T/R Switching

A quick post from the day’s experiments: the transmit/receive power switching arrangement for my next project. The scheme is very much like that in the KN-Q7; I wouldn’t have stumbled across this (very simple) setup without an excellent write-up by Andrew Woodfield, Zl2PD.

Here’s the simple schematic:

The power switching arrangement for a new project.

The circuit itself is straightforward – when the “key” pad is left floating, current can flow through the 2k2 resistor attached to the 3904, providing a small base current and driving the 3904 in conduction and powering anything connected to the +12R pad. At the same time, there is nowhere for base current to flow in the 3906, so no current is provided to the +12T pad.

When the key pad is grounded (by a morse key, or other TR switching method), the base of the 3904 is pulled to ground through the small signal diode, and very little current will flow through the 3904 and into the +12R section, effectively killing receive functions. At the same time, a small current will flow through the 2k2 resistor attached to the 3906, allowing it to conduct and powering anything attached to the +12T circuit.

In short: when the key line is floating/disconnected, the circuit is powering receiver functions. When it is grounded, the circuit powers transmitter functions.

I added the little 7808 regulator to this power board, which will be powering some NE602 mixers on receive only. I put both circuits together on a little piece of copper-clad, in something like Island-Pads-meets-manhattan-construction:


I like how the layouts ends up showing off the inherent symmetry between PNP and NPN transistors. The whole thing looks quite nifty on the bench. I left the input and output pads deliberately large, to accommodate however many connections end up being made.

I did begin construction and testing on the next project, but was stymied by my SLA-battery power source running down (it was down to 7.4 out of 12V by the time I thought to put a meter on it). Rather than switch to the bench power supply (an old computer power supply with the 12V and 5V rails brought out), I took it as a sign to call it a night.

Hear you over the sizzle of solder!


An Si5351-Driven QRP Amplifier

This work follows directly on from my initial thoughts on an Si5351-Based Transmitter. Check out that post for background.

This project ulimately got packaged up into the SI5351 Signal Generator/VFO.

This week I finally got around to improving the transmitter/amplifier I started work on back in March. With a shipment from KitsAndParts, I replaced the J110s that had been part of this amplifier and replaced with BS170s. The differences between the parts are striking – the J110 is a general-purpose JFET (which is by definition a depletion-mode FET) with a rated power disipation of a few hundred milliwatts. The BS170 is a fast-switching enhancement-mode MOSFET with a rated disipation of 800 mW.

Here’s how the schematic looks now. Pretty similar to the last time, with a couple key changes:

CW Transmitter BS170

I stripped the heat sinks that had been on the J110s off and threw them on the BS170s. These puppies still do get hot, and even with the ability to dissipate almost a Watt, I think they’d not be pleased too much heat. The heat sinks, plus some thermal goop from Microcenter, are easing my mind a bit.

As built, the amplifier had an output impedance of about 10 ohms at 7 MHz, as measured by the method suggested by W2AEW in this Youtube Video. To bring that up to the standard 50 Ohms, I built a little L-match, using the values suggested by this L-Match calculator, about .43 uH in series with the amp and a little over 1 nH shunted to ground. Because I don’t know the voltage ratings on most of my miscellaneous caps, I put for 4.7nF caps in series to form the fun capacitance. The inductance is 12 turns on a T37-6 (I did some experimenting to find the ideal number of turns). Re-measuring the output impedance (again with the W2AEW method) showed an output impedance of around 47 ohms, which is close enough for my purposes, I figure. I

I also recently bought a little power-supply kit from Ebay, and have been using that to supply the drive current. It’s adorable:

Now, being driven by the Si5351, the amplifier puts out about 1W on a 5V supply, and about 5W on a 12V supply. Not a particularly clean signal, mind – the 5W is as measured in the power meter of an MFJ-949 – but it is an actual 5W!

Hear you on the air!


Transistor Tidbits

I got another little shipment of components today from KitsAndParts. Namely:

  • 25 @ 2N7000 N-Channel MOSFETs – A general use enhancement-mode FET with a few hundred milliamps current handling.
  • 6 @ BD139 NPN Transistors – Able to handle 1.5 amps DC at 80V, these form the driver or final stages of a number of VK3YE’s portable rigs, and I’m excited to experiment with them.
  • 12 @ J310 FETs – a VHF/UHF amplifier JFET, which turn up in a lot of the regen-recevier designs I’ve been reading.

A little progress has been made on stripping the silver chassis from the DeKalb Hamfest to turn it into something. And I’ve put a little more work into optimizing the amp in the Si5351-based transmitter. But neither have enough actual results to write-up yet. So, for today, just a shopping list and a ping.

Hear you on the air,


DeKalb Hamfest 2016

I’ve been trying to think how long its been since I’ve been to a ham radio swapmeet. Certainly the last one I visited was the Electronics Flea Market at De Anza College with W6KWF sometime during my college days. So it’s been a good long while, and it seemed time to rectify that. And thus, I’m just back from the DeKalb Hamfest. First some details, and then I’ll get to today’s haul. A bit of a long post today, but there’s much to tell.

A brief overview – the DeKalb Hamfest is hosted by the KARC – the Kishwaukee Amateur Radio Club – and overseen by Bob W9ICU. It’s hosted at the Sandwich County Fairgrounds in Illinois around this time (early May) each year. $8 for admission, and easy parking.


It wasn’t a huge event – maybe 10 or 12 indoor vendors and perhaps three times that many tailgate-ers. The indoor vendors, while they had lots of nice products, didn’t particularly entice me, as I think you could find just as good value on, say, banana-jack-to-BNC tails or audio connectors anywhere online. There was one vendor with a huge spread of every RF adapter and then some, as well as individual toroids for sale, including some big 100- and 240- size ones in unusual mixes. I’ll look for them at future fests.

