Chirp and My Michigan Mighty Mite

I’ve been doing lots with my recently brewed Michigan Mighty Mite.  By lots I mean listening to my Tecsun PL-600 with SSB turned on and twiddling the BFO knob to find the tone being emitted by this new transmitter.  What I noticed early on is that the tone emitted started as what sounded like a pretty pure tone would quickly degrade into a warble before settling out at a slightly lower tone which became mostly stable.  After I little research I confirmed that this is what I had heard referred to as “chirp.”  That all makes sense — a variability to the tone emitted might sound like a like a bird’s chirp.

But what was causing it?  It sounds like a loss of power, so I looked into the stability of the power supply, a 9v heavy duty battery.  Watching the voltage across the battery, I saw an approximately 1v drop.  That looks like a culprit!  (Right?)  So, I proceeded to try using a more substantial 7.6v battery pack I made out of 1.2v AA NiMH batteries.  Did that resolve the issue?  Well, if by the dropping voltage, yes.  The voltage dropped less with the NiMH battery pack.  However, I was sad to find, the chirp was still there.  Next idea!

Maybe the transistor is overheating?  After all, I’m using a plastic case 2N2222a TO-90 case transistor.  Everyone says to use a metal can packaged transistor.  So, I figured it could be the transistor for many reasons.  I tried a different 2n2222a.  No dice.  The chirp was still there.  I tried a 2N3904.  Still there.  A MPS A14… Wait a minute, the sound changed.  It still has a chirp, but it is delayed.  So, I learned that the transistor has SOME effect on the chirp.  Next.  2N4401.  Now the chirp is shorter in duration and softer (it doesn’t get as quiet before returning to a tone).  More effects while only changing the transistor.  9287 — Wait, virtually no chirp.  That’s different.  Still there, but greatly diminished.  Am I on to something?  Maybe.  Next, I got some BD139s from Mouser and tried those since they can handle a lot more heat.  What!?  Still a chirp.  One time I held the BD139 body while keying to see if it was getting warm and something surprising happened — no chirp.

Here’s what my bench looks like right now:

Five transistors laid out on postit notes labeled with their part numbers and their effects on chirp.
These are the transistors I tested in my Michigan Mighty Mite to see if the transistor affected the chrip. The BD139 is in-circuit.

From: http://arcarc.xmission.com/PDF_Electronics/Transmitter%20Faults.pdf

CHIRPING
Chirping only applies to CW (Morse) transmitters. Keying chirps are quick changes in the frequency of a transmitter and occur each time the telegraphy key is closed. […]

In CW mode, if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed. These quick changes in frequency sound very much like a canary chirping, hence the term ’chirping‘. Chirping can also occur if the voltage regulation to the oscillator stage is inadequate.

So what’s going on when I hold the transistor?  Well, lots of little things.  I’m not sure which one is causing the change. One thing I realized right after that was that my key is conductive and connected into my circuit.  And, remember, in the Michigan Mighty Mite (MMM, from now on), the circuit is little more than an oscillator…  Hmmm, “if the stage being keyed is too close to the oscillator, then the oscillator may shift slightly off frequency each time the key is closed”  So, by keying and touching the transistor and key I’m altering the electrical (capacitive and possibly feedback) properties of the oscillator.  How can it not chirp?  It’s like licking a tuning fork or something!?  Maybe that isn’t a fair analogy.  I wouldn’t know.  But, it made me smile for a minute 🙂

So, how do I remove the chirp with my hands off the circuit?  I did some reading about Hartley oscillators, which the MMM is styled after, and found that the position of the tap in the inductor in the tank circuit decides how much feedback is delivered to the oscillator.  Then I remembered that the air core inductor I wound on a 1.25″ form for the MMM wasn’t center tapped — it was tapped after like 1/3 of its turns.  So, the orientation of the inductor affects the feedback and therefor stability of the oscillator.  I tried the inductor both ways.  No change.

