Cool Stuff, Ham Culture, Homebrewing

Nerd Famous

It’s nice to see we hams, who I think suffer from a bit of an image as throwbacks in the larger maker community, get some recognition for the good stuff we’ve accomplished. Today on Hackaday, a nice article about Manhattan and Ugly construction was posted, with ample coverage given to the fact that a lot of the best exemplars of these techniques come from the world of amateur radio builders. I’m not certain about how others feel on this topic, but it seems to me that Hackaday is one of the preeminent blogs relating to our hobby, so I get quite excited when we get repped there.

Featured in this article are two names well-known in our circles, and guys that I’m proud to call my friends (although I have never personally met either in real life yet!). Todd VE7BPO, is renowned for his rigorous empirical work in circuit design, as well as his beautiful Ugly circuit creations. They feature one of his designs near the top of the article.

The other is Dave AA7EE, who is probably familiar to almost every reader, unless you just crawled out from living under a rock for the last decade. It’s not difficult to see why they chose Dave’s work for to illustrate Manhattan construction, as his is some of the best out there. Period. Also unsurprisingly, this is not the first time that Dave’s creations have made it to Hackaday.

Well done, gentlemen! Way to show the maker world at large that we’ve got relevant skills for the 21st century hacker community!



30 Watt Dummy Load

30 Watt Dummy Load
30 Watt Homebrew Dummy Load

First off, I want to apologize for the sparse updating on the blog lately. The muse has not been my friend in the last few weeks. I try to keep the content mostly original, but perhaps I will have to turn to that time honored blog tradition of short blog posts repeating something cool I heard on the Internet.

Anyway, on to the good stuff. I remembered in my last Digi-Key order to grab two 100 Ω Caddock power resistors (MP930-100-1%) to make simple dummy load. These were mentioned in a QRP Quarterly article a little while ago as a great non-inductive resistor to use for RF dummy load applications. The datasheet looked good and the price was cheap ($3.51/piece), so I figured I would give ’em a try when I got a few spare moments.

30 Watt Dummy Load
Dummy Load Close-Up

I wasn’t sure what kind of heat sinking was needed, so I used my most scientific method and took a wild guess. The resistors (in TO-220 packages with a ceramic contact pad) were mounted to a piece of copper clad measuring 2″ x 4″. A bit of thermal grease was smeared on the copper clad before mounting the resistors with 4-40 machine screws. I mounted the resistors so that one lead could be soldered directly to the center pin of the BNC bulkhead connector, while the other lead was soldered to the copper clad ground plane. I figured this should minimize stray inductance.

I gave the dummy load a test drive on the IC-718 set for 30 watts power output. A keydown period of 30 seconds showed a nice SWR on the the rig meter, although my LP-100 showed a reading of about 1.8. The dummy load was fairly warm, but could be handled. I wouldn’t want to key it for much longer at that power level, but I bet it could handle <20 watts quite easily. I expected to see a reading of nearly 50 Ω purely resistive on the LP-100, but surprisingly there was a fair amount of inductive reactance (hence the SWR of 1.8). Now I’m a bit doubtful that I’ve calibrated my LP-100 correctly since I wouldn’t expect such such a lousy reading. I’m going to wait to declare the LP-100 reading bad until I can get the dummy load on a calibrated VNA to make sure that it doesn’t really have this problem. Stay tuned for VNA measurements on the dummy load when I am able to make them.

Homebrewing, QRP

W8DIZ 5 Watt CW Amplifier Analysis

I finally got the proper binocular ferrite cores that I needed to build the W8DIZ 5 watt amp correctly. You can see my previous post on this amplifier here. In my last post, I noted that I was seeing some strangeness in the drive level circuitry. I found that I had a very bad connection through my ammeter to the DC power supply, and once it was corrected the drive circuitry worked as it should.

For this basic analysis of the amplifier, I took measurements of the RMS voltage of the amplifier output into a 50 Ω dummy load with a constant input amplitude of 0 dBm. I also measured the total current draw of the circuit, which allowed me to calculate the amplifier efficiency. Note that no low-pass filtering was used at the output of the amplifier. The output waveform was not sinusoidal, but my DSO is able to do a good job measuring RMS voltage.

