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Three simple AM transmitter plans for Part 15 broadcasting

Back in the not-so-distant past the primary mode of transmitter diaspora was electronics magazines articles by circuit gurus like Rudolph Graf, William Sheets and Doug DeMaw.
     If you missed an issue with a great circuit, you could always order reprints for $2.00.
     The bad, and somewhat ironic, news is that the publishers of electronics magazines couldn't adapt to the new electronic publishing environment brought about by the medium we are using now and all those stories publications have perished.
     The good news is that an enterprising person has resurrected all of those dead issues in a delightful site called American Radio History
     Having been an avid reader of our dearly departed physical media, your faithful editor was able to take you back in time to bring back some great little Part 15 AM transmitter projects featured in that chosen medium of yesteryear.         
     So with that in mind, please to be enjoying three low power AM transmitter circuits from 80s and 90s electronics magazines.
     Parts lists included but you'll likely need to do some substitutions - more's the fun!

Doug Demaw's "Mini Broadcaster," with five 2N2222 transistors in this intermediate level project, Monitoring Times, December 1996

Doug Demaw's "Transmitter Basics," featuring a low parts count for this intermediate level project, Monitoring Times, October 1988

Joseph Smolski's "Build a 1930's transmitter," A tube powered mini-transmitter with fewer than 20 components for this beginner level project, Popular Electronics, November 1991
     So grab some of the new lead-free solder, heat up the iron and make some radio! Lots of articles on this very blog about antennas you can use to extend range. 

5 watt AM tube type transmitter plan

Created for carrier current use, this tough little transmitter could serve a variety of low power radio uses.
     Besides, we all know tubes are cool!   
     While a conventional antenna is tightly regulated, limited use of the ac power line is permitted as a low-power, carrier-current antenna.
     Any radio plugged into or near the AC circuit will receive the signal and range is usually limited by the utility transformers.
     This schematic, parts list and instruction sheet come from 1973 and offer a simple, robust three vacuum tube transmitter circuit that might make a nice project.
      Parts substitution skills will be important but it's likely every part needed is still available.
      So get out the bread board and heat up the soldering iron, your next transmitter awaits!

Click here for the 5 watt AM tube type transmitter plan

1 watt AM transmitter, available assembled

It's the item our visitors most search for and now it can be yours!
     6v6 Electronics, home of Vintage Components and a wide variety of radio topics, components and kits, has done it.
     6v6 Electronics now offers the "Mosquito" a ready built, ready to run one watt AM transmitter for the US market.
     Vintage Components first entered the low power radio scene with an innovative Part 15 compliant transmitter called the "Gizmo."
     While no longer available, this first unit served as a clever and robust effort, prologue to an exciting selection of innovative low power transmitters.
     The Gizmo gave way to the "Metzo" (now also discontinued) and finally the "Spitfire" a truly remarkable Part 15 compliant unit.
     We still have the Gizmo we ordered from Vintage Components back in our SCWIS days and our Gizmo works as well today as it did 16 years ago - a testament to the quality you can count on from 6v6 Electronics.
    The "Mosquito" offers an amazing array of features and you can read about those on the Mosquito US and Mosquito EU product pages.
     Even better, your faithful editor was able to get an email interview with 6v6 Electronics for an in depth look at some of the insider secrets behind this exciting product:

Q. Why did you decide to offer the Mosquito?

The Mosquito 1 Watt AM Transmitter was designed to cater to
  • the new 1 Watt Low Power AM standards (MW) being introduced worldwide, (for example in  Scandinavia and the Netherlands).
  • and also for use on 160m SW (Top Band) for radio amateurs (HAMS)
Q. Do you recommend any specific antenna configuration? E.G., air coil vs iron core matching coil, use a 365pf variable capacitor, base loaded or center loaded, is a ground rod recommended?

We have no specific recommendations for type of antenna

  • the rules in Europe & Scandinavia are different (as are the rules for MW & SW)
  • antenna choice is largely governed by individual users circumstances (live in flat, house with garden, open land etc etc)
  • as an example some Scandinavian countries have no limit to antenna size so theoretically you could use a half wave 110 meter long wire with matching balun (space permitting)
  • far more likely is base loaded verticals with suitable loading coils
We do however require the antenna to
  • present a 50 Ohm load and ideally be tuned/matched to the frequency of choice
  • a suitable ground plan or counterpoise (depends on antenna selection)
  • suggest the transmitter connected to Earth

Q. Was there a technical reason you chose 1000 - 1710 kHZ or is that the typical band range for an EU MW XMTR?

The reasons are
  • with short antenna the higher the MW operation frequency the more efficient the antenna will be
  • to minimize wasted power in the transmitter operate in class E mode where possible
  • and don't forget the unit will also run SW Top band 160M as well
  • in  fact the unit will cover 450khz to 1000khz working in class C!
Q. How does the Mosquito differ from the Spitfire? e.g., the Spitfire includes an antenna matching section but there's no mention of that for the Mosquito, the Mosquito doesn't seem to have the same audio management functions, etc.