Then, I remembered something I never should have forgotten: The tuning capacitor.  Doh!!!  I know, it’s a totally noob thing to do.  I did it.  I confess.  So, I tuned the tuning capacitor with the BD139 in place and BAM!  I got a stable pitch (to my ear at least.  I’ll check it on the oscilloscope soon).  That was such sweet music!  Now, I went back and tested all my previous transistors with this current arrangement and used the tuning capacitor.  I found I could make all of the transistors sound a little better than before (with all this experience), and, while there IS variation between different transistors, the BD139 IS still the best.  This leads me to the hypothesis that temperature is a factor.  Am I sure?  Nope.  It is just a likely factor.  If I figure it out 100% for sure later, I’ll share my findings.  For now, it is more stable in my circuit.

My next step is to build this transmitter in a compact and robust package that I can take on the road.  I need to test the limits of my ability to transmit!

Disclaimer: I’m likely to be amazed if I can receive my transmissions just 20 miles away from the town I work in, Orono, ME.  Why?  I’ve never transmitted on anything other than VHF/UHF so far.  My plan is to leave a device recording the output of my Tecsun PL-600 tuned to 7.030MHz and drive to work.  Then, while in Orono, transmit a sequence of code.  Cross my fingers, and when I get home that night, seek to that time of the day and listen!

Low power operation and homebrew

I’ve always been fascinated with leveraging the benefits of computing and electronic communications without consuming too many resources in the process.  I feel, deep inside, that if it isn’t sustainable, that it will likely go away.  Think Rome.

With that said, I recently built a Michigan Mighty Mite transmitter for 40m according to the outline here: http://www.qsl.net/wb5ude/kc6wdk/transmitter.html  Honestly, to be sure I understood the schematic which has some quirks (at least to my lightly trained eye), I had to read lots of other blogs and watch some videos.  The information about winding the air core inductor was the weakest.  I think some of the lightness of documentation is because it will most likely work somewhat even if you mess it up pretty badly.  So, this marks my first ever homebrew transmitter!

Michigan Mighty Mite transmitter wired on a breadboard.
This is the first homebrew transmitter I’ve built. It has been an adventure.

Testing it involved my first homebrew dummy antenna (or dummy load, depending on your vernacular) a pair of 100 ohm resistors in parallel inside a 6oz tomato paste can.

A pair of 1/4 watt 100 Ohm resistors in parallel connected to alligator clips at the bottom of a 6 ounce tomato paste can
This is my first run at assembling a dummy load to prevent this transmitter from radiating its signal far.  No, the leads are not touching the can 🙂

Then my first homebrew low pass filter to attenuate the higher frequency harmonics.  I followed the recipe for 40m here: http://www.gqrp.com/Datasheet_W3NQN.pdf

ceramic capacitors and hand wound ferrite torroid inductors arranged as a low pass filter for 7MHz ham radio operation
Simple LC harmonic, low pass, harmonic filter for 40m

The one site on the Michigan Mighty Mite I encountered suggested the input voltage be between 10 and 14v.  Mine didn’t work when I first assembled it on the breadboard so I wondered if maybe my 9v battery wasn’t enough to start the oscillator (noobs, huh?). I was pretty sure that wasn’t true, but not understanding the theory enough yet, I couldn’t rule it out.  So I threw together a homebrew LM317 based linear power supply.  I set the voltage to 10.7v and it still didn’t work.

Then I got down to the business of triple checking my connections and trying a little rearranging of some connections to shake things up a bit.  When I triple checked every connection and made sure I was following the schematic 100%, sure enough, I got it to work.  One homebrew project turned into four.  Now I have four interchangeable modules that can be put to use in other projects and I’ve learned a lot while refreshing some of my skills like reading datasheets, soldering, and translating schematics to a breadboard.

Bottom line: this was an adventure and, since I knew it would take some hard work and time, I did not get frustrated.  I just set my mind to work through each challenge and learn from their solutions.  This was a HUGE success for me.

What’s more, I feel a strong push to follow this super low power ethos and see where it takes me.  I’m used to thinking of low power as laptop vs. desktop.  Raspberry Pi vs. Laptop.  Baofeng vs 100watt desktop transceiver.  The thing I was reminded of here is that fundamental, empowering, and exciting communications can take place with mW and mA from small dc power supplies (ex. very small solar panels or 9v heavy duty batteries).

