G3PJT’s pages on Amateur Radio

These pages describe some of my Elecraft K2 projects.

An antenna switch for the Elecraft K2.

A keypad for the Elecraft K2.

A Second Receiver for the Elecraft K2 Version 2 (Nov 2006)

Although the K2 is a great transceiver the addition of a second receiver adds a lot of fun and effectiveness when using the K2 for both DXing and contesting. I have two K2s which I like to use as a linked pair. I am not at the moment interested in SO2R operation - I just use the second K2 for receiving, but I did want to interface them to each other and to the logging programmes I use. After I had gained some experience with the first version of the controller I decided that a number of changes needed to be made. This version uses much better software and has a couple of new features. Let's call the two K2s K2T ( the transmit K2 or your 'main' K2) and K2R (the 'second receiver' K2)

Basically the new controller :

- allows frequency and mode info to be ccopied from either K2 to the other.

- has a 'SWOP' feature which swops frequuency and mode between K2s but on releasing the button or footswitch it swops back.

At the same time it also swops the headphone sense (see later)

Being able to copy frequencies between the two K2s makes a very effective combination to break split frequency pile ups.

And with these new feature you can use the combination for contesting too. The arrangement would probably work with any brand receiver if the appropriate software changes were made. The method of operation is as follows.

Both K2s are controlled via their KIO2s. On wishing to copy frequency info. Each K2 is first interrogated by the controller which then sends the required commonds to the second K2. The Controller manages at the sametime any switching of the various RS232 lines between the radios. Logging programmes normally poll your radio for frequency/mode information via an RS232 interface (KIO2). using the same commands. The Controller interleaves these commands and avoids confusion. It is the status information of K2T which is sent to the logging programme. The controller however scans buttons and a footswitch to check if any has been pressed. The two K2s seem to be able to complete any changes within a few 10s of mS and less than the cycle of the poll from the logging programme. As written this programme will now recognise most commands from a logging programme -including direct frequency jumps from Writelog/LOGGER etc.

The arrangement block diagram and controller I use is shown below. The DB9 sockets go to the PC, K2R and K2T respectively. The push buttons control K2T to K2R and the reverse and 'Swop' The switch controls audio, K2T in both ears or K2T in one and K2R in the other.


The specification of the two K2s is:-

Transmit and receive K2 - K2TSecond receiver K2 - K2R

My logging programme is LOGGER and for contests I use WRITELOG. They are both set to the Kenwood radio specification.

The Controller.

The controller allows one to copy frequency and mode information between the two K2s. At the same time it interfaces with my PC logging programme.

The Controller is quite complex due to the need to control timing and to ensure data integrity (these were problems with the first version)

K2T, K2R and the logging PC are interfaced togethervia RS232 to CMOS converters. These are to buffer the controller from the RS 232 lines. It would be possible to dispense with these but its better to have the right voltage levels and the buffer between the circuitry.

The system uses the RS232 interfaces on the K2 and the commands described in the Elecraft KIO2 Programmer's Reference. The audio section to the left uses a DIL relay to switch the audio between the left and right ears - the relay is controlled and sequenced with the 'SWOP' commands.


A new electronic switch is installed in K2T which allows internal connection of K2R to the antennas in use. Connecting the K2R in parallel across K2T does not appear to affect the receiver performance. The electronic switch circuit is based on the K2 antenna switch and is switched with 8R. The switch is mounted on the RF Board transverter header (J13 RF Board) as a plug in module, see picture. The components used are the smallest I could find. A new BNC socket was fitted to the lower back panel of K2T close to the RX Antenna socket.

electronic switch circuit


This is set up as follows.

The RX Ant socket is connected to the new RX Out socket on K2T in 4. K2R is set up as follows:-

RX Ant is selected for ALL bands

Port ON

Power Control zero

STL is 000

Normal antenna terminal connected to dummy load ( this is for protection only)

Select TEST (this needs to be selected each time since it is not a remembered setting for K2, but see software below)

As K2R does not have a SSB module the filters are set up is as the manual, with three wide settings for SSB as well as settings for CW which are the same as those for K2T.

NOTE. Do NOT attempt to use the main antenna connection on K2R for reception. For sure you will inadvertently transmit into the K2T with disastrous results.

Digital Controller Hardware and Software.

The controller is built on a small piece of perf. board and a separate board carries the glue logic to switch the I/O data to the correect K2 or PC. The PIC is a 16F877 running at 20MHz. The code can will be made available together with a PCB and assembly instructions as soon as I can do the layout.

Headphone Control.

A switch is included to allow the selection of K2R in right headphone and K2T in the left or K2T in both. Care has to be taken to avoid an earth loop when this is in the same enclosure. In my layout the earth loop is broken by insulating the input jack from the chassis of the controller. And only earthing the audio to the panel of the controller at the stereo out jack on the front panel.

Spurii and other nasties.

Interconnecting two K2s so closