The Elecraft K2 Transceiver Kit

This article describes the basic, CW-only K2 kit which I built in 1999. My K2 now has a full complement of options including the KAT-2 automatic ATU, KSB-2 SSB module, K160RX 160m module, KAF-2 audio filter, KIO-2 computer interface board and KBT-2 internal battery pack.

People enjoy ham radio for all sorts of reasons. But for me what makes the hobby unique is that it allows you to build your own equipment and try it out on the air. I'm not an RF engineer, though, so I need a circuit to work from and preferably a kit of parts as well. Since the demise of Heathkit several years ago, most of the kits available were for QRP CW-only gear, often with a fairly limited specification. So when, in the spring of 1999, I read about a company in the USA called Elecraft that was planning to offer a complete, commercial quality 160 to 10 metres CW and SSB transceiver in kit form, it seemed just what I wanted.

pic The Elecraft K2 is the brainchild of Wayne Burdick, N6KR, and Eric Swartz, WA6HHQ, names that will be familiar to many QRPers as designers of a number of popular and successful low-power rigs. Not surprisingly, given its ancestry, the K2 comes as a high performance QRP (10W maximum) CW-only transceiver covering 80 to 10 metres (ham bands only.) But it is expandable by means of plug-in options to add 160m capability, SSB transmit with VOX and speech processor, noise blanker, transverter drive capability, internal gel-cell battery pack and auto ATU. In place of the battery pack and auto-ATU, a 50W or 100W PA can be installed. The first of these add-in options were only just becoming available at the time of writing, so the review that follows describes only the basic transceiver.

The K2 was designed during 1998 and around a hundred kits were field tested by willing pioneers. Feedback from the field testers was incorporated into the final product: this included not only circuit modifications but changes to the comprehensive construction manual to help reduce the chances of builder error. Production kits began shipping in early July 1999. By this time a sizeable waiting list had built up, so it was the end of July before I became the owner of K2 kit serial number 392.

picThe first thing that struck me - apart from the shock of paying the UK import duty and VAT (see later) - was how small and light the box was. The K2 has no chassis as such: the main RF circuit board performs the function of the chassis or motherboard and the case parts - olive green spray-painted aluminium panels - are attached to this, and to each other, to form a rigid box.

There are three circuit boards in the basic transceiver. The front panel board sits behind the front panel and contains switches, the mike and headphone sockets and the LCD display and S-meter bar graph, plus associated electronics. Immediately behind this comes the control board, which contains the main microcomputer, voltage regulators, AGC circuit and audio amplifier. Both these boards plug in to the RF board which as well as containing the synthesizer, receiver and transmitter circuits, is the motherboard that connects the different parts of the transceiver together. Multi-pin sockets are provided for all of the K2 options, completely eliminating inter-board wiring.

Design features

Microprocessor control is used in the K2 to a much greater extent than in other transceivers. For example, the microcontroller monitors the supply voltage, current consumption and power output and can display them on the front panel. There's even a user-programmable over-current limit on transmit: useful during testing, as I found out. The K2 has built-in test equipment - a voltmeter and frequency counter - so you don't need your own DMM or counter to successfully align it (though a separate DMM would certainly be useful.)

Other features provided by the firmware include four programmable IF filters for each mode, a memory keyer and band scanning. Many options are changed using a menu, which uses the main display to show the option name and the tuning knob to change the selection. But you don't need a degree in computing to operate this rig. The ergonomics are excellent, with all the functions you need day-to-day instantly accessible using knobs and buttons.

picRunning through the whole transceiver is a serial data bus called the AuxBus. This is used to convey commands from the main processor to satellite processors on other option boards (this is partly how the elimination of inter-board wiring has been achieved.)

The RF board has one such satellite processor called the I/O Controller. This carries out functions associated with band-switching. Satellite processors sleep when not in use, so they don’t generate digital noise that can be heard in the receiver. Latching relays are used for all band-switching and option selection, so no relay current is drawn during normal operation. Again, this helps to make the K2 an excellent portable rig: current consumption on receive is typically 200mA, but can be reduced to almost half that by measures like turning off the LCD backlight and S-meter.

The K2's state-of-the art design isn't confined to its lightweight construction and computer control. This rig has a receiver that will outperform others costing much more! The receive path starts with the band pass filters and moves on to either a 10dB attenuator or a low noise 14dB preamp. Signals are then mixed to the 4.915MHz IF in a diode ring mixer. This is followed by a 2N5109 post-mixer amplifier. This amplifier can handle high power (it has its own heat sink!) to achieve good dynamic range, though the power (and dynamic range) can be reduced if low power consumption is a greater priority than strong signal handling.

The synthesized local oscillator has been designed to have very low phase noise. The VCO uses eight relay-switched capacitors which, together with a single high-Q inductor and a tuning varactor diode, allows it to tune the appropriate segments in the range 6 to 24MHz. The synthesizer chip provides coarse tuning in 5KHz steps. Fine tuning (10Hz steps) is provided using a varicap tuned VXO (variable crystal oscillator.) A firmware calibration routine records the voltage required for each 10Hz step and records it in EEPROM memory.