Here’s the parting shot of my fully prototyped transmitter

A breadboarded Michigan Mighty Mite transmitter with an external breadboarded LC filter feeding a dummy load in a tomato paste can all keyed by a keyer made on a slice of maple sapling with a strip of flattened copper pipe
Building this was an adventure. I’m so glad I tried the mighty mite first.

Computer interface cable for Baofeng – First try

In this post, I will share some info about the tx/rx computer interface cable I made for my Baofeng radios that is compatible with the Direwolf software TNC and probably others.  Here’s a peak at the mess I have made (that actually works):

Baofeng connected through homebrewed interface to computer to enable APRS packet transmission and reception tuned to my testing frequency of 147.120MHz with the standard 144.390MHz APRS frequency.
This is crude but works and was built entirely with parts I had laying around tuned to my testing frequency of 147.120MHz.

But first, a little background…

Now that I’m receiving beacons in Direwolf and I’ve setup a Raspberry Pi to send and receive APRS, it is time to begin the holy grail of 2 way testing with two APRS nodes — one RPi and a laptop.  Then I will be able to experiment with more advanced messaging capabilities of APRS as well as a BBS system.  BPQ is looking like the top contender right now (Thank you Joe DeAngelo AG6QO for the suggestions and excellent packet BBS implementation log!)

The radios

My first radios where a pair of 2m VHF radios from a thrift shop.  I like those HTX-202 radios.  However, I believed that the VOX feature on newer radios would make it easy for me to send packet data as well as receive it so I bought a couple Baofeng radios.  I bought the super cheap UV-5R VHF/UHF (about $25) and the BTECH UV-5X3 VHF/1.2m/UHF (about $65).

So how did that work out for me?  I found early on that the VOX feature, does not seem to function well.  I can’t find much about others having the issue.  Am I the only one who is either confused or has two radios with a poorly functioning VOX?  I’ve tried setting VOX as low as 1 and as high as 9 (just for giggles).  I’ve tried both the UV-5R and the UV-5X3; both have the issue!  When I yell, close to the mic, the VOX detects audio and enters transmit mode.  Less than a yell and it stays in receive.  Worst of all, I haven’t found an output level from my packet software TNC (Direwolf) that will trigger transmit.  No dice!

So, that felt raw.  I bought those radios mainly for VOX but they wouldn’t work that way.  Consolation: I have two more radios (reasons stated above), I have two more frequencies 1.2m and 0.7m.  Overall, I’m happy with them.  I may have just expected a little too much for my money.

VOX isn’t reliable for transmitting packets

Moving forward, I know that the VOX solution was not the best solution to transmitting packets with a radio.  Just think about it: The radio VOX circuit must hear the audio of the packet to determine it needs to turn on, before it is transmitting.  That means that the start of the packet, no matter how fast the detection and switching, must miss somewhere between microseconds and milliseconds.  That would require more workarounds depending on the detection and switching time.

The best solution is to have the software TNC (or hardware if that’s what you’re using) actually activate the PTT circuit.  That will place the radio in transmit mode prior to transmitting the packet.  Perfect.

The homebrew interface

Eventually, (a month later…) I accepted this reality and committed myself to building an interface for my laptop.  This is different than the way I setup the Raspberry Pi since my laptop does not have easily accessible GPIO pins.  It turns out, it is only a little bit more challenging the the RPi interface — score!   I’ll share my RPi setup later.  However, for now, suffice it to say that the RPi setup is very well documented especially inside the Direwolf documentation.  Here is a bad pic, just for the curious:

Raspberry Pi in old plastic parts box with breadboarded PTT circuit for Baofeng radio
This is my Direwolf-Raspberry Pi setup acting as my home APRS station.

The biggest difference between the RPi and the laptop PC interface is the need to control the on/off state of a wire to turn on the PTT circuit when it is time for Direwof to transmit.  One common way to do this is by plugging in a USB<->Serial (RS-232) adapter and using one of the pins that are part of hardware handshaking in the RS-232 protocol like RTS (Request To Send).  If you have an old enough computer, it might just have one installed already.  This is a DB9 or DB25 connector that looks like this

USB to Serial DB9 RS-232 glued to a PTT circuit
USB to Serial DB9 RS-232 glued to a PTT circuit

I was able to cobble this interface together to connect my Baofeng radio to my PC soundcard’s mic and headphone jacks for transmitting and receiving APRS packets using a transistor, a capacitor, some wires, an old PC board, and an USB<->RS-232 serial adapter.  I was back on the trail of something good — transmitting digital packets!

