Capstan Winch Central To This All-Band Adjustable Dipole Antenna

The perfect antenna is the holy grail of amateur radio. But antenna tuning is a game of inches, and since the optimum length of an antenna depends on the frequency it’s used on, the mere act of spinning the dial means that every antenna design is a compromise. Or perhaps not, if you build this infinitely adjustable capstan-winch dipole antenna.

Dipoles are generally built to resonate around the center frequency of one band, and with allocations ranging almost from “DC to daylight”, hams often end up with a forest of dipoles. [AD0MZ]’s adjustable dipole solves that problem, making the antenna usable from the 80-meter band down to 10 meters. To accomplish this feat it uses something familiar to any sailor: a capstan winch.

The feedpoint of the antenna contains a pair of 3D-printed drums, each wound with a loop of tinned 18-gauge antenna wire attached to some Dacron cord. These make up the adjustable-length elements of the antenna, which are strung through pulleys suspended in trees about 40 meters apart. Inside the feedpoint enclosure are brushes from an electric drill to connect the elements to a 1:1 balun and a stepper motor to run the winch. As the wire pays out of one spool, the Dacron cord is taken up by the other; the same thing happens on the other side of the antenna, resulting in a balanced configuration.

We think this is a really clever design that should make many a ham happy across the bands. We even see how this could be adapted to other antenna configurations, like the end-fed halfwave we recently featured in our “$50 Ham” series.

Teardown of a quartz crystal oscillator and the tiny IC inside

Inside the oscillator package, showing the components mounted on the ceramic substrate.The quartz oscillator is an important electronic circuit, providing highly-accurate timing signals at a low cost. A quartz crystal has the special property of piezoelectricity, changing its electrical properties as it vibrates. Since a crystal can be cut to vibrate at a very precise frequency, quartz oscillators are useful for many applications. Quartz oscillators were introduced in the 1920s and provided accurate frequencies for radio stations. Wristwatches were revolutionized in the 1970s by the use of highly-accurate quartz oscillators. Computers use quartz oscillators to generate their clock signals, from ENIAC in the 1940s to modern computers.1

A quartz crystal requires additional circuitry to make it oscillate, and this analog circuitry can be tricky to design. In the 1970s, crystal oscillator modules became popular, combining the quartz crystal, an integrated circuit, and discrete components into a compact, easy-to-use module. Curious about the contents of these modules, I opened one up and reverse-engineered the chip inside. In this blog post, I discuss how the module works and examine the tiny CMOS integrated circuit that runs the oscillator. There’s more happening in the module than I expected, so I hope you find it interesting.

The oscillator module

I examined the oscillator module from an IBM PC card.2 The module is packaged in a rectangular 4-pin metal can that protects the circuitry from electrical noise. (It is the “Rasco Plus” rectangular can on the right, not the square IBM integrated circuit.) This module produced a 4.7174 MHz clock signal, as indicated by the text on the package.

The quartz oscillator module is in the lower right, labeled Rasco Plus. 4.7174 MHZ, © Motorola 1987. The square module is an IBM integrated circuit. Click this (or any other image) for a larger version.

The quartz oscillator module is in the lower right, labeled Rasco Plus. 4.7174 MHZ, © Motorola 1987. The square module is an IBM integrated circuit. Click this (or any other image) for a larger version.


I cut open the can to reveal the hybrid circuitry inside. I was expecting a gem-like quartz crystal inside, but found that oscillators use a very thin disk of quartz. (I damaged the crystal while opening the package, so the upper part is missing..) The quartz crystal is visible on the left, with metal electrodes attached to either side of the crystal. The electrodes are attached to small pegs, raising the crystal above the surface so it can oscillate freely.

Inside the oscillator package, showing the components mounted on the ceramic substrate.

Inside the oscillator package, showing the components mounted on the ceramic substrate.


On the right side of the module is a tiny CMOS integrated circuit die. It is mounted on the ceramic substrate and connected to the circuitry by tiny golden bond wires. A surface-mount capacitor (3 nF) and a film resistor (10Ω) on the substrate filter out noise from the power pin.

The IC’s circuitry

The photo below shows the tiny integrated circuit die under a microscope, with the pads and main functional blocks labeled. The brownish-green regions are the silicon that forms the integrated circuit. A metal layer (yellowish white) wires up the components of the IC. Below the metal, reddish polysilicon implements transistors, but it is mostly obscured by the metal layer. Around the outside of the chip, bond wires are connected to pads, wiring the chip to the rest of the oscillator module. Two pads (select and disable) are left unconnected. The chip was manufactured by Motorola, with a 1986 date. I couldn’t find any information on the part number SC380003.

The integrated circuit die with key blocks labeled. "FF" indicates flip-flops. "sel" indicates select pads. "cap" indicates pads connected to the internal capacitors.

The integrated circuit die with key blocks labeled. “FF” indicates flip-flops. “sel” indicates select pads. “cap” indicates pads connected to the internal capacitors.


The IC has two functions. First, its analog circuitry drives the quartz crystal to produce oscillations. Second, the IC’s digital circuitry divides the frequency by 1, 2, 4, or 8, and produces a high-current clock output signal. (The division factor is selected by the two select pins on the IC.)

The oscillator is implemented with a circuit (below) called a Colpitts oscillator, which is more complex than the usual quartz oscillator circuit.43 The basic idea is that the crystal and the two capacitors oscillate at the desired frequency. The oscillations would rapidly die out, however, except for the feedback boost from the drive transistor.

Simplified schematic of the oscillator.

Simplified schematic of the oscillator.


In more detail, as the voltage across the crystal increases, the transistor turns on, feeding current into the capacitors and boosting the voltage across the capacitors (and thus the crystal). But as the voltage across the crystal decreases, the transistor turns off and the current sink (circle with arrow) pulls current out of the capacitors, reducing the voltage across the crystal. Thus, the feedback from the drive transistor strengthens the crystal’s oscillations to keep them going.

The bias voltage and current circuits are an important part of this circuit. The bias voltage sets the drive transistor’s gate midway between “on” and “off”, so the voltage oscillations on the crystal will turn it on and off. The bias current is set midway between the drive transistor’s on and off currents so the current flowing in and out of the capacitors balances out.5 (I’m saying “on” and “off” for simplicity; the signal will be a sine wave.)

A large part of the integrated circuit is occupied by five capacitors. One is the upper capacitor in the schematic, three are paralleled to form the lower capacitor in the schematic, and one stabilizes the voltage bias circuit. The die photo below shows one of the capacitors after dissolving the metal layer on top. The red and green region is polysilicon, which forms the upper plate of the capacitor, along with the metal layer. Underneath the polysilicon, the pinkish region is probably silicon nitride, forming the insulating dielectric layer. The doped silicon (not visible underneath) forms the bottom plate of the capacitor.

A capacitor on the die. The large faint square to the left of the capacitor is a pad for connecting a bond wire to the IC.
The complex structures on the left are clamp diodes on the pins. The cloverleaf structures on the right are transistors, which will
be discussed later.

A capacitor on the die. The large faint square to the left of the capacitor is a pad for connecting a bond wire to the IC. The complex structures on the left are clamp diodes on the pins. The cloverleaf structures on the right are transistors, which will be discussed later.


Curiously, the capacitors aren’t connected together on the chip, but are connected to three pads that are wired together by bond wires. Perhaps this provides flexibility; the capacitance in the circuit can be modified by omitting the wire to a capacitor.

The digital circuitry

The right side of the chip contains digital circuitry to divide the crystal’s output frequency by 1, 2, 4, or 8. This lets the same crystal provide four different frequencies. The divider is implemented by three flip-flops in series. Each one divides its input pulses by 2. A 4-to-1 multiplexer selects between the original clock pulses, or the output from one of the flip-flops. The choice is made through the wiring to the two select pads on the right side of the die, fixing the ratio at manufacturing time. Four NAND gates (along with inverters) are used to decode these pins and generate four control signals to the multiplexer and flip-flops.

How CMOS logic is implemented

The chip is built with CMOS logic (complementary MOS), which uses two types of transistors, NMOS and PMOS, working together. The diagram below shows how an NMOS transistor is constructed. The transistor can be considered a switch between the source and drain, controlled by the gate. The source and drain (green) consist of regions of silicon doped with impurities to change its semiconductor properties and called N+ silicon. The gate consists of a special type of silicon called polysilicon, separated from the underlying silicon by a very thin insulating oxide layer. The NMOS transistor turns on when the gate is pulled high.

Structure of an NMOS transistor. A PMOS transistor has the same structure, but with N-type and P-type silicon reversed.

Structure of an NMOS transistor. A PMOS transistor has the same structure, but with N-type and P-type silicon reversed.


A PMOS transistor has the opposite construction from NMOS: the source and drain consist of P+ silicon embedded in N silicon. The operation of a PMOS transistor is also opposite from the NMOS transistor: it turns on when the gate is pulled low. Typically PMOS transistors pull the drain (output) high, while NMOS transistors pull the drain low. In CMOS, the transistors act in a complementary fashion, pulling the output high or low as needed.

The diagram below shows how a NAND gate is implemented in CMOS. If an input is 0, the corresponding PMOS transistor (top) will turn on and pull the output high. But if both inputs are 1, the NMOS transistors (bottom) will turn on and pull the output low. Thus, the circuit implements the NAND function.

A CMOS NAND gate is implemented with two PMOS transistors (top) and two NMOS transistors (bottom).

A CMOS NAND gate is implemented with two PMOS transistors (top) and two NMOS transistors (bottom).


The diagram below shows how a NAND gate appears on the die. The transistors have complex, meandering shapes, unlike the rectangular layouts that appear in textbooks. The left side holds the PMOS transistors, while the right side holds the NMOS transistors. The polysilicon that forms the gates is the slightly reddish wiring on top of the silicon. Most of the underlying silicon is doped, making it conductive and slightly darker than the non-conductive undoped silicon along the left and right edges and in the center. For this photo, the metal layer was removed with acid to reveal the silicon and polysilicon underneath; the yellow line illustrates where some of the metal wiring was. The circles are connections between the metal layer and the underlying silicon or polysilicon.

A NAND gate as it appears on the die.

A NAND gate as it appears on the die.


The transistors in the die photo can be matched up with the NAND-gate schematic; look at the transistor gates formed by polysilicon and what they separate. There is a path from the +5 region to the output through the large elongated PMOS transistor on the left, and a second path through the small PMOS transistor near the center, indicating the transistors are in parallel. Each gate is controlled by one of the inputs. On the right, a path from ground to the output connection must go through both of the concentric NMOS transistors, indicating they are in series.

This integrated circuit also uses many circle-gate transistors, an unusual layout technique that allows multiple transistors in parallel at high density. The photo below shows 16 circle-gate transistors. The copper-colored cloverleaf patterns are the transistor gates, implemented with polysilicon. The inside of each “leaf” is the transistor drain, while the outside is the source. The metal layer (removed) wires all the sources, gates, and drains together respectively; the parallel transistors act as one larger transistor. Paralleled transistors are used in the output pin drivers to provide high current for the output. In the bias circuitry, different numbers of transistors are wired together (e.g. 6, 16, or 40) to provide the desired current ratios.

Sixteen circle-gate transistors with four gate connections.

Sixteen circle-gate transistors with four gate connections.


Transmission gate

Another key circuit in the chip is the transmission gate. This acts as a switch, either passing a signal through or blocking it. The schematic below shows how a transmission gate is constructed from two transistors, an NMOS transistor and a PMOS transistor. If the enable line is high, both transistors turn on, passing the input signal to the output. If the enable line is low, both transistors turn off, blocking the input signal. The schematic symbol for a transmission gate is shown on the right.

A transmission gate is constructed from two transistors. The transistors and their gates are indicated. The schematic symbol is on the right.

A transmission gate is constructed from two transistors. The transistors and their gates are indicated. The schematic symbol is on the right.



