EM Theremin- Using hand wavy science to create music

I first heard of the Theremin during a class in my undergraduate studies when another group of students decided to build one for their end-of-class project. I didn’t think much else on this hands-free musical instrument until recently when I had some down time in my graduate studies and decided to do a bit of research on a project to kick winter off until it was warm enough for another hike.

I found a schematic based on the original design on the internet (these days, most folks are abandoning the pitch and volume antennas in favor of optical sensors and microcontrollers), pulled from an early 1996 issue of Electronic Musician magazine, which is still in print as of this posting. The article itself was a nice read, not only detailing the construction of the Theremin including tuning and playing tips for those that take on the challenge of this build themselves, but also providing a bit of the history and theory of this instrument that dates back to 1928. After placing my order with my favorite online electronics component provider, I anxiously awaited for my package to arrive, passing the time by hunting for a partner in crime with which to build this project.

The build went well for the most part, though I found myself wanting to build another custom PCB since the perf board soldering was pretty time consuming. After completing the circuit (and a few hours of debugging) I was ready to tune it up and start practicing.



Winter SOTA: Genesee Mountain FR-194

Summits on the air in the winter in Colorado can be a great opportunity to nab a few lower-point peaks; during the winter months, the W0C region allows for 3 bonus points on all peaks, which makes the 2 or 3 point summits even more enticing. That, coupled with the accessibility of these lower elevations makes them great for activating even when snow lines the trails.

This morning I decided to head up north to Genesee Mountain, along the I-70 corridor about 30 minutes East of Denver. With the drive less than an hour and a half from Colorado Springs, I left around 8:15 and stopped for coffee and made my way out to the summit trailhead. Though it had snowed within the last week, there were plenty of tracks that packed the snow and made the trail easy to find. After only 10 minutes, I made my way to the top of the mountain, indicated by a large flagpole. The temperatures at the top were in the mid 30’s, but with the sun shining and only a slight breeze the conditions were excellent. Though this is usually a busy peak during the summer due to the proximity to downtown, I found myself alone at the top of the summit. I spent half an hour or so on a rock and made 12 contacts on my HT, a Yaesu FT-60R which maxes out at 5 Watts. I know it’s not much on VHF but today I had my first pileup, which makes me even more amped to improve my Morse code skills so I can bring along an HF radio and make contacts across the US.



2m/70cm Duplexer

Last year I bought a dual band Arrow antenna with the intent of hitting the local repeater with my HT, but also leaving open the possibility of using it on SOTA activations and for satellite work. Since then our club has switched repeaters to a more central 2m location, so I looked into how I could get on the air with other hams on VHF throughout the country via satellite. Though Arrow sells duplexers for a modest $60, I intended to make a project out of this and investigate how I could over engineer a solution to this problem myself.

Although I am confident in my ability to synthesize two 5th order passive filters for frequencies <500 MHz, it is much quicker to use some google-fu and find values for the capacitors and inductors needed to create a passband in the 2 meter and 70 cm ham bands. I used 0603 and 0402 components to minimize the space and insertion loss that would be seen for each branch of the duplexer.

After using ADS to layout and simulate the expected performance for my new gadget, I sent my design to Advanced Circuits, a quick turn PCB fab located just outside of Denver. Being the first time I’ve sent out a PCB for my own personal use, I found the process to be easy enough, though if you aren’t familiar with PCB manufacturing processes, it may be difficult to navigate their online forms without dialing their number and getting some help (the help topics throughout the form left much to be desired). While waiting for my PCBs to arrive I modeled up an enclosure for my duplexer and sent it out to the machine shop at work.

After receiving my PCB and machined aluminum enclosure, I quickly assembled the board and headed to the local hardware store to get the fasteners needed to complete my project.

Overall, I am pleased with how everything turned out. I was able to measure 1.3 dB insertion loss through the 2m path and less than 2 dB of insertion loss in the 70 cm path, though admittedly I performed this measurement using a edge launch connector to measure the performance before the enclosure arrived. The next step will be to get this on the air!

2m/70cm Duplexer


Barrett WØASB


Inexpensive APRS Mobile Beacon

APRS, or Automatic Packet Reporting System is an amateur radio-based system for real time digital communication of information. This information may be weather data, GPS location information, text messages, digital repeating (digipeating), or other queries. There are existing APRS options available in commercial radios, but I wanted to see if I could drive the price down to a bare minimum. This post walks through the steps I took to create an APRS GPS beacon as well as how I installed this beacon in a mobile station.

Snapshot of APRS showing digipeaters, WX stations, and mobile stations in my area from www.aprs.fi

I have been considering adding a mobile APRS setup for a number of months, but as my schooling ramped up I had to put the project on the shelf and return to it again when things cooled off between work and school. Luckily my thesis defense finished up near the beginning of the semester and I was able to get some time at the start of December to put some more thought into getting this project going.