As I refine it, I’ll provide more detail.  For now, I just wanted to share some of the things I learned along the way.

Lessons learned/Tips

  • If you plug the radio’s speaker output into the mic of your computer sound card and the mic input of the radio into your computer’s speaker y0u will activate PTT and the radio will go into transmit mode constantly.
    • The solution is to prevent DC current from flowing between the ground wire of the PC line out (headphones, etc.) and radio line in (mic).
    • Solve this by placing a capacitor in series with the ground between pc line out and radio mic.  You will probably see an initial PTT activation when you first plug it in due to the initial flow of current to charge the capacitor.  You should see the transmit turn off quickly (1-2 seconds).  If not you’ll have some troubleshooting to do 🙁
  • Test your transmissions on a VHF or UHF frequency that you have the privilege to transmit on and does not have activity on it.  Use the lowest power possible.
  • Use a NPN transistor with a 2-5k ohm resistor between the base and the RTS pin to trigger PTT.  I’ve seen 2N2222 recommended.  I didn’t have one handy and I grabbed one I did have.  It was this one
    • PTT Switching transistor (9287 N830) and capacitor (10uF electrolytic)
      This is a train wreck. But it does work.
  • The configuration for Direwolf that initiates PTT using the RTS pin of the USB serial port looks like this:

#
# If not using a VOX circuit, the transmitter Push to Talk (PTT)
# control is usually wired to a serial port with a suitable interface circuit.
# DON’T connect it directly!
#
# For the PTT command, specify the device and either RTS or DTR.
# RTS or DTR may be preceded by “-” to invert the signal.
#

#PTT COM1 RTS
#PTT COM1 -DTR
PTT /dev/ttyUSB0 RTS

Finally, here is a crude schematic of my circuit when I wired it for my Raspberry Pi.  The one I’ve “crafted” for PC is identical except the RCA jacks are replaced with 1/8″ male stereo plugs and the “GPIO25” pin is replaced with RTS on a USB-Serial adapter.  YMMV

My circuit diagram/schematic depicting a very rough draft of a circuit to connect from a Baofeng mic/speaker connector to a PC Mic/Headphones port using RCA female jacks.
This is really crude. But, I’m sharing it just in case it helps someone figure something out for their setup.

APRS: Social Media for Hams

While exploring APRS, I’ve found inspiration to help guide my adoption.  This inspiration comes straight from the developer of APRS, Bob Bruninga WB4APR,  who said recently in a post to the TAPR APRS Mailing List that “APRS and its APRS-IS cloud are not being used to their full potential” and that “The original intent of APRS in its 1992 paper at DCC was to be a single information resource channel”  I’m inspired by this because I’m feeling like I want to push beyond the usage I see locally here in central Maine, near Bangor, ME.

The Original Vision for APRS

Bob continued to outline his original vision in more detail by saying:

[it is] to maintain real-time situational awareness of *everything* going on in HAM RADIO is an area.  Everything we could think of to identify our activities, station capabilities and actions was included in the format.

All we got instead, was a wasted decade of one-way GPS trackers with no one even looking at what was going on in their area.

Are you doing something useful right now in Ham radio?  Are you beaconing it?

Social Media for Hams

I am so happy Bob said this.  It supports my evolving notion that APRS should be the advertisement free, totally non-commercial Twitter and Facebook of Ham radio!  You know, with that athentic amateur radio twist 😉  I know I’ll talk more about this usage pattern in future posts.  Especially while I’m implementing some packet BBS (BPQ?) and advertising it with APRS.  I may even post a “listening frequency” and begin monitoring it…  THIS is starting to look a lot like the Ham radio I dreamed of as a teenager at the end of the 1980s.  Hang on tight!

By the way, Happy New Year!

 

SDR’s killer app?

