Over the last few weeks I’ve been working on a new project, a prototype of a new product. I don’t have a name for it yet, but it is essentially a beacon transmitter. Beacons are frequently used in the amateur microwave radio hobby as location markers and as a means for testing receivers. During a contest, a beacon in a known fixed location can be used to establish a heading. Since microwave antennas are typically high gain, it is really important to know your heading. Additionally a beacon can be useful in studying propagation between two points. In Southern California, there are several beacons on the 10 GHz band including one with wide coverage on Frazier Mountain.
Historically, beacons have been built by individuals or clubs who have access to some great spot on a tower or a mountain, as well as access to the parts and test equipment required to get one on the air. The goal of my beacon project is to put together a kit that will allow more people to put up beacons in more places. I am basing the kit on my two existing products, the OpenSynth PLL synthesizer, and the TimesFour multiplier. Since the OpenSynth is based on a fractional PLL, it is possible to achieve the narrow frequency steps required for use in crowded bands. Also, since the OpenSynth has a microcontroller, it is relatively easy to program in a CWID routine that uses FSK to generate a Morse code identifier.
My first major design choice was packaging and location of the electronics. There are two ways to make a beacon, one in which the electronics are located indoors, and the other has the electronics on the tower next to the antenna. I decided that for ease of installation and lower cost, I would have the electronics on the tower. For the 10 MHz reference though, I chose to keep that indoors. This allows builders to use their choice of high stability 10 MHz sources, from an ovenized crystal to a GPS locked Rb standard.
To get power to the beacon, as well as the 10 MHz, I designed a small bias tee board which also has ESD and reverse voltage protection as well as a fuse. Microwave beacons are often installed in harsh locations and it is important to protect the sensitive electronics appropriately.
For an antennas, it is hard to beat the slotted waveguide antenna. A beacon transmitter should have an omni-directional horizontal pattern, but very little vertical radiation. The slotted waveguide antenna does this very well, and a local ham Dan, W6DFW makes some very nice ones!
Finally, a suitable enclosure is necessary. Suitable means that it provides protection against a harsh environment (rain, dust, wind, snow, ice, EMI, RFI). The enclosure I’m using is a die cast aluminum housing which is both waterproof and has an EMI/RFI gasket as well. Beacons can be installed on the same tower as high powered broadcast transmitters, so it is critical to keep everything very well shielded.
After assembling all the pieces and programming the synthesizer, I put the prototype beacon on my apartment roof in Redondo Beach, California. I have it mounted to an existing DirecTV dish mount. I’m interested to see how long this will last before the landlord starts asking questions!
In one week of operation I have gotten reception signal reports from as far away as La Jolla, CA (100 miles south), and Gorman, CA (70 miles to the north). Other hams more local to me have also reported hearing the beacon from their homes. I would call this first prototype succesful, and am very interested to see how things hold up in the weather over the next few weeks.
Below is a screen capture of my beacon being received by Rein W6SZ at his home in Rancho Cucamonga, a distance of almost 50 miles.
Here’s a recording of my beacon as heard from Mt. Soledad in La Jolla, CA by Kerry, N6IZW.
There’s plenty of work to do before this becomes a product including the design and construction of an amplifier. The beacon currently puts out ~75 mW of power which is enough for some applications, but it is handy to have more power, so that will be coming. I also need to look at humidity venting options, RF filtering, and issues related to getting a good 10 MHz signal up different lengths of coax.
Tony – KC6QHP