I came up with the idea of using a Raspberry Pi as a mobile beacon after seeing the TNC-Pi in a post on Reddit. I have always considered the Raspberry Pi to be neat way to play retro arcade games, though I didn’t see much practical use for it until I considered the Raspberry Pi contained more than the processing power of a Terminal Node Controller, the gateway to bringing my mobile beacon on air.

TNC-Pi, available from Coastal Chipworks

The drawback was I was not able to find anything on the web that addressed the deficiency in transmitting APRS location data, so that meant I could create my own solution. I found the least expensive solution was was to piece together a using the TNC-Pi, a Baofeng Radio, and a modified version of the TNC-Pi to Baofeng radio cord sold on the Coastal Chipworks website. After exchanging a few emails with John Hansen, W2FS who designed the kit, I discovered though setting up an internet gate (iGate) or a digipeater was well covered, there might be some room for including a couple of pointers for setting up GPS location data with the Raspberry Pi system, so that’s where I would need to start my research.

Originally I wanted to use a standalone GPS chip to interface with the Raspberry Pi’s Python configurable IO ports, but I quickly realized banging bits in python to decode the National Marine Electronics Association (NMEA) 108 protocol that GPS uses would be more than I wanted to take on for this project, so I hopped on the web and found a USB GPS for around $30 that would plug into the Pi. Turns out the Xastir software supports innate GPS decode and radio transmit with the Pi and only needed a few settings adjustments to start transmitting as a GPS beacon.

As for the install in my vehicle, I wanted my APRS tracker to automatically power up when the car started, but I didn’t need the rest of the controls on the radio to be immediately available. I also wanted to be able to manually switch off the beacon from transmitting with a switch near the drivers seat. Finally, I wanted to to be able to monitor the transmit PTT indicator from my driver’s seat due to an intermittent issue I encountered with the PTT not releasing after transmission ended. With these goals in mind I began by putting together a couple of simple scripts that would wait until the GPS module locked onto a satellite and then run Xastir on boot.

The suggested fix for the latching PTT was a 2k resistor between the radio and the TNC-Pi; Poor grounding within the HT is the suspected fault for this issue

Well, I wasn’t expecting snow here in Colorado between Christmas and New Years (because, why would it be snowing when there’s time to go to the mountain?), so I had to put the installation of this project off until the weekend after New Years when it warmed up a bit. I made quick work of the install and soon was ready to test my system.

I installed the main power switch near the driver’s seat in the event I want to turn my APRS system off. The second switch is used only for the indicator light when the radio is transmitting
There is a hole on the passenger’s side where I was able to route my electrical through the firewall
The final installation, before looming all of the wires together

I have tabulated the final cost for this installation below. If you don’t already have a spare Pi and a few cheap handhelds sitting around, you will have to include that in your cost. It is probably worth mentioning that this is not the ideal solution to this problem; Xastir is a GUI based application that has more overhead than is needed for a simple transmitting beacon. Though power draw and processing power were considerations for this project, I would rather deal with a few extra mW of power than to redesign an interface that would boot from the command line. This concept can be easily modified to act as a home weather station, iGate, or APRS reader/display with minimal extra hardware.

USB GPS: $28.85
TNC-Pi/TNC-Pi Cable + shipping: $48
Battery Eliminator for UV-5R: $6.99
Wires, Power Plugs, Fuses and Connectors for mobile install: ~$80

Total: $163.84

As you can see I spent a few extra dollars getting connectors and switches to minimize drilling extra holes in the interior, but you depending on your preferences, this line item can be reduced.

Barrett WØASB

First Activation of Indio Mountain, W6/CT-211

Earlier this month I visited family in Southern California for a wedding. I was interested in activating another mountain range as an attempt to make progress for unique ranges activated; of course, I was looking forward to getting out on a hike and ham radio is a great excuse to get some time away from the family.

I decided on a VHF activation of Indio Mountain, W6/CT-211, just a few miles from my hotel. I read some pointers for this hike on summitpost and decided this was the hike for me. I drove out to the trailhead (I parked at the corner of Avenida Monezuma and Calle Colima) and starting making my way to the base of the mountain.

Indio Mountain from the far side of the wash between the road and the mountain

I decided to start climbing up the closest mountain to the road, thinking I could retrace the steps in the opposite direction of the summitpost reference I was using. Unfortunately, the batteries on my new GPS can’t hold a charge, which is a drag, because I was just starting to get the hang of how to use it. I was able to navigate using my cell phone, but at the expense of no cell battery on the way down (notice a trend, heh?).

I found a number of these shells during my climb up- it’s easy to forget the whole desert was once covered in water

The entire hike took a bit longer than expected, but the views were worth it. It just so happens that my only VHF contact was another operator sporting a 0 in his call, also activating a peak (NØOI on CT-007). Though I was a bit bummed I didn’t get the minimum 4 contacts to qualify for the points for the activation, I enjoyed the exercise and time outdoors. As always, I will take this experience and make improvements for my next climb.



Barrett, W0ASB