Andres Vahter, whom I discovered through rtl-sdr.com’s blog, has created command line apps to do Doppler corrections based on a Ham satellite’s red shift (wavelength stretch) and blue shift (wavelength compression) IN REAL TIME using satellite data (current Keplerian elements).

The idea that an SDR coupled with the right right software tools can automatically tune to a satellite’s downlink frequency even as it compresses or stretches its radio transmission as it approaches us and then recedes is just the most empowering application I’ve ever heard or conceived of for SDR — beyond the simple fact that SDR is an amazing mathematical reality.  Wow.  Thank you Andres for strengthening our tool set!

I am new to the field because of my concerns with Software Defined Radio (SDR) and, let’s be honest, time and energy constraints, so forgive me if de-Dopplering as signal is an old SDR topic.  It’s new to me and maybe some of you as well.  Either way, since I find comfort on the Linux/BSD/Unix command line, this implementation is right up my alley.

Here are links directly to Andres’ blog:

Further reading on Ham satellites

First digital traffic received! APRS on 144.390

Yes, yes, and yes.  It has been a long time since I last posted.  I was aiming to post more frequently, too.  Right?  However, this summer my family and I moved from in-town Orono, Maine to “in-town” Charleston, ME.  Wow, we were busy.  Still are.  However, now I’ve had just enough time to setup a functional, outdoor antenna for 2m.  I’ve been looking for digital signals to decode recently.  I found plenty.  However, I haven’t figured out what the lower pitched sounds I’ve encountered on 145.305MHz are.  I’ll share a recording soon.

With my new antenna freshly setup and my trusty, thrift store HTX-202 in hand, I tuned to 144.390 to listen for APRS traffic.  WOW!  I hear a steady stream of bursts!  Yes!  What I’ve been waiting for — a recognizable sound representing a know digital signal encoding that I could try to decode.

I’ve been reading about Direwolf and APRS a little and thought I’d give it a shot.  Look at what I got!  My first, digital, Ham traffic decoded:

This was truly a thrilling moment. I have been hearing data in many places around 2m. However, my signal was always too weak and my setup wrong. Last weekend I put up a simple 1/4 wave antenna about 12 feet off the ground based on a mobile antenna cable to exposed conductor connector and VOILA!
This was truly a thrilling moment. I have been hearing data in many places around 2m. However, my signal was always too weak and my setup wrong. Last weekend I put up a simple 1/4 wave antenna about 12 feet off the ground based on a mobile antenna cable to exposed conductor connector and VOILA!

Stratospheric Balloon Experiment – A Start

I first encountered a Ham who launched a high altitude weather balloon in about 2006. At the time I was 10 years from getting my Amateur radio Technician’s license. Regardless, I saw the wisdom of using a radio to transmit location data (APRS) from a GPS to enable tracking of the balloon and your payload. I’ve been busy with work and family during the time in between then and now. Since then, things have slowed down a little. I’ve removed some of the chaos of my work by downsizing my career. Also, my kids are a lot older now leaving more time to experiment with things. Sometimes I feel like I’m 17 again :)Moving right along… This winter my two younger sons and I bumped into the UMaine High Altitude Ballooning group (http://umaine.edu/hab/) at the Maine Science Festival in Bangor, ME (http://www.mainesciencefestival.org/). I talked with the professor who runs the program with a NASA grant. He even indicated that they work with homeschool kids (Win! We homeschool our kids.) That sparked my research into different experiments my kids could carry out and send up on a flight to the “edge of space.”I found lots of info online about it. There are lots of resources like http://arhab.org/index.php Then, in the course of deciding how to involve my kids’ homeschool science with a HAB project I remembered we already had exactly what we needed! A “Sun and Sky Monitonring Station” from RadioShack based on Forest Mimms III’s work using LEDs as narrow-band photo diodes (http://makezine.com/projects/make-36-boards/how-to-use-leds-to-detect-light/). It’s really crazy, because my wife bought the old kit at a thrift store for $3.99 last year thinking it would come in handy. How right she was!