A multiplexer is used to select one of the four clock signals. The diagram below shows how the multiplexer is implemented from transmission gates. The multiplexer takes four inputs: A, B, C, and D. One of the inputs is selected by activating the corresponding select line and its complement. That input is connected through the transmission gate to the output, while the other inputs are blocked. Although a multiplexer can be built with standard logic gates, the implementation with transmission gates is more efficient.

The 4-to-1 multiplexer is implemented with transmission gates.

The 4-to-1 multiplexer is implemented with transmission gates.


The schematic below shows the transistors that make up the multiplexer. Note that inputs B and C have pairs of transistors. I believe the motivation is that a pair of transistors presents half the resistance to the signal. Since inputs B and C are the higher-frequency signals, the pair of transistors allows them to pass through with less distortion and delay.

Schematic of the multiplexer, matching the physical layout on the chip.

Schematic of the multiplexer, matching the physical layout on the chip.


The image below shows how the multiplexer is physically implemented on the die. The polysilicon gate wiring is most prominent. The metal layer has been removed; the metal lines ran vertically connecting corresponding transistors segments. Note that the sources and drains of neighboring transistors are merged into single regions between the gates. The top rectangle holds the NMOS transistors while the lower rectangle holds the PMOS transistors; because PMOS transistors are less efficient, the lower rectangle needs to be larger.

Die photo of the multiplexer.

Die photo of the multiplexer.



The chip contains three-flip-flops to divide the clock frequency. The oscillator uses toggle flip-flops, that flip between 0 and 1 each time they receive an input pulse. Since two input pulses result in one output pulse (0→1→0), the flip-flop divides the frequency by 2.

A flip-flop is constructed from transmission gates, inverters, and a NAND gate, as shown in the schematic below. When the input clock is high, the output passes through the inverter and the first transmission gate to point A. When the input clock switches low, the first transmission gate opens, so point A holds its previous value. Meanwhile, the second transmission gate closes, so the signal passes through the second inverter and transmission gate to point B. The NAND gate inverts it again, causing the output to flip from its previous value. A second cycle of the input clock repeats the process, causing the output to return to its initial value. The result is that two cycles of the input clock result in one cycle of the output, so the flip-flop divides the frequency by 2.

Implementation of a toggle flip-flop.

Implementation of a toggle flip-flop.


Each flip-flop has an enable input. If a flip-flop is not needed for the selected output, it is disabled. For instance, if the “divide by 2” mode is selected, only the first flip-flop is used, and the other two are disabled. I assume this is done to reduce power consumption. Note that this is independent from the module’s disable pin, which blocks the module output entirely. This disable feature is optional; this particular module does not provide the disable feature and the disable pin is not wired to the IC.

The schematic above shows the inverters and transmission gates as separate structures. However, the flip-flop uses an interesting gate structure that combines the inverter and the transmission gate (left) into a single gate (right). The pair of transistors connected to data in function as an inverter. However, if the clock in is low, both power and ground are blocked so the gate will not affect the output and it will hold its previous voltage. This provides the transmission gate functionality.

Implementation of a combination inverter / transmission gate.

Implementation of a combination inverter / transmission gate.


The photo below shows how one of these gates appears on the die. This photo includes the metal layer on top; the reddish polysilicon gates are visible underneath. The two PMOS transistors are on the left, as concentric loops, while the NMOS transistors are on the right.

One of the combination inverter / transmission gates, as it appears on the die.

One of the combination inverter / transmission gates, as it appears on the die.



While the oscillator module looks simple from the outside, on the inside there’s a lot more complexity than you might expect.6 It contains not just a quartz crystal but also discrete components and a tiny integrated circuit. The integrated circuit combines capacitors, analog circuitry to drive the oscillations, and digital circuitry to choose a frequency. By changing the wiring to the integrated circuit during manufacturing, four different frequencies can be selected.

I’ll end with the die photo below showing the chip after removing the metal and oxide layers, showing the silicon and polysilicon underneath. The large pinkish capacitors are the most visible feature in this image, but the transistors can also be seen. (Click the image for a larger version.)

Die photo of the oscillator chip with metal removed to show the polysilicon and silicon underneath.

