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 Post subject: One transistor part 15 AM Transmitter!
PostPosted: Sep Sat 25, 2021 12:09 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
This is nothing more than a one transistor colpitts oscillator with 1MHz crystal. There's no need to buy those crystal can oscillators. As always the LM386 is the modulator. Output is taken from emitter tap and fed to a low impedance tap on a ferrite rod auto transformer also tuned to 1 MHz to supress the harmonics. Select the value of the resistor feeding ferrite rod coil to maintain high modulation index and good quality audio, that is, not overload the oscillator. Tap should be at 10% from ground end. With 23ma of current and 6V from LM386, it's compatible with part 15 rules.

What is the impedance of a 6 foot of wire at 1MHz? Probably 30 to 50K. The tuned circuit needs to develop a large AC voltage to feed the antenna for any reasonable range. A high Q pot core could be used from the Wenzel circuit for maximum range. Feed it with a 7812 voltage regulator to remove all power supply ripples. The circuit is very small and self explanatory with good sounding audio, a perfect mini radio station with just a handful of components.


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 Post subject: Re: One transistor part 15 AM Transmitter!
PostPosted: Sep Sat 25, 2021 4:16 am 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 711
Location: Colorado Springs, CO 80917
Quote:
with good sounding audio


You built this one, then? Or did someone else build it? I thought that these hand drawn designs were things you usually offered & hoped someone would build.


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 Post subject: Re: One transistor part 15 AM Transmitter!
PostPosted: Sep Sat 25, 2021 4:31 am 
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Joined: Feb Fri 01, 2019 11:35 pm
Posts: 710
Location: Riverside Ca. 92504
Would you need something to match the input impedance or is this a direct feed in.


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 Post subject: Re: One transistor part 15 AM Transmitter!
PostPosted: Sep Sat 25, 2021 4:44 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
hwhall wrote:
Quote:
with good sounding audio


You built this one, then? Or did someone else build it? I thought that these hand drawn designs were things you usually offered & hoped someone would build.


Built in 2005. All are my designs and ideas


Last edited by Dare4444 on Sep Sat 25, 2021 10:38 pm, edited 1 time in total.

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 Post subject: Re: One transistor part 15 AM Transmitter!
PostPosted: Sep Sat 25, 2021 4:45 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
tomb123 wrote:
Would you need something to match the input impedance or is this a direct feed in.


To lm386? The audio is fed directly to the 820n.


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 Post subject: Re: One transistor part 15 AM Transmitter!
PostPosted: Oct Fri 01, 2021 2:26 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1573
Dare4444 wrote:
A high Q pot core could be used from the Wenzel circuit for maximum range.


When I did a series of experiments on loops vs whips, for antennas that were a fraction of the size of the wavelength of the transmission frequency, a number of issues came up, most relate to the radiation resistance. One was it is much easier to accelerate charges in a conductor formed into a loop, with tuning capacitor across its terminals and create a reasonable current in that conductor. Much more difficult say for a 3 or 4 foot long whip, trying to establish any reasonable current at frequencies below a few MHz. As you pointed out, the drive voltage needs to be very high.

I have attached a photo of an experimental 3.3 foot long antenna I made for 1500kHz operation. It used a 1:10 ratio ferrite pot core auto-transformer in its base, and also contained a neon bulb that would light up when the voltage at the base of the rod exceeded 70V. Despite the negligible energy loss into the neon, it did create some carrier distortion due to the neon's conduction.


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