The result of this is a receiver that immediately impresses you with how quiet it is, yet which can hear signals that are inaudible on other modern transceivers. Reports from other builders suggest that the only rig that comes close to the K2 in terms of receive performance is the Ten-Tec Omni VI, a premium-priced transceiver. A couple of hams have successfully used K2s as tunable IFs for EME work - a very demanding application.

picFollowing the IF buffer is a five-pole crystal ladder filter. The filter bandwidth and centre frequency are both continuously variable. This is again achieved using microprocessor controlled varicap tuning. Four programmable settings are available for each mode (CW, CW Reverse, USB and LSB) and each setting is user-selectable: the voltages required are stored in EEPROM. As standard, the settings are 1.5KHz, 700Hz, 400Hz and 100Hz (CW) and 2.2KHz, 2.0KHz, 1.8KHz and 1.6KHz (SSB). How many rigs come with 8 switchable filters as standard? At the wider settings used for SSB reception the filter passband is not optimal. However, the SSB module will have its own dedicated filter, still with four programmable settings, making the K2 a superb rig for data modes.

On transmit, the driver and PA stages use inexpensive CB type PA transistors. The bias voltages are regulated by the firmware to give better efficiency in CW mode. The PA transistors - a pair of 2SC1969s in push-pull - can comfortably give 10W output though the heat-sinking, which just uses the bottom panel of the case, makes this inadvisable for long periods. For QRP purists who don't intend to exceed 5W output, the output transformer can be wound differently to give even greater efficiency at low power.
Receive-transmit switching uses diodes, so changeover is completely silent. Full break-in keying is possible. Again thanks to the microcontroller firmware, the transmit-receive delay is completely user-programmable from 0 to 2.55 seconds. As one kit builder said: "It makes an FT-1000MP look poor."


This brief run through some of the design highlights should give you the idea that the Elecraft K2 is an outstanding transceiver that is in several respects unique. But for most people it won't matter how good the K2 is if it's beyond their ability to build it successfully.

The good news is that it isn't hard to build at all. Elecraft calls the K2 "an intermediate-to-advanced kit" but it's easier to build than that implies. The main challenge is simply the size of the undertaking. You have to solder in a lot of components before you can do any testing and if you make a mistake, finding it can be hard. However, the construction manual is first-class, just like the Heathkits of yesteryear: download a copy from Elecraft's Web site and see for yourself. The components are all conventional, lead-in-hole components (no SMD devices) and the PCBs are of excellent quality, clearly labelled and well masked. If you can solder without making solder bridges or dry joints there's no reason why you couldn't build your own K2.

Construction takes place in three main stages. First, you build the control board which contains the microprocessor, voltage regulators and audio amp. On completion, you check for shorts by performing some resistance checks using a DMM. Next you assemble the front panel board with its switches, pots and the display. Care must be taken to get the switches at the right height. But all the steps are carefully spelt out. At the end, you perform another batch of resistance checks. Then you mount the board into the front panel and attach the knobs. Now you can start to see what the transceiver will look like.

Next, you add the relays and a small number of other components to the RF board. After this is done you attach the side plates to the RF board, plug in the front panel and the control board, and can run the first series of tests with the rig plugged in to a power supply. These tests verify that microprocessor is working, along with the audio amplifier, the sidetone generator, the keyer and the built-in test equipment.

picIn the second main stage you add the synthesizer, BFO and receiver components plus the band pass filters for 40m. You need to wind some toroids during this step. If this is the first time you have wound RF inductors, don't worry: it's easier than it looks. None of the inductors are difficult to wind (only a few have a secondary winding) so it's just a matter of counting the turns and getting them evenly spaced around the toroid. There are even pictures of what the inductors should all look like on Elecraft's web site. You install a lot of components during this stage so it takes some time, but the reward on completion is a fully working receiver on 40 metres. It's a thrill to hear signals on a receiver you've built yourself, and even better when you immediately realise just how good the K2's receiver is.

In the final stage you add the transmitter stages and the band pass filters for the remaining bands. It's a shorter stage than stage 2 - though more toroid winding is involved - the only problem is keeping your impatience in check. After final assembly you calibrate the VFO and then align the band pass filters by peaking the output power into a dummy load. You can use the K2's own built-in power meter for this, so you don't even need one of your own. Your K2 transceiver is now complete.

It was at this last stage that I hit trouble. When I keyed the transmitter, the K2's over-current protection operated, even with the power control set at zero. The PA stage was oscillating. Despite all the care I had taken, resulting in a faultless construction, I was stymied by what turned out in the end to be an open-circuit RFC in the driver feedback circuit. It was a very disheartening moment, which was followed by several hours of careful checking and signal tracing as I homed in on the culprit. An invaluable resource during this time was the Elecraft e-mail reflector, which provides a forum for communicating with other builders (and the rig's designers) and getting help if you are stuck.

I was unlucky. Most of the problems other builders have experienced have been due to unsoldered component leads and misplaced or omitted components: problems that could have been avoided by taking a bit more care. If you've constructed kits successfully before, there's no reason why you couldn't build a K2 and have it work first time, which is the trouble-free experience most builders have.

How to buy

Elecraft doesn't have a dealer in the UK, or anywhere else for that matter except Japan. However, it's easy to order the K2 kit and its accessories direct by post or by phone, using a credit card. If you're on the Internet you can use the self-calculating order form on Elecraft's web site which, once completed, you simply print out and send off. Shipping to the UK takes about a week, and new orders are now being met from stock.

Currently, the basic K2 transceiver cost $659 (about £340 at present exchange rates.) If you're in Europe, shipping costs and VAT will push the price a fair bit higher. And that's just the basic model. A fully loaded K2 with 100W PA will cost more than many ready-built rigs.

However, the K2 will outperform anything you could buy at this price, or indeed a lot more. Even more importantly, you can tell everyone you work that you are using a rig you built yourself. Is there any ham who wouldn't get a buzz of pride from that?

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