Die photo of the oscillator chip with metal removed to show the polysilicon and silicon underneath.


I announce my latest blog posts on Twitter, so follow me at kenshirriff. I also have an RSS feed.

Notes and references

  1. Modern PCs use quartz crystals, but with a more complex technique to get multi-gigahertz clock frequencies. A PC uses a crystal with a much lower frequency, and multiplies the frequency using a circuit called a phase-locked loop. Computers often used a 14.318 MHz crystal because that frequency was used in old television sets, so crystals with that frequency were common and cheap. 
  2. Why does the board use a 4.7174 MHz crystal, a somewhat unusual frequency? In the 1970s, the IBM 3270 was a very popular CRT terminal. These terminals were connected with coaxial cable and used the Interface Display System Standard protocol with a 2.3587 MHz bit rate. In the late 1980s, IBM produced interface cards to connect an IBM PC to a 3270 network. I obtained the crystal from one of these interface cards (type 56X4927), and the crystal frequency of 4.7174 MHz is exactly twice the 2.3587 MHz bit rate. 
  3. The terminology used for crystal oscillators is confusing with “Colpitts oscillator” and “Pierce oscillator” used in contradictory ways. I looked into the history of oscillators to try to sort out the naming, and I’ll discuss it in this footnote.

    In 1918, Edwin Colpitts, the head researcher at Western Electric, invented an inductor/capacitor oscillator, now known as the Colpitts Oscillator. The idea is that the inductor and capacitors form a “resonant tank”, which oscillates at a frequency set by the component values. (You can think of the electricity in the tank as sloshing back and forth between the inductor and the capacitors.) On their own, the oscillations would rapidly die out, so an amplifier is used to boost the oscillators. In the original Colpitts oscillator, the amplifier was a vacuum tube. Later circuits moved to transistors, but it can also be an op-amp or other type of amplifier. (Other circuits, such as the module I examined, ground an end and provide feedback to the middle. In that case, there is no inversion from the capacitors, so a non-inverting amplifier is used.)

    A simplified schematic of a Colpitts oscillator, showing the basic components.

    A simplified schematic of a Colpitts oscillator, showing the basic components.


    The key feature of the Colpitts oscillator is the two capacitors, which form a voltage divider. Since the capacitors are grounded in the middle, the two ends will have opposite voltages: when one end goes up, the other goes down. The amplifier takes the signal from one end, amplifies it, and feeds it into the other end. The amplifier inverts the signal and the capacitors provide a second inversion, so the feedback strengthens the original signal (i.e. it has a phase shift of 360°).

    In 1923, George Washington Pierce, a professor of physics at Harvard, replaced the inductor in the Colpitts oscillator with a crystal. The crystal made the oscillator much more accurate (higher Q factor), leading to its heavy use in radio transmission and other applications. Pierce patented his invention and made a lot of money off it from companies such as RCA and AT&T. The patents led to years of litigation, eventually reaching the Supreme Court. (For more information, see this thesis on crystal history.)

    For several decades, the common terminology was that a Pierce oscillator was a Colpitts oscillator that used a crystal. (See Air Force Manual, 1957 and Navy training, 1983 for instance.) The Pierce oscillator often omitted the characteristic voltage-divider capacitors, using the stray capacitance of the vacuum tube instead. But then terminology shifted, with “Colpitts oscillator” and “Pierce oscillator” indicating two different types of crystal oscillator: Colpitts with the capacitors and Pierce without the capacitors. (See, for example, the classic electronics text Horowitz and Hill.)

    Another change in terminology was to describe the Colpitts oscillator, Pierce oscillator, and Clapp oscillator as topologically identical crystal oscillators, just differing in what point in the circuit was considered AC ground (the collector, emitter, or base respectively). (See Frerking’s Crystal Oscillator Design and Temperature Compensation (1978, p56) or Maxim’s crystal oscillator tutorial.) Alternatively, these oscillators can all be called Colpitts, but common-collector, common-emitter, or common-base (details).

