The personal website of Scott W Harden

Screwy Oscillator Idea

Can you believe it’s been almost 3 months since my last post? A lot’s been going on since then, namely the national board dental exam. I’m happy to report I prepared for it and performed above and beyond my expectations on the exam, and I’m quite satisfied. The last few weeks were quite a strain on my life in my aspects, and during that time I realized that I didn’t appreciate the little things (such as free time) that I would have loved to experience instead of studying. I guess it’s the feeling you have when you’re really sick and think to yourself “remember this moment so that when you’re well again, you can appreciate feeling well”. Now that it’s all behind me, what do I do? I sit at my work station, play some light music, grab an adult beverage, turn on the soldering iron, and make something special.

I’m resuming work on my simple transmitter/receiver projects, but I’m working at the heart of the device and experimenting with oscillator designs. I built various Colpitts, Hartley, Clapp, and other oscillator designs, and I think I landed on a design I’m most comfortable with replicating. I’m actually creating a voltage controlled oscillator (VCO or VFO), with a frequency that can be adjusted by rotating a dial or two. It’s always a balance between stability and tunability for me. I don’t want to use polyvaricon variable capacitors (expensive!), and LED-based varactor diode configurations only give me a swing of about 20pf. What did I come up with?

I had tremendous success using a variable inductor for coarse tuning! The inductor is nothing more than a screw entering and exiting the center of an air core inductor. I can’t claim all the credit, because I got the idea from this photo on one of the coolest websites on the planet, Alan Yates’ Lab. It looks like Alan got the idea from this page… This is so useful! Is this common HAM knowledge? Why am I, someone who’s been into RF circuitry for a couple of years now, JUST learning about this? I’m documenting it because I haven’t seen it out there on the web, and I feel it should be represented more! Here’s a video of it in action:

This is the circuit I was using:

This is what it looked like before the glue or screw:

Here’s the variable inductor enveloped in hot glue before it cooled and turned white:

At the end of the day, it looks nice!

Band changes can be accomplished by swapping the capacitor between the inductor and ground. It couldn’t be any easier! I’ll see if I can build this in a more compact manner…

UPDATE (2 days later): Apparently this is called a “Permeability Tuned Oscillator”, or PTO. It’s an early design for radios (earlier than variable capacitors) and I guess therefore not described often on the internet. Knowing it’s official title, searching yielded a few pages describing this action: Dave, G7UVW did some analytical measurements using a mercury core!The Tin Ear uses a PTO as its primary tuning method (also McDonalds straw?) This guy made a PTO out of PVC with a nice screw handle! This PTO kit seems to be used in many projects.The Century 21’s VFO is a PTO! I love that rig and had no idea it tuned like that… This guy used a PTO in his MMR-40 radio.

Someone on Hackaday recommended This ARRL Challenge winner with an almost identical design as mine!I guess this bright idea was so bright, it was thought of by many people long ago…

QRSS and WSPR Presentation for GARS

Last night I had the honor of speaking for GARS, the Gainesville Amateur Radio Society. Their club call (K4GNV) is used by the 146.820 MHz FM repeater (the most active in Gainesville). There were a few dozen attendees, and it was interesting to give a lecture on advanced radio topics to a body that had a lot of experience with RF. This group was a couple generations older and a lot more experienced and than the groups of students I’m used to speaking for! Overall it went well and I’m glad I had the opportunity to present. It’s probably one of those things I’ll remember for quite some time.

The talk was an overview of QRSS, WSPR, and ultra-narrowband data transmission, with a focus on home made transmitters and simple radio/PC/software receiving stations. I did a little more research into WSPR in the process of preparing for it (including familiarizing myself with the details of how the data is encoded into 4 tones) and I am glad I understand more about the mode than I did before I started. I grinned as I showed a few slides of Hans Summers’ kit pictures. I wonder if anyone in the group will buy one? I hope so! Below are a few links I put together where people can go for more information about QRSS and WSPR:

6 Day QRSS Stitch

Has this been done before? I don’t know how useful it is, but I did it just for kicks and it turned out pretty cool. I realized that my strong signal (what is that?!) which is desentizing the the receiver is repetitive by day. Perhaps it’s something at the hospital! Days are 3/7/2011 through 3/12/2011, separated by black lines.