Test Equipment

  • Tektronix TDS 1012 Digital Storage Oscilloscope (100 MHz bandwidth)
  • Tektronix SG 503 Leveled Sine Wave Generator
  • Tektronix DM 502A Digital Multimeter
  • Tektronix PS 503A Power Supply
  • M3 Electronix FPM-1 Frequency Counter/Power Meter

Test Conditions

The DC power supply to the amplifier was set to a loaded voltage of 13.5 VDC. The signal generator for the input signal was set to 0 dBm power output into 50 Ω, which was verified with the FPM-1 each time the frequency was changed. Two sets of measurements were taken, one with R6 set to minimum and the other with R6 set to maximum.


Min Max
MHz VRMS (V) PO (W) IDC (mA) VRMS (V) PO (W) IDC (mA) Eff.
1.8 13.7 3.75 440 22.4 10.03 957 0.776
3.5 20.2 8.16 768 23.9 11.42 992 0.852
7.0 13.8 3.81 470 21.7 9.42 844 0.826
10.1 9.3 1.73 300 19.8 7.84 732 0.793
14.0 5.24 0.55 174 17.6 6.20 635 0.723
18.7 3.12 0.19 103 14.4 4.15 514 0.598
21.0 2.07 0.09 78 12.6 3.18 452 0.521
24.9 1.53 0.05 65 9.05 1.64 326 0.372
28.0 1.19 0.03 60 6.56 0.86 232 0.274


As Diz states in his original post, the efficiency of the amplifier is quite good. However, both the power output and the efficiency starts to droop a bit above 20 meters. It’s my belief that this is a function of the gain-bandwidth product of the two PA transistors. According to the datasheet, the FT of a 2SC5739 is 180 MHz. Given the rule of thumb of having a FT at least 10 times the output frequency, it makes sense that the output starts to get a bit weak around 18 MHz. I do have some similar devices (2SC5954) with a slightly higher FT of 200 MHz that I will probably substitute in the circuit to see if I can improve the upper HF response a bit. There seems to be some kind of strangeness at 3.5 MHz, which doesn’t allow me to get much power output range. I’ll have to check with Diz about this. Regardless, this would still make a very fine QRP amplifier up to the 15 meter band. The amplifier is extremely stable and the PA transistors don’t get very hot during long periods of use. I currently have the transistors floating freely, but a modest heat sink would probably be a good thing if running the amp at full power output. This kit will be a great addition to the RF Toolkits line.


NT7S Code Practice Oscillator

NT7S Code Practice Oscillator - In Altoids Tin

Here it is, a completed version of the simple discrete component code practice oscillator that I promised. I tweaked the circuit just a little bit and made a Manhattan layout that will enable the CPO, a 9 volt battery, and all of the required controls to fit into a standard sized Altoids tin. This CPO produces a nice sine wave at about 600 Hz, unlike many of the other CPOs that output a buzzy square wave tone. There are no exotic parts used in this project, only a couple of generic NPN transistors, a handful of common resistors and capacitors, and a trim pot. The output level is sufficent for headphone use, although it will not blow your eardrums out, even at full volume. If you need to use a speaker with this oscillator, just plug it into a set of amplified speakers, like those used for a computer. This project would also make a good oscillator for CW practice on a VHF/UHF FM repeater. The volume control should allow you to adjust the output level to one that is appropriate for the microphone jack of a FM rig.

NT7S Code Practice Oscillator - Tin Open
Inside view of the CPO

I’ve attached a PDF schematic and layout diagram below. I haven’t created any build instructions, but it should be an easy build for anyone who has any experience with Manhattan construction. Let me know if you plan on using this design for a group build to help people learn CW, I might be able to work with you to create such a document. Print out the layout diagram at 100% scale, and you should be able to use it to size your copper clad board and mark the locations of your pads. I hope this is helpful to you and can help you to introduce new operators to CW.


Design, Homebrewing

Progress is Made

Baxter Is Unsure
Baxter Is Uncertain

I finally got a few days of decent sleep (decent meaning more than 4 hours), so I had a little energy to work on the simple DC transceiver. A few days ago, I got the remainder of the audio chain working. The emitter follower on one of the outputs of the differential mixer was yanked, and I connected a class-A audio amp directly to the mixer. Then I stuck the emitter follower on the output of the class-A amp to enable the receiver to drive low-impedance headphones. No, it’s not extremely efficient, but it is simple and it works. Best of all, no transformers are needed. As an afterthought, I added a simple shunt-to-ground mute circuit with a 2N7000. That might have to be tweaked a bit later

The Yet-Unnamed DC Rig
Unnamed on the Bench

The transmitter is also a simple design. The second output of the differential mixer is tapped with an emitter follower that will have its VCC line keyed to control transmit. Directly following this is a 2N7000 class-C PA. After a bit of work tweaking the impedance matches to get the right amount of drive to the PA, I can easily get 2 watts out of the amp (before low-pass filtering). What’s neat is that the emitter follower puts out about +10 dBm, and it gets amplified up to +33 dBm in one stage. A very compact design that can generate a decent amount of power.