    The point of this history is that oscillator terminology is confusing, with different sources calling oscillators Colpitts or Pierce in contradictory ways. Getting back to the oscillator module I examined, it could be described as a common-drain Colpitts oscillator (analogous to common-collector). It would also be called a Colpitts oscillator using the terminology based on the ground position. Historically, it would be called a Pierce oscillator since it uses a crystal. It’s also called a single-pin crystal oscillator since only one pin of the crystal is connected to the circuitry (and the other is grounded). 

  4. The typical quartz oscillator is built using a simple circuit called the Pierce-gate oscillator, where the crystal forms a feedback loop with an inverter. (The two capacitors grounded in the middle make this very similar to the classical Colpitts oscillator.)

    The Pierce oscillator circuit commonly used as a computer clock. Diagram by Omegatron, CC BY-SA 3.0.

    The Pierce oscillator circuit commonly used as a computer clock. Diagram by Omegatron, CC BY-SA 3.0.


    I’m not sure why the module I disassembled uses a more complex oscillator circuit that requires tricky biasing. 

  5. The voltage bias and current bias circuits are moderately complex analog circuits built with a bunch of transistors and a few resistors. I won’t describe them in detail, but they use feedback loops to generate the desired fixed voltage and current. 
  6. If you want to learn more about quartz oscillators, there are interesting videos at EEVblog, electronupdate, and WizardTim. Colpitts oscillators are explained in videos at Hackaday.

The $50 Ham: Digital Modes With WSJT-X

As it is generally practiced, ham radio is a little like going to the grocery store and striking up a conversation with everyone you bump into as you ply the aisles. Except that the grocery store is the size of the planet, and everyone brings their own shopping cart, some of which are highly modified and really expensive. And pretty much every conversation is about said carts, or about the grocery store itself.

With that admittedly iffy analogy in mind, if you’re not the kind of person who would normally strike up a conversation with someone while shopping, you might think that you’d be a poor fit for amateur radio. But just because that’s the way that most people exercise their ham radio privileges doesn’t mean it’s the only way. Exploring a few of the more popular ways to leverage the high-frequency (HF) bands and see what can be done on a limited budget, in terms of both cost of equipment as well as the amount of power used, is the focus of this installment of The $50 Ham. Welcome to the world of microphone-optional ham radio: weak-signal digital modes.


Just a Regular Joe

First things first, let me make it clear that there are a ton of modes available to amateur radio service licensees that don’t require talking into a microphone, going right back to the beginning of radio with continuous wave (CW) modes. Banging out dits and dahs with a straight key is perhaps the original digital mode, if we stretch the meaning of the term just a wee bit from its current modern connotation of transmitting and receiving encoded messages using computers, either built into the radio or attached as a separate component. I’ll use that as my definition of “digital mode” for the purposes of this article.

But even with that stricter definition, there is still a huge ecosystem of digital modes that have cropped up over the history of ham radio;  the desire for communications without the need to be a conversationalist goes way back, it seems. But for this article, I’ll be focusing on a couple of modes within the “weak signals” family of modes, mainly because I find them fascinating and incredibly useful, and I get a real kick out of seeing what kinds of contacts are possible using less power than it takes to light up an LED light bulb.

When you get into the weak-signal space, one name keeps popping up: Joe Taylor (K1JT). Joe is a ham based in New Jersey, and when you first start hearing about him, you figure he’s just a, well, regular Joe, an old school ham who has come up with some clever software to make low-power signals easier to pull out of a high-noise environment. And while that’s certainly true, it quickly becomes apparent that Joe is a lot more than that. Joseph Hooton Taylor, Jr. earned his Ph.D. in astronomy from Harvard in 1968. He joined the physics faculty at Princeton in 1980, and has won pretty much every major prize in physics and mathematics, including the Draper Medal, the Wolf Prize, and in 1993, the Nobel Prize in Physics.

The Magic of WSJT-X

For all these lofty achievements, in many ways Joe is very much a “ham’s ham”, and since his retirement in 2006 he has turned his considerable experience in digital signal processing toward an all-encompassing weak-signals package called “WSJT“, for “weak signals, Joe Taylor.” Actually first written in 2001, the program has undergone nearly constant revision and updating by Joe and a cadre of digital-modes enthusiasts, with the latest incarnation, WSJT-X, which implements ten different weak-signal digital modes.