  • The Spitfire is designed to "Run out the box" using 3 Meter antenna... Requires virtually no technical Knowledge...
  • With the Mosquito you will need the some technical skills to fabricate a suitable antenna
  • Both transmitters have the same audio features.
Q. Anything else you'd like to include?
  • The Mosquito transmitter is ready built and ready to run..
  • The Mosquito employs a crystal controlled frequency synthesizer for accurate tuning and drift free frequency stability.
  • The Mosquito employs "Class E & C" output technology and provides the full 1 Watt carrier, 
  • The modulator stages includes both over current and thermal shutdown.
  • The modulation stage can provide 0 to 110% modulation.
  • The Mosquito also has a two channel mixer for either Stereo to Mono or twin channel Mono audio sources.
  • The Mosquito printed circuit board uses a grounding technique that splits the RF and Audio grounds, as well as providing and external grounding point. 
  • In addition a new external "universal"power supply provides regulated DC power to the transmitter, supplied with county specific power leads for the EU,US,UK & Australia.
  • Fully Built and Tested: High Quality PCB, Plastic CASE to IP56
  • AC Power Adapter: Power Supply 12-15V DC 90-240V AC 50-60 Hz (EU,US,UK & Australia.)
  • Audio Cables included.
  • Stereo 3.5mm jack to twin RCA (Phono)
    The "Mosquito," and most of the other exciting products at 6v6 Electronics is available for shipping to:
  • The Netherlands: for use in the Netherlands for a “laagvermogen AM” (LPAM) license granted by Agentschap Telecom.
  • Scandinavia: Sweden, Norway, Denmark & Finland
  • Worldwide subject to any country specific regulations
     Wow! You won't want to swat this Mosquito. If you'd like to join the experimenters who enjoy the relatively lax enforcement in the AM band and try your hand a low power radio the easy way, check out the "Mosquito" at 6v6 Electronics

Quick and simple FM radio station goes solar

Seeking to build an easy, quick and inexpensive low power FM radio station, I first experimented with the Scosche FMT4 transmitter. I paired the FMT4 with a cheap MP3 player for a self-contained, low power radio station.
     You can read about this player/transmitter configuration on this blog, here: Quick, simple FM radio station.
     Like many of you who have tried the Scosche FMT4 FM transmitter, I soon realized that battery power wasn't going to be practical. 
     It was nice that the AAA batteries for the transmitter and portable MP3 player lasted over 3 days but the down side was that the batteries only lasted 3 days - not ideal for a 24/7-365 operation.
     Also like many of those who have used the FMT4 for Part 15 broadcasting, I've added  an old cell phone charger and voltage drop resistor to provide full time power - but that means you're tethered to an outlet.
     Using this simple FM station model, plug in power isn't all bad. Most of us have noticed an occasional 110 V outlet in intriguing public places and adding the Simple FM Station to a location like that could be fun, all by itself.
     Looks like I need to do a blog post on guerilla FM station placements!
     To really liberate this guerilla radio operation, I wanted to give solar a try.
     Looking through my junk box, I found some items that could give me the chance to try solar power in a low power radio configuration.
     Item 1 was a solar battery maintainer, providing up to 22 volts and 120 MA, around 12 - 13 volts under load.
     Item 2 was a very old portable jump start box that no longer had enough oomph to turn over a car engine.
     After charging up the jump starter with the wall wart charger included with the starter, I was happy to discover it still held a charge - not enough to start a car but plenty to run the station.
     With a solar panel and a means to store solar power, I then needed to match the power available to the needs of the player and transmitter.
     My first effort was to use voltage drop resistor circuits to get my 1.5 V and 3 V sources for the player and transmitter. Unfortunately, I found that as the battery level dropped, my dropped voltage fell below the  operating range of the connected devices.
     The next option, Item 3, was to try  a DROK DC-DC Step-down Voltage & Current Regulator Module from Amazon. Two were definitely overkill, as I could have used one to get down to 3 V and a voltage drop resistor to get down to 1.5 V but this is experimental broadcasting, so why not experiment?
     I connected both regulator inputs to the 12 V source and then trimmed the output on one to 1.5 V and the second to 3 V for the player and transmitter, respectively.
     The regulators created a bit of audible noise, barely perceptible when listening on an FM receiver, but the noise abated as the capacitors on the regulator boards charged up. The regulators have been quiet since shortly after being connected to power.
     The MA consumption of the transmitter and player are estimated, based on the 700 or so MAH in a AAA battery. These devices ran about 70 - 80 hours continuously before the batteries went dead.
     The 120 MA output for the solar panel is, of course, in full sunlight. Fortunately, I've been impressed with the output, measured by a VOM, even in less than ideal conditions. The panel is a no-name unit I bought online about 8 years ago but it seems to be working well. The stated output is 1.5 Watts.
     I'm located not far from the Canadian border so we have long days from late February until early October. I might add a second 1.5 Watt panel during the winter.
     So, how's it working?
     So far, it's been encouraging. The solar panel has kept the battery charged enough to run the system for nearly two weeks.
     Without any post processing, the audio quality is a bit weak but the station comes through A-OK when tuning an analog receiver. I have an old guitar compressor that helps quite a bit and that runs on a 9 V battery. The next upgrade might be to put the transmitter and player on one regulator, with a voltage drop resistor for the player, and connect the compressor to the second regulator.
     True solar power aficionados might be asking "Where's the controller for the solar panel?" and the answer is that there isn't one. The input is pretty marginal and with the steady drain on the battery from the regulators, transmitter and player it seems that there should be adequate headroom.
      If the battery does overcharge and become damaged, that's OK - it was headed for the junk pile anyway. I have two surplus 10 AH gel cells waiting to take over and I'll add a controller when I switch over.
     When high and in the clear, one of these Scosche units with a hacked antenna can get out several hundred yards. I'm hoping I'll be able to finish testing and arrive at a final configuration that makes ideal placement possible, with the unit totally self contained and having no need to connect to power or audio.
     Of course, this is merely a low cost demonstration project but a project that suggests some fascinating steps forward.
     A little bigger solar panel, a properly sized controller and battery matched to an FM transmitter with a little more output power could be very interesting. Pair that with a low current draw audio source like a full feature MP3 player and the opportunities are multifold.
     A more refined setup could really present some interesting opportunities for placement near a larger potential audience.
     Experimental broadcasting for a better tomorrow!