Higher resolution image here:

Cool stuff. All right, I’m outta here.

$10 Frequency Counter Finished!

Wow, what a cool project start to finish. Simple, cheap, and absolutely useful! … and not to mention big green numbers which make it look more impressive than it actually is! This is my super-simple frequency counter designed to be used for amateur radio, all for about $10. It was a project I developed over the last few months and documented all along the way. It’s finished I guess, so this will probably be the last post about it! Now for some vids and pics:

__Sure there’s room for improvement,__but that’s the fun part! This is a solid start and it’s cheap as can be. Simply improving software would greatly improve its accuracy. This doesn’t use any time-averaging at all! If you had it average 20 readings, it’d probably be much smoother, update every second, and have a higher precision. Also, there’s ample room left in the case to build in a transmitter or receiver!

There’s the finished project! It looks pretty good, considering it was built mostly out of junk box components, and everything it’s made from can be purchased cheaply online. I’m happy with it! I could improve my metal cutting, but that was the first time I ever cut a square window in aluminum so I’m still proud of myself.

As you can see the enclosure is made from sheet metal bent and cut into 2 pieces. The enclosure was from RadioShack, and was $2.99! Yeah it might be cheaper online, but when you add shipping it’s pretty convenient to get it locally. My local RadioShack didn’t carry these metal ones (they have stupid plastic ones), but I found these in Orlando and after asking the workers I learned that anyone can find any product online (such as the case I used) and request that their local store order them. When they arrive, you can buy them with no extra charge!

Here are some of the internals after being mounted. Heck, these are ALL the internals! You can tell I could have gotten away with a case one third this size if I had one available. Oh well, it’s still cool.

There are a few random photos of the build. It’s just a microcontroller reading (and resetting) a counter a bunch of times a second and displaying the result on the multiplexed display. That’s it! It was a lot of work, but a truly simple concept. The micro-controller is an ATMEL Atmega 16 AVR which is a little costly (around $5) but I had it on hand. I imagine you could accomplish the same thing with a far less intricate microcontroller! I’ll bet you could pull it off with an ATTiny2313, especially if you had a LCD display rather than a multiplexed LED like mine. The counter is a 74lv8154 chip, a 32-bit (dual 16-bit) counter IC at a bargain $0.50 - why when I google for home made frequency counters do I not see people using these? They daisy-chain multiple 8-bit counters! What a shortcut I stumbled upon…

Thinking of making your own? Go for it! Here are some of my other posts which describe the development of this thing (including stuff I tried that didn’t work). Everything I ordered should be stocked at mouser.com.

I guess that sums it up! What a fun hack. If you have any questions feel free to contact me (link in the menu on the right), and if you make one of these of your own I’d LOVE to see it! I’ll even slap a photo of yours on my site to share with everyone. I had fun working on this project. If you’re at all into radio, I recommend you try attacking a project like this too! It’s more efficient at determining frequency than turning on a commercial radio receiver and spinning the dial until you hear your transmitter ^_^


Upon request here’s the code! It’s nothing special, and certainly not very efficient, but it’s quite functional. If you re-create this project, I recommend writing your own code rather than flat copying mine. You’ll learn a heck of a lot more… and my code for this is really crap XD

#include <avr/io.h>
#include <avr/delay.h>
#include <avr/interrupt.h>

#define A PC5
#define B PC0
#define C PC6
#define D PC7
#define E PC2
#define F PC4
#define G PC1
#define P PC3