Joseph H. Taylor, Jr. (K1JT), 1993 Nobel Prize in Physics. Source:

We’ll skip a deep dive into the DSP techniques underpinning WSJT-X — although it’s fascinating stuff and probably worthy of an article all by itself — and suffice it to say that the package implements various multiple frequency-shift keying (MFSK) modulation methods, each of which is optimized to work under different propagation conditions. The ten modes currently implemented cover everything from high-noise ionospheric propagation to tropospheric scatter, with modes that support bouncing signals off meteor ionization trails or even listening to your own signals bouncing off the Moon.

Even though WSJT-X modes are separated into broad “fast” and “slow” categories, by modern networking standards, they’re all pretty slow. Typical bit-rates range from a dozen characters per second to 400 baud or so. The low-throughput nature of these modes is entirely by design; by not attempting to achieve blazing speeds, WSJT-X makes very efficient use of the spectrum. Some modes only need a few hertz of bandwidth, with the tradeoff being that even very short messages can take multiple minutes to transmit.

The mode that I’ve been playing with most lately, FT8, is a relatively recent addition to the WSJT-X suite. FT8 was written by Joe Taylor and Steve Franke (K9AN), hence the “FT” in the moniker. The “8” refers to “8-FSK”, which means that the modulation scheme uses eight different tones spaced 6.25 Hz apart. Each FT8 signal therefore occupies 50 Hz, a huge chunk of bandwidth when compared to other weak-signal modes, but still pretty compact. All that extra bandwidth means that FT8 transmissions can be much shorter than, say, a 30-minute transmission on JT9. That makes FT8 suitable for quick QSOs and contesting, which is sort of the contact sport of amateur radio.

Speed Dating for Hams

While FT8 is fast, the tradeoff is message length. Each FT8 transmission encodes only 75 bits, with a 12-bit cyclic-redundancy check (CRC). That and the rapid turnaround time means that most operators rely on automation built into WSJT-X, as well as standardized messages, to make their FT8 contacts.

Setting up WSJT-X and getting a transceiver ready for FT8 is highly dependent on your computer and your radio. In my case, I built a dedicated Raspberry Pi 4 to run my ham radio operation, using the excellent Ham Pi image by Dave Slotter (W3DJS). I also attempted to use KM4ACK’s equally excellent Build-a-Pi image, but I had trouble getting my Icom IC-7200 transceiver talking to WSJT-X, and rather than devote a lot of time to troubleshooting I just tried the Ham Pi build. Both images have outstanding communities that will help you get spun up, as does WSJT-X, which has a forum where you’ll often see Joe Taylor pop in to answer questions. A community that has a Nobel laureate as a frequent contributor is a strong community indeed.

The video above shows why I call FT8 “the speed dating of ham radio.” The waterfall display at the top shows about 2,500 Hz of passband — the transceiver must be set up to allow as wide a possible band of frequencies through to WSJT-X (tip o’ the hat to Josh KI6NAZ for the help getting that right.) The FT8 algorithm decodes every 50-Hz wide FT8 signal in the passband at once; that along with the fact that each transmission is 15 seconds long followed by 15 seconds idle results in the characteristic checkerboard appearance on the waterfall display.

The characteristic FT8 checkerboard pattern develops on the WSJT-X waterfall. The 40-meter band was pretty good tonight, but I couldn’t make any QSOs.

Decoded messages are displayed in the left window of WSJT-X, with operators generally looking for stations calling CQ. Clicking on an entry in the Band Activity window starts a series of automatic messages, with WSJT-X keying up the transmitter and sending a minimal QSO — basically just the two call signs, a grid square locator, and the received signal strength. It’s important to note that the two sides of the conversation don’t have to be, and in fact shouldn’t be, on the same frequency — the other operator’s copy of WSJT-X will decode the entire passband if it can. Once the acknowledgment of the CQ is received by the other station, the exchange of messages is entirely automatic, until the final 73s are sent and WSJT-X gives both sides a chance to log the QSO.

Since I’ve set up my end-fed half-wave antenna for the HF bands and gotten WSJT-X installed, I’ve made quite a few contacts. Most of them have been in the continental US and Canada, but I did manage to bag Japan on 30 meters last week, which was a treat. The fact that I could do all of this without once picking up the microphone, struggling to think of something to say, is a godsend to me, and the fact that WSJT-X is able to decode signals that are so far down into the noise floor is an intoxicating technical feat.  It’s also really nice to sit down for a half-hour or so before dinner and bang out a couple of low-effort QSOs without having to invest too much in the process.

As mentioned, FT8 isn’t the only weak-signal mode that Joe Taylor and his collaborators built into WSJT-X. Next time on The $50 Ham, we’ll look at the equally addictive WSPR mode, and see how you can actually work HF bands for far less than $50, transmitter included.


3D Printer Makes Ham Antenna Portable

You don’t normally think of a 3D printer as a necessity for an antenna project. However, if you are interested in making a handy portable antenna, you might want to melt some plastic. [N2MXX] has an end fed antenna winder design that also contains the necessary matching toroid. This would be just the thing to throw in your backpack for portable operation.

The end-fed configuration is handy for portability too, because you can easily secure one end and feed the other end. Compare that to a dipole where you have to feed a high point and secure both ends.


Of course, you also need wire and some other components — we don’t know how to 3D print a usable ferrite toroid. Honestly, there is some controversy about how these antennas actually work, but people swear that they work well.

There are quite a few ways to operate a portable station, depending on your definition of convenient is. Verticals are popular, although laying out ground wires can be painful. A dipole isn’t that hard to erect, especially if you are staying in one place for a while. However, we really like how small this design is and it should be easy to clip one end and just play out the wire to operate. Our only concern is how plastics will fare in the elements over the long term. Then again, if it wears out, you can just print a new one.

Our own [Dan Maloney] has made these sort of antennas and had good luck. If you want to go really tiny, try surface mount.


Bouw je eigen big-ass powerbank voor velddagen!

Op velddagen is een 12V voeding natuurlijk noodzakelijk maar waar haal je die vandaan? Een kleine 7Ah accu is al redelijk snel uitgeput, zeker bij wat hoger vermogen. En de laptop? En de telefoon?

Thijs PE1RLN bedacht er het volgende op: een draagbaar accupack met een shitload aan features:

– 12V accu met 50Ah capaciteit (2 x 25Ah parallel)
– 2 stuks 20A outlets
– 1 aansteker-socket 10A
– dual USB socket
– 24V output voor surplus radio’s
– 230V @ 300W stopcontact

Door alles in te bouwen in een gereedschapskist, is het geheel draagbaar én veilig. Een ingebouwde lader zorgt ervoor dat je het accupakket na de velddag weer kunt opladen en de diverse zekeringen voorkomen dat er te grote stromen gaan lopen: 30A voor de interne bekabeling en 20A resettable fuses voor de 4mm aansluitblokjes. De 230V is schakelbaar zodat de inverter niet voortdurend stroom trekt terwijl deze niet wordt gebruikt en een ingebouwde ventilator zorgt voor afzuiging naar buiten zodra het te warm wordt.

De voorzijde is voorzien van een hoofdschakelaar die intern twee hoofdrelais bedient en een spanningsmeter.

Bovenop zitten de aansluitingen onder klepjes om ze tegen de regen te beschermen.

Het binnenste met de dikke bekabeling rondom de twee forse gel-accu’s.

Deze powerbank heeft al menig keer z’n diensten bewezen waarbij gewoon op vol vermogen kan worden gewerkt met de gangbare transceivers. Zelfs tijdens de recente stroomuitval in het Heuvelland zorgde de 230V inverter ervoor dat bij Thijs de CV-ketel bleef functioneren en kon Thijs via de 12V output de draagbare Nespressomachine gebruiken. Niks aan het handje dus.

Zelf bouwen? De koffer is van de Praxis, de accu’s van Nedis, de lader van de Action en de rest komt via de post uit China.