brainwagon

I’ve been interested in low power beacon transmitters for a long time.  If you’ve followed my blog, you’ve probably read about my experiments with WSPR and QRSS operation.  Most of those took place on 30m using my FT817 transmitter, sometimes with software that I wrote myself.  But I’ve long thought that I should homebrew a transmitter, and the idea of making it solar powered has always been in the back of my mind.

These operations have all been on ham frequencies, but even if you are unlicensed in the United States, you can get experiment with low power beacon operation under what is known as “Part 15“.   These rules are part of the Code of Federal Regulations, Title 47, and specify certain bands and limitations as suitable for unlicensed devices.   Rather than reading the boring regulations, you can go to Long Wave Club of America website and read up on experimentation under Part 15.  You’ll generally find operation divided into “LowFER” (operation below 200khz, usually in the 2200m and 1750m bands), “MedFER” (operation in the AM broadcast band, usually between 1600khz and 1800khz) and “HiFER” (mostly centered around 13.55Mhz).  

I was initially most interested in the LowFER bands, but I’ve never really gotten off the ground with the project, but recently have become more interested in operation in the HiFER bands.  It presents some significant limitations.  The most desirable band is limited between 13.553 and 13.567 Mhz.  The total power output is specified in terms of a field strength at 30m distance from the antenna, but is on the order of one milliwatt into a half wave dipole at the frequency.   You also are supposed to maintain frequency stablity to about +/- 0.01% over a temperature range of -20 to 50 degrees C. 

This isn’t a lot of power, so it can be challenging to get an effective signal out, but it also makes for some interesting opportunities for home brew construction.  Most beacons of this sort are small crystal controlled transmitters, and they can be powered by reasonably small solar cells, and controlled by inexpensive, low power, eight bit microcontrollers that are readily available. 

Dave Richards (AA7EE) wrote up an awesome description of his Boris Hifer beacon, named after his cat.  It’s a very straightforward design: a Collpitt’s crystal oscillator, a small two stage low pass filter, and an ATtiny85 microcontroller driven by a LP2950 5V regulator that is powered directly from his solar panel.  (He also points at this $20 kit from Chris Smolinksi at blackcatsystems that would be dead simple to put together).   As it happens, I have almost all the parts in my junk drawer, and could easily put such a beacon together.  I even have a stash of the 13.56 crystals somewhere.

Dave’s construction is gorgeous, and he mounted it fairly low at the top of a fence.  I have a similar fence at the back of my property.   I would be tempted to use a more sophisticated solar setup perhaps utilizing a small battery.  Propagation on this frequency is probably better during the daylight hours, but the solar panel likely generates an excess of power each day, and being able to survive small dips in cloud cover and the like would be good.  But then his experience is that even on cloudy days his beacon wakes up, so perhaps I’m overthinking it.  He also has a good description of using the BOD (brown out detection) in the ATtiny85 to ensure proper startup in varying voltage conditions.  His write-up is really great, and will no doubt save me from a lot of hair pulling.  

Well worth reading.  You might also read his page about making a temperature beacon which was inspired by K7TMG and also this short experiment I did back in 2012 (apologies for the jittery phone video):