char sendDigit(char row, char num, char dot){
    char val=0;
    if (num==0) {val|=(1<<A)|(1<<B)|(1<<C)|(1<<D)|(1<<E)|(1<<F);}
    if (num==1) {val|=(1<<B)|(1<<C);}
    if (num==2) {val|=(1<<A)|(1<<B)|(1<<D)|(1<<E)|(1<<G);}
    if (num==3) {val|=(1<<A)|(1<<B)|(1<<C)|(1<<D)|(1<<G);}
    if (num==4) {val|=(1<<B)|(1<<C)|(1<<F)|(1<<G);}
    if (num==5) {val|=(1<<A)|(1<<C)|(1<<D)|(1<<F)|(1<<G);}
    if (num==6) {val|=(1<<A)|(1<<C)|(1<<D)|(1<<E)|(1<<F)|(1<<G);}
    if (num==7) {val|=(1<<A)|(1<<B)|(1<<C);}
    if (num==8) {val|=(1<<A)|(1<<B)|(1<<C)|(1<<D)|(1<<E)|(1<<F)|(1<<G);}
    if (num==9) {val|=(1<<A)|(1<<B)|(1<<C)|(1<<F)|(1<<G);}
    if (dot==1) {val|=(1<<P);}

void showNumber(unsigned long val){
    if (val==0) {return;}
    int i;
    int array[6]={10,0,0,0,0,0}; // NUMBER OF DIGITS
    int dly=10;
    while (val>0){
      val /= 10;

#define byte1 PB4
#define byte2 PB3
#define byte3 PB2
#define byte4 PB1

unsigned long val=123456;
void readFreq(){
    unsigned long b4,b3,b2,b1;

int cnt=0;

int main(){

    TIMSK|= (1 << TOIE1); // Enable overflow interrupt
    sei(); // Enable global interrupts
       TCCR1B|=(1<<CS11); // Set up timer at Fcpu/8


… and I know it’s unrelated, but:

(I watched this four times - it’s so random I love it!)


This project was featured on a couple of my favorite sites, Hack-A-Day and Electronics-Lab!

AJ4VD Portable - Spring Break QTH

So here’s the deal: I’m staying in Orlando, FL at my parents’ home for a large portion of spring break. I wanted to take some radio equipment, and set up a simple antenna to allow me to operate. This post documents what I did, and only time will tell how well my configuration actually works!

Let’s start with the video. It summarizes the antenna, and even shows me making my 1st contact with it to a guy in Vermont, and I’ve been getting good signal reports from Europe and South America as well.

This is what I started with. It’s pink because it’s the bedroom of my younger sister Leah. It might be a a little embarrassing to operate in such an environment, but my ego is up for the challenge!

The first thing I did was set up a table and run some wire out the window. I chose a window with a tree near by. The wire is not coax, but rather some el-cheapo speaker wire I found. It’s not 50 ohm impedance, but I imagine it will make the tuner happy anyhow.

I tied some random objects to the end of a fishing line and used a fishing pole to cast them up and over the tree branch I wanted. Believe it or not this took a lot of time, effort, broken lines, and frustration. Have someone help you do it, or practice doing it! I now appreciate how in an emergency situation getting something like this up might not be as easy as it seems…

The end of the wire splits in two and connects to two quarter-wavelength legs of a dipole resonant at the 40m wavelength (7mhz frequency), with each orange piece of wire being about 33ft in length.

When the fishing line looped over the branch (tied to the speaker wire) is hoisted up, the branch acts like a pulley and pulls the whole system up. You can see the junction of the wires if you look closely in the image…

The thing isn’t that visible, which is a plus. I don’t want anyone to know I operate! Notice how the orange wires are virtually invisible, even against a blue sky. Slick!

From far away the most noticeable wire is the speaker wire. I wish it weren’t reflective with a white stripe! Oh well, it’s all I’ve got. It’s more dramatic since the setting sun is behind me. Also keep in mind that there is a tree/shrubbery between my nearest neighbor and me, so I don’t think they can even see it from their house.

The final setup from inside. Poor Leah, I hope she doesn’t cry when she sees this! With the blinds closed I’m confident I can operate with confidence any time of the day. I don’t think I’ll push the setup beyond 40W (even though it can do 100W) just to keep things simple and safe. I look forward to operating CW (Morse code) later tonight! I wonder how this would do as a QRSS receiver? Much experimentation is left to perform! I’m happy knowing that in a very short time I set up a functional antenna to talk across the country at virtually no cost. Since I retained the pulley system, I can lower the antenna when it’s not in use, and hoist it up whenever I want. I’ll post a log of my contacts at the end of the week.