RF adapters and connectors for miles.

The tailgate-ers were more my speed, though none of the big-ticket items were really what I was looking for (or in my price range) Several old Heathkit receivers and transceivers for sale ($200-$600), lots of microphones, mobile units, CB radios. A few newer items, but mostly old tube equipment. At least four tables had the same Heathkit GD-1B Grid Dip Meter, which I found curious. Lots of vibroplex keys marked over $100.

But lots of treasures too. Maybe only three or four tables with proper parts bins, but I spent a good couple hours pouring through these to find some treasures. Most table owners were happy to make small talk and chat about their projects and where their goodies came from. I had a really laugh chatting with some of the nice folks running the tables, while piling up the day’s haul.

And what a haul!:


Here’s the short list, starting at the bottom-left and working clockwise:

  • A pack of five unlabeled compression trimmer capacitors. These turned out to have a range of about 2-25pf.
  • A bag of ~10 small toggle switches with assorted mounting hardware. These range from SPDT to DP4T.
  • A large bag of ~20 rotary pots of assorted values, with a few rotary switches thrown in for luck.
  • A couple RG-58 jumpers with BNC connectors (free!)
  • A large air-spaced variable capacitor – unlabeled, turned out to be about 28-152pF. More on this below.
  • A nicely boxed step-attenuator with 50-ohm input/output impedance. I’d been thinking about building one of these, but running across one of them at the fest (for only $3) was a dream! It has a maximum of 59 dB of attenuation in steps of 20, 20, 10, 6, and 3 dB, as well as a variable attenuator (“3 dB?”) for fine tuning. It has Walter Schwartz’ name on the bottom, who was the nice gentleman who sold it to me. A very cool older ham.
  • A mysterious silver box… more on this below as well.
  • A bag of twenty 47pF NP0 capacitors. For future oscillators.
  • A couple bags of trimmer pots. One bag had five 500 Ohm trimmers, the other turned out to have a mix of 1K and 1M trimmers. I have these in mind as balance pots for diode mixers, but we’ll see where they end up.
  • A couple vacuum-molded packs of trimmer capacitors, 10-. These actually came from the Fry’s Electronics in Downer’s Grove – I couldn’t resist a visit on my way back to Chicago. And I got to help a dad pick out resistors for his ten-year old’s first electronic’s project. Neat!

All of the above for less than $20 total. Hamfest, for the win.

I’m particularly excited about the air variable capacitor. After a big of digging around, I pulled this one out of a cardboard box burried underneath a giant box of tubes:


Since I didn’t have either a multimeter or an LCR meter with me, I had to make my best guess as to the voltage rating and capacitance of this unit. When I got it home, it turned out to have a capacitance that varies from about 28pF fully open to 152pF fully closed, with stops at either end. The plate spacing (measured with a caliper) is about 0.060 inches, which means it should start to arc-over at around 2400-2600V.

I have a specific project in mind for this guy: building a small transmitting loop antenna. A range of ~28-150pF should be enough to tune a 10′ circumference loop for the 20m, 17m, 15m, and 12m bands, as well as the 10m and 30m bands with a little fudging (this according to the 66 Pacific Loop Antenna Calculator). With some added capacitance in series or parallel and it could possibly be pushed to 10m, 40m, or 80m bands with reduced efficiency. The air-variable capacitor is key here, since it seems the voltages in a loop antenna can peak over a Kilovolt with just 10-20W of output power! In any case, more thoughts on that antenna as it comes together, but for now I’m drawing ideas from AA5TB, Nonstop Systems, KR1ST, W8JI, and VK3YE’s various projects.

In any case, one of the nice things about this particular air-variable is the reduction drive on it. You can see the worm-gear and larger reduction gear in the picture above – it takes 46 full revolutions of the small stud to shift the capacitor full fully open to fully closed. With a difference of 122pF between the two extremes of the capacitor, that’s an average of 2.7pF per revolution. Even using the bamboo-stick tuning method used by KR1ST (to avoid detuning the antenna by getting close to it), if I can manage rotational accuracy to about 10 degrees of the tuning stud, that’s a resolution of .07pF. Not bad!

One of the things I was on the hunt for this morning was a metal chasis or two to mount projects in. As nice as the Virgin Receiver turned out, I was hoping to find something a little more robust for future projects – maybe some old CD-drive casing or a gutted something or other. There wasn’t a whole lot of that sort of thing around, even in the big-piles-O’-parts, but then this silver beauty caught my eye.

I could see a couple of tubes inside, and from the ‘RCA’ and 1/4-inch jacks on the back (an no jack for a key or mic) I had a pretty good guess it was some kind of receiver, but other that, it seemed like a gamble. When I asked the owner of the table what it was, he very gamely said “I have no clue, but for $2, it’s yours!”

Well, at that price, it’s worth it just for the box! And what I found inside is definitely worth more than two bucks to me:

Judged by the two parts labelled “10.7 MCycle Interstage”, I feel pretty confident that this was indeed a receiver of some kind. More than half of the tube-sockets are empty, so I don’t think it’s worth trying to figure out what they were and bringing this thing back to life.

But check out the nifty components! Up front is a 4.5:1 vernier reduction drive, controlling a two-section air-variable capcitor for some kind of tuning. There’s a 30pF NP0 trimmer cap and a coil mounted directly to the air-variable, which is interesting. There’s a couple of 500K log-scale pots, a bunch of high-voltage capacitors, some old carbon-composition resistors, miniature RF chokes… this thing is going to be a great source of parts, as well as a keen chassis for a future project. Perhaps a regenerative receiver? I could see leaving that classy vernier drive capacitor in place as is and working around it…. hmmm….

Hear you on the air, and see you at the fest!