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Saturday, July 18, 2020

Overcoming the Complexity of the Michigan Mighty Mite: Walter's Sunrise Net Special


From Walter KA4KXX
June 21, 2020

Michigan Mighty-Mite:  Why So Complicated?

The April 2020 issue of QRP Quarterly magazine featured an article by Bob Rosier K4OCE which included a schematic for a “Ten Minute Transmitter” by G4RAW (SK), which apparently first appeared in SPRAT 82 in 1996. 
It is even simpler than the Michigan Mighty Mite, so this transmitter can truly be built on a solderless breadboard in about 15 minutes, because a complex coil is not required.  
The only tuning needed was for me to establish the correct value of the output series capacitor. 
This rig allowed me to check-in to the Sunrise Net (see details in blue text on my QRZ page) today on my very first attempt, and landed me a 549 signal report from 250 miles away.
The first photo shows the transmitter connected to a Transmit/Receive Switch mounted in an Altoids box. In the Transmit position the antenna is disconnected from my 1979 Heathkit HR-1680 receiver, which then coincidentally supplies a sidetone at an ideal volume level.  That little black pushbutton which can be seen in the second photo serves as my key, and works just fine for a five-minute daily QNI on the Sunrise Net.
Of course, part of the secret is having a crystal exactly on the Net frequency, and I have a few left, free to whomever in the Eastern U.S. is interested in building one of these simple Sunrise Net Special Transmitters and participating in our Net. 


6 comments:

  1. And what's the harmonic output look like? A single transistor driven hard, and no tuned circuit on the output.

    Sometimes simplicity is not the route to go.

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  2. I was wondering how long it would take for someone to point this out. OK, as with the MMM, you really should add a low pass filter.

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  3. I just tried the circuit with some parts on hand and measured 0.7-0.8W in to a 50 ohm load after a low pass filter (7th order Chebyshev). Even with the filter, the next harmonic peak is only down ~25dB. The unfiltered waveform is non-sinusoidal to say the least.

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    Replies
    1. By the way, one should make sure to heatsink the transistor, as did the original poster, or else the magic smoke may be released.

      Delete
  4. Actually I transmitted through a double-tuned antenna tuner connected to an End Fed antenna, which I probably should have mentioned even in a short article. I did not measure the transmitted harmonics, but considering the poor propagation these days and the low power level I do not suspect that I interfered with anyone. Certainly I will work to refine the design and add appropriate harmonic suppression so even inexperienced Soldersmokers can build it and transmit a clean signal. Thanks for the comments. --Walter KA4KXX

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  5. A example for a possible ARDF transmitter.
    I'll have a look at the relevant SPRAT article from 1996.

    Frank , EI7KS

    ReplyDelete

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