UPDATE: I’ve been making contacts on 40m, 20m, 17m, 12m, and 10m. I’m impressed how well I’ve been doing on 10m! Even with a dipole not cut for 10m, and oriented for north-south propagation, I’m getting Europe on this thing. I don’t know if it’s an improved antenna, or less noise being in a residential neighborhood. (Compare this to the University of Florida Gator Amateur Radio Club station located on top of a hospital). Here is a video showing some random 12m/10m contacts…

ANTENNA UPDATE - I ended up stringing a SECOND (much longer) antenna. Although it’s configured somewhat like a dipole, I consider it more of a long wire antenna. It’s ridiculously long (about 100 ft) and high (about 20 ft). The other wire is more like a RF ground, and is about 150ft. Here’s a photo of the new antenna (yellow) and the old antenna (orange). Both of them are functional, and I can switch between them quickly.

Although I intended the longer antenna for the lower bands (40m, 80m, and why not try 160m?), it seems to work surprisingly well on the higher bands too! I just worked Spain (EA5GPQ) on 24.967 MHz (12m), how cool is that? I can’t wait until later tonight when the lower bands open up. This is pretty fun…

March 10, 2011

While operating from a portable station in Orlando, Florida, I’m taking time to relax and investigate (through experimentation) different methods to rapidly deploy wire antennas. The antenna system described in my last post was taken down, half by me, and half by the storms that slammed central Florida last night and this morning! I also visited a store and purchased some flat 300-Ohm antenna wire (usually used between old TVs and TV antennas) that I hope will serve me well (better than speaker wire, probably less than 50-ohm coax). I used a fishing rod / line to string up the antenna, and here are some photos of the build. At the end you can see a video of my first QSO with this new antenna (OP2A in Belgium, over 4,500 miles away).

March 12, 2011

The end of the week is nearing! Tonight my family flies in from Colorado, and I’m holding my fingers crossed for several reasons. First, this is the first time that anyone in my entire family has seen an amateur radio. They understand it’s a big part of my life, but obviously they don’t really know what it is/does. I imagine I’ll give a demonstration later, and I hope that everything functions well (and I can have a few QSOs), and that it doesn’t come across as fickle. I think there’s quite a disconnect in interests between most of my family and me. For example, the things that I find fascinating (the RX/TX circuitry, my home brew designs, antenna theory, and digital exchanges) are not things I think they can understand or appreciate in the way that I do. Therefore I’m limited to common ground - talking to a few random people out there - and I don’t think that’s very impressive.

I did a quick estimate and it looks like I have a little over 200 contacts from this portable station! I haven’t broken it down by state or country, but I’m excited to dive into the logs a little further down the road. I grabbed a few random videos along the way, so I present the following:

Here I cam sending/receiving some very slow CW on 30m. This is actually one of my first CW contacts on 30m, as I usually just play with QRSS on 10.140MHz!

This is just a demonstration part for informational purposes and part just for me to have and look back on in the future. I’m sure in the future I’ll laugh at the way I operate (I already find it funny how my voice goes up an octave when I pick up a hand mike!), but it is what it is. For newcomers to HF, this represents what can be done just using about 50W of power and a wire antenna hung in a tree!

I also tried my hand at QRSS reception with this antenna (almost forgot to post it). I realized the shortcoming(s) of my QRSS VD software when I couldn’t get it to run on my linux box! The netbook ran Ubuntu and used the pulse audio system and QRSS VD used PyAudio for linux audio support (runs fine on windows though) and PyAudio likes ALSA and not pulse! It’s so frustrating - I wish there were a better cross-platform solution than pyAudio. Anyhow, in a humbling moment I ended-up actually using Argo (my “competitor” program) fired up with WINE under linux (burn, lol) to grab QRSS and save captures, then I used the QRSS Stitcher (my software) to assemble a few hours worth while the XYL and I went out to lunch. Frequency is 10.140MHz. Here’s the result!

AJ4VD Portable Logbook

March 6-12, 2011 (Orlando, FL):

39 States (146 QSOs)

35 Countries (62 QSOs)