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 Post subject: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Thu 03, 2019 2:01 am 
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I just found this and thought of those with SW radios missing out on homebrew transmitter's frequency. I'm sure it needs a lot of audio input, unlike our LM386 transmitters with the amp built in.
Add the LM368 module and you have my "Muntzed AM Transmitter" viewtopic.php?f=12&t=340927&p=2865396&hilit=muntzed#p2865396

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https://www.ebay.com/p/5033852682


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Thu 03, 2019 6:46 pm 
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Interesting.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Mon 07, 2019 4:22 am 
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What's the legal part 15 power limit at 3 and 5 MHz?

That said it is pre-assembled. So unless it is FCC certified it isn't legal.

That said I do like it.

A simple transmitter.

My guess is the transformer may be custom wound to provide the necessary output voltage for a cellphone, MP3 player ETC... as the source.

Not sure the frequency response of the small audio transformer though.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Tue 08, 2019 9:58 pm 
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Tube Radio wrote:
What's the legal part 15 power limit at 3 and 5 MHz?



15.223 Operation in the band 1.705-10 MHz.
(a) The field strength of any emission within the band 1.705-10.0 MHz shall not exceed 100 microvolts/meter at a distance of 30 meters. However, if the bandwidth of the emission is less than 10% of the center frequency, the field strength shall not exceed 15 microvolts/meter or (the bandwidth of the device in kHz) divided by (the center frequency of the device in MHz) microvolts/meter at a distance of 30 meters, whichever is the higher level. For the purposes of this section, bandwidth is determined at the points 6 dB down from the modulated carrier. The emission limits in this paragraph are based on measurement instrumentation employing an average detector. The provisions in §15.35(b) for limiting peak emissions apply.

(b) The field strength of emissions outside of the band 1.705-10.0 MHz shall not exceed the general radiated emission limits in §15.209.


There is no DC power limit, you would have to do some good faith engineering to ensure that you comply with the radiated field strength requirements. Unless you are doing fairly wideband (300 and 500 kHz) transmissions the limit would be a field strength of 15 uV/m at a distance of 30 m. Any power input and antenna structure that complies with the limit is OK. But of course this is for home built devices. Since these are commercial they have to be certified and you don't have to worry about it ... assuming they actually are certified!


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Wed 09, 2019 2:04 pm 
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Exactly.

I do hope these are certified.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Wed 09, 2019 2:29 pm 
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If I had to guess, I would say that the likelihood of these being FCC certified is zero.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Wed 09, 2019 7:22 pm 
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Its 3 clock oscillators connected to an antenna. Legal? Who knows. I've tested many clock oscillators in a test fixture that radiated as much and I don't think we worried about legality per Part 15.

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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Wed 09, 2019 7:37 pm 
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I suppose the clock oscillators don't put out near enough to violate part 15 which also means they won't have much range unles a tuned antenna is added.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Wed 09, 2019 7:41 pm 
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A loopstick would do it.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Thu 10, 2019 1:02 am 
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Clock oscillators aren't made to be modulated, so the modulation is probably accomplished by modulating the supply voltage. If that is the case, giving it too low voltage would make it stop oscillating. So you won't get 100% modulation; perhaps not even 50%.

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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Thu 10, 2019 12:44 pm 
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I once built one of those that ran on a 9 Vdc battery.

I of course ran it on 5 Vdc since that's what the oscillator ran on.

Actually sounded fairly decent.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Thu 10, 2019 2:32 pm 
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Jim Mueller wrote:
Clock oscillators aren't made to be modulated, so the modulation is probably accomplished by modulating the supply voltage. If that is the case, giving it too low voltage would make it stop oscillating. So you won't get 100% modulation; perhaps not even 50%.

I dunno about osc in this transmitter but the ECS we were using in the 386 xmitt will oscillate at less than 2v. They perform fine modulating B+ voltage in a range of 2.5 to 4.5v. Give a easy 75-80% modulation.

I keep one on 24/7 connected to cable TV box. Running steady for over four years, no issues.

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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Sat 12, 2019 3:36 am 
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One thing is that the crystal oscillators put out square waves. So unless there is filtering, it will put out harmonics, and that may be more immediate than "too much output".

Part 15 was originally about incidental radiation, like oscillators in receivers. I don't know if low power transmitters were specjfically listed, or were added with time. Though deliberate radiators like "phono oscillators" and license free walkie talkies have been around for decades. I know there was a reworking, I think in the seventies, that changed or clarified the rukes about low lower transmitters. But digital electronics was probably the main thrust, generating more noise but also using more oscillators.


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 Post subject: Re: 3-channel Am Transmitter 1mhz 3mhz 5mhz $16
PostPosted: Oct Sat 19, 2019 2:34 am 
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I received this transmitter today, installed a battery, screwed in the 2½' antenna and plugged in a small mp3 player. No power switch, turned on by input plug. Tuned to 1mHz
With an AM radio within a foot and the player maxed the radio sounded OK.
Jumpered to 3mHz and 5mhz worked better. Adding an amp between the player greatly improved everything.

Connected a ground, and an outdoor antenna and range was increased to barely 20'. But as mentioned above it was heard on 3, 5 and 10mhz. Using the whip limited it to a few feet. It's just this basic circuit with 2 extra channels powered by an 18650.
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