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 Post subject: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Fri 24, 2021 9:36 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 779
16 years ago I was experimenting with >100% modulation in an AM transmitter. Adjust the 4.7k pot for final DC input of 100mW.
Then set VR1 until modulation clips at 50% , then lower a little
. Now adjust input pot of LM386 feeding the final stage for full
300% mod. depth on an oscilloscope. Used 1KHz tone from a YouTube video to calibrate it. No modulation = final input power should be 100mW
. . Confirm it again. A 6 foot wire antenna matching network was not used, just a 50ohm dummy load.

It was weird that I was able to achieve 300%
positive modulation by:

Modulating both the oscillator and final from the same audio supply. It didn't sound bad at all, and the range increased manyfold times! 300% positive modulation is a lot and with a proper ground and matched wire antenna 1Km range maybe possible? This might be pushing it. 1/2 a mile is a more realistic. Worth a try.

Beyond 100% modulation most IF stages in AM receivers saturate but mine sounded fine and the audio had quite a punch and even noise couldn't drown it. Speech was perfectly readable.


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sat 25, 2021 1:31 pm 
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Posts: 18896
Location: Utica, NY 13502 (USA)
Back in the "old" days, there was no limit in the rules for positive modulation of AM broadcast transmitters. Some stations found that, with proper audio processing, they could achieve very high positive modulation while still limiting negative modulation to 100% which was required by the rules. This gave certain stations a competitive loudness edge over the competition. Eventually, because of interference to adjacent channels due to increased sideband power, the rules were changed to limit positive modulation to 125%. I don't know if this applies to homebrew (non-certified) AM Part 15 transmitter operation. I couldn't find anything in the Part 15 rules about AM modulation limits but I may have missed it.

Dave


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sat 25, 2021 3:39 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 779
Dave Doughty wrote:
Back in the "old" days, there was no limit in the rules for positive modulation of AM broadcast transmitters. Some stations found that, with proper audio processing, they could achieve very high positive modulation while still limiting negative modulation to 100% which was required by the rules. This gave certain stations a competitive loudness edge over the competition. Eventually, because of interference to adjacent channels due to increased sideband power, the rules were changed to limit positive modulation to 125%. I don't know if this applies to homebrew (non-certified) AM Part 15 transmitter operation. I couldn't find anything in the Part 15 rules about AM modulation limits but I may have missed it.

Dave



Vectronics 1290k output network should match a wire ant, right? Isn't 300% modulation just too much?


Last edited by Dare4444 on Sep Sat 25, 2021 10:36 pm, edited 1 time in total.

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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sat 25, 2021 4:22 pm 
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Posts: 5002
Location: Texas
There is a circuit somewhere online for a homebrew asymmetrical modulator that shifts the modulation point from center to more positive than negative. So the positive would be over 100% in the negative less than 100%. It uses a transformer with center tap but with a resistor in line on the negative side so it doesn't modulate as much as the positive does. I never got around to building it but I was wondering how well it would work.

Here's the article, and in the article is a link to the schematic.

http://amfone.net/AMPX/61.htm


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sat 25, 2021 10:44 pm 
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Location: Somers, CT
Most AM detectors in vintage radios have problems with 100 percent mod. levels.

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Just because it can be done doesn't mean it should be done.


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sun 26, 2021 12:07 am 
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Peter Bertini wrote:
Most AM detectors in vintage radios have problems with 100 percent mod. levels.


Good point. Envelope detectors are not good for more than 100% modulation.

Hmmm. I'd bet a synchronous detector would work. What about infinite impedance detectors? What about plate detection? And is there a way to use slope detection in AM? While we're at it, what other kinds of detection are there?


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Sun 26, 2021 1:24 pm 
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Location: Utica, NY 13502 (USA)
Macrohenry wrote:
Peter Bertini wrote:
Most AM detectors in vintage radios have problems with 100 percent mod. levels.
Good point. Envelope detectors are not good for more than 100% modulation.
I can understand how this is true in the negative direction because the carrier is reduced (to 0 at 100%). But in the positive direction, the carrier is increased.

Dave


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Mon 27, 2021 1:49 pm 
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Dave Doughty wrote:
Macrohenry wrote:
Peter Bertini wrote:
Most AM detectors in vintage radios have problems with 100 percent mod. levels.
Good point. Envelope detectors are not good for more than 100% modulation.
I can understand how this is true in the negative direction because the carrier is reduced (to 0 at 100%). But in the positive direction, the carrier is increased.

Dave


http://www.valveradio.net/radio/low-dis ... ector.html


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Tue 28, 2021 7:19 pm 
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Location: Seattle, WA
My favourite is the 4 transistor 1.5V transmitter.

Btw, you could just post your ideas in a single thread (E.g. name it simple low power am transmitters), that way the forum wouldn't have many-many threads going on a very similar subject.


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 Post subject: Re: 300% positive modulation from 4 transistor AMTX
PostPosted: Sep Tue 28, 2021 11:43 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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szoftveres wrote:
My favourite is the 4 transistor 1.5V transmitter.

Btw, you could just post your ideas in a single thread (E.g. name it simple low power am transmitters), that way the forum wouldn't have many-many threads going on a very similar subject.


It didn't cross my mind, I'll do it. Thanks for the excellent idea which 4 transistor 1.5V TX you're talking about? Link to schematic pls!


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