So, we spent a few weeks working through some of the basic atmospheric science presented in the book. Then, I got started building a prototype UV data logger based on a couple of RadioShack UV (405nm) LEDs. I started with the logger running on a PicAxe 08M chip. I encountered problems that I later found had NOTHING to do with the PicAxe. I was working my way to the bottom of my sporadic voltage reading issues where the numbers coming out of the ADC would randomly climb and then drop back to 0. I figured it was related to my poorly structured circuit and/or the very small number of electrons produced by the LEDs.

I switched to an Arduino but I still had the same issues — that didn’t surprise me. But it made me feel better. Then I was determined I needed to handle the very low ampere output of the LED better and wired it to a unity gain OpAmp circuit based on the one that Forest Mimms III presents in that Makezine article I referenced above. That worked like a charm! I extended it a little bit because I reason that, since the LEDs are so directional in their detection of light, that I’ll need at least four pointing outward spaced 90 degrees apart. My hope is that, if I average the values of all four LEDs we’ll be able to detect the trend in UV light intensity and factor out the angle of the LEDs with respect to the sun.

Here is a sneak peak at my very first prototype that seems to work:

This is an arduino with a custom op amp circuit to drive the Arduino's ADC circuits from the UV LEDs acting as UV sensors.
This is an arduino with a custom op amp circuit to drive the Arduino’s ADC circuits from the UV LEDs acting as UV sensors.

Here’s how it looked when I figured out how to put the circuits inside the the official UMaine provided enclosure:

Our UV Experiment packaged up in the official UMaine HAB enclosure
Our UV Experiment packaged up in the official UMaine HAB enclosure

I’ve got a few different pieces of code I’ve written for handling this datalogging and testing process. However, I’ll save that for another post.

And just in case you haven’t seen the Sun and Sky Monitoring Station:

Sun and Sky Monitoring Station - Vintage, by Forest Mimms from Radio Shack
Sun and Sky Monitoring Station

Two Meter VHF Antenna and Transceivers on the Cheap

Since getting my Technician class license in April of this year, 2016, I’ve done some playing with a pair of 2m radios (Realistic HTX-202) I got at my local thrift shop for $10US for the pair. Luck, I know. However, I have watched for radios at thrift shops and yard sales for a L O N G time in human terms.

Realistic HTX-202 2m Ham transceivers in decent shape with rubber duck antennas
Realistic HTX-202 2m Ham transceiver

To solve that problem, I’ve constructed some antennas. The most effective one so far is one I can place outside my rented house based on simply cutting 1/4 wavelength of casing, shielding, and dielectric away from one end of a long run of 75ohm RG6U TV coax. Of course, if I don’t want to lose all the signal in reflections between the radio and the antenna from the impedance mismatch, I had to find out how to match the radio’s 50ohm impedance with the cable’s 75 ohm impedance.

I overcame that obstacle, anecdotally at least, using the technique of cutting the coax so that it is approximately 1/2 wavelength multiples of the frequency I want to operate on. To be clear, that means the length of my coax was some multiple of 1/2 wavelengths + 1/4 wavelength. I learned about this technique from http://www.jpole-antenna.com/2015/01/28/using-rg-59-or-rg-6-catv-cable-with-an-amateur-radio-antenna/
This made it so that when I removed the top 1/4 wave of shielding to form the antenna I had a multiple of 1/2 wavelengths of cable remaining. I thought this was kinda cool since I got out of needing any hardware to connect the antenna to the transmission line.

Of course, you need a way to connect the male F connector of the CATV cable to the BNC connector of the average 2M transceiver. I splurged on that at my not-so-local RadioShack in Ellsworth, Maine. There they had a pretty cool old-school selection of electronics DIY components — I was amazed. Anyway, I spent about $6 on an Female F <-> Male BNC and I was in business! Here is the antenna on a piece of copper pipe with an old light fixture base as a reflector/ground plane that I later placed up off the ground a little temporarily:

First elevated, outside, 1/4 wave 2m band antenna.
Made from 75 ohm CATV coax, carefully measured, in 1/4 wave increments with a 1/4 wave of shielding stripped from the end as the antenna. It uses an old steel lamp base as a radial-ground-reflector.

At a later date I’ll do some testing to see how well this antenna works. SWR and field strength measurements anyone? I think I’d like to call my new antenna a “C-Pole” for Cheap-Pole. Making a terrible play on the “J-Pole” name 🙂