So in order to make this a true transceiver, I have a few things left to do. First thing, of course, is to get a low-pass filter on the transmitter. I’ll also need to provide a keying circuit and T/R switching. I’m still not sure what I’m going to do about a sidetone. I also think that I’ll put an RIT circuit in there and not worry about a fixed transmit offset (that would be very hard to get right in such a simple transceiver). Keep watching for another update, hopefully soon.

Design, Homebrewing

Yet Another DC Receiver

80 Meter DC RX Prototype
Unnamed Simple Discrete 80 Meter Rig

Yes, its a post about another simple, low-performing direct conversion receiver. However, I think that this one is slightly unique. I was inspired to give this a try based on the Flea minimalist transceiver that was introduced on the EMRFD Yahoo group. These little rigs are fun to build in an evening, but just how usable are they? Would you feel comfortable giving it to a new ham and believing that they even had a small chance of success? For me, these Pixie-class rigs are nearly unusable due to the horrible AM broadcast interference that blows right through the rig. While a minimalist rig is an admirable thing, they are only useful in limited circumstances. I figure that a few things have to be added to these rigs in order to make them more than a novelty. KD1JV also shares this viewpoint, and has created his own answer to the Pixie.

I’ve started with a similar philosophy, but built the rig around a different topology. The basic strategy is to use a differential amplifier as an active mixer. The rig is designed for the 80 meter band, which is probably the easiest for homebrewing. The LO is a Colpitts ceramic resonator oscillator, but is not separate from the mixer. Instead, the oscillator is built around the third transistor which acts as the constant current source. I know that this is certainly not a new idea; it’s used all of the time in NE602-based QRP circuits. However, I don’t think this topology is seen very much in discrete component use. It saves quite a bit of circuit space and is composed of very common components.

The rest of the receiver is very simple. I placed a standard double-tuned circuit bandpass filter in front of the RF port of the mixer to filter out all of the AM BCB crud. The output of the mixer feeds a dirt-simple emitter follower to transform the relatively high collector impedance of the diff amp mixer to a low impedance output. I haven’t designed the final AF amp yet, but I don’t think it will take much to get the signal up to headphone levels. When the emitter follower output is connected to my test bench AF amp, I have to have the amplifier AF gain control turned nearly all the way down, lest the whole thing start oscillating wildly.

Tonight, I connected the RX to the bench AF amp and the antenna to see how it would work. Tonight was an excellent night to try, since we are right in the middle of Sweepstakes. Pleasantly, the receiver immediately came to life with a cacophany of CW signals in the unfiltered audio output of the receiver. I’ve attached a recording of the receiver output so that you can get a feel for how well it works for such a minimalistic design. The ceramic resonator osc tunes from nearly 3.500 MHz to 3.580 MHz, and I tune across the entire band in this clip.

All I have to do to finish the receiver is to add on a discrete component AF amp. I think that a single class-A stage of amplification will be enough to get the audio up to headphones level. After that, I’m going to try to tack on a transmitter by picking the VFO signal off of the other unused collector port of the diff amp. I think that I can get away with another emitter follower as a buffer, followed by a class-C PA. I’m shooting for around 1 watt of output power, which is enough to snag QSOs without too much difficulty. I think this could be a lot of fun to build as a kit. It’s will be quite a bit more complex than a Pixie or Flea, but also quite a bit more usable. Stay tuned for further developments on this rig.


Preliminary CPO Schematic

Code Practice Oscillator Schematic
Code Practice Oscillator Schematic

Here’s quick update to post the schematic of the code practice oscillator that I mentioned previously. As you can see, I just paired a twin-T sine wave oscillator with a buffer amp that feeds directly into headphones. The twin-T provides plenty of voltage, so the buffer is all we need to provide enough current to drive headphones. You can also download a PDF file of the schematic here.

Thanks to a suggestion from David KB0ZKE, I’ve decided to rework the layout to fit in an Altoids tin (an excellent idea!). I’m also going to try to come up with an easily reproduced straight key made from a paper clip, knob, wooden base, and wood screws. This idea was inspired by KE6GS, who has a great example of such a key right here next to his Willamette. I’ll try to get the layout changes finished real soon. The detailed documentation will have to wait until after I finish the Willamette documentation (but I have started on it again, so it will come in the near future).