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Very simple low power AM transmitters

A transmitter with fewer than 10 components? No, four transmitters with fewer than 10 components! Here are four ways to have fun with low power radio, quickly. Low-component-count transmitters probably won't deliver spectacular range and they will be a bit fussy to tune but enjoyment is virtually guaranteed. These circuits are easy to put together and fun to try. Be careful - I built one of these in the early 90s and I've been hooked ever since!
    One transistor transmitter - This circuit has a fairly low input impedence and will operate in the 1200 to 1600 KHz range. The frequency is adjusted with the variable capacitor G1.
    A more robust one transistor transmitter - Another circuit that works best on the 800-1200 kHz range, but with changes to the inductor L or the variable capacitor C1 it can transmit to the whole range of the AM dial.
    A 555 IC used as an oscillator - This circuit uses a 555 timer IC, resistors, capacitors, mic and an wire antenna. The 555 timer IC is used a free running multi-vibrator with the frequency pulled down to 500 - 600 KHz.
    A crystal oscillator based 1000 KHz transmitter - In this circuit, the transformer isolates the input from the oscillator and provides a better impedence match. The signal from the secondary coil modulates the DC power to the oscillator. A wire connected to the oscillator output works as an antenna. Another 1000 KHz crystal oscillator page and this new addition, the 1000 KHz Oscillator transmitter YouTube Video!

So, you might ask, where on earth would one get these components, even if there are less than 10? Your Low Power Radio blog has answers! The four suppliers listed below are great places to start your search:
    Dan's Small Parts & Kits is a legendary ham and hobbyist supplier from big sky country (Montana!). Dan's is a "money order or wait for your check to clear only" vendor, probably a key to his long term survival selling small electronic items at low prices. Far too many items to summarize - if it's for radio, it's here.
    Midnight Science, home of the Crystal Set Society. Here you'll find links to FAQs, science fair suggestions, and introductory articles, along with a nice selection of low power radio parts like alligator clips, brass binder posts, air variable capacitors, coil forms, diodes and toroids.
    Peeble's Originals comes from the mind of Mike Peebles who has been producing crystal radios, tube radios, transistor radio kits, and informative instruction articles, since 1988. Here you will also find capacitors, audio match transformers, fixed variable coils & coil forms, connectors, enclosures and other hardware.
    Science Toys offers a whole selection of fun science projects, kits, parts and supplies, as well as the crystal oscillators, audio transformers and other items needed for the crystal oscillator circuit above.



Jay said...

Since you're on the subject. I found a new in the box Radio Shack Science Fair AM Broadcast Station Kit in a box of my crap in my parents garage. It's supposed to transmit at 1000khz on a 9volt battery. Can I change the frequency to something higher? Would a long wire antenna increase my range?

Blog Author said...

That kit would be fun! It's difficult to say if you could easily change the frequency without seeing the schematic. If the oscillator uses discrete components then the component values can be changed to change the frequency.

If the kit came with a 1000 KHz crystal, you would only be able to easily change the frequency by a small amount. Not the worst thing in the world unless another station sits on it, of course.

A long wire antenna would increase range a bit, but not by much until you cleared around 115 feet, or 1/8 wave length - not that fun to do. Most AMers add a loading coil to a 10 foot to 50 foot antenna to increase range.

An antenna over 10 feet long (3 meters, to the letter of the law) is not permitted by the FCC, so folks who want to operate for a long period of time often keep the antenna at 3 meters or 9"8". Lots of info about that here, just search for "AM antenna" or "loading coil" on this blog.

140 turns of light wire around a tube 8" long and 4" in diameter, connected between the transmitter and a ten foot length of copper or aluminum tubing or a 10' length of wire, would be an approximate starting place for 1000KHz.

A good RF ground really helps range as well, search for "Ground" on this blog for more info.

It also helps range to send an amplified audio signal to an AM transmitter. Earphone connector voltage levels are too low for good modulation.

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