that the input impedance of AM radios that used external antennas was all over the place and very unlikely to be 300Ω. With a direct connection it wouldn't matter anyway, even with a drastic mismatch you would have more than enough signal strength.

They would probably legally be considered a transmitter as they produce an RF output whether your intention is to connect an antenna to the output of the unit or not. Therefore, you should keep in mind the FCC limits for power and note that you can't sell them unless they are FCC type accepted.

It starts with 3 100mW transmitters and then I apply attenuators and op-amps to reduce the amplitude. Everything is impedance-matched, so I should not have much radiated emissions from the box itself. As for impedance of the radio, I will do some reading on my new radio to see what was recommended. I should be able to find a balun that works, or make one.

If it's many dB below 100mW, it should not be an issue for FCC. I want enough power to just peg the strength meter on my modern receiver when direct-connected, but not much more than that, maybe double.

Good grief... Simply broadcast it, then any radio without a wired connection will work. Not that we are bathed in WiFi now whats the issue with three legal part 15 AM stations???

No fussing with receiver, attenuating signal from a legal broadcaster, ugly wires to restrict placing receiver or for the cat to chew...

Well, I don't want to erect an antenna and I want to be able to take the radio to the high-school and play it after I plug it in. I don't want to erect an antenna in my house or at the high school.

Then maybe you should build a self contained AM transmitter with an MP3 player that you can set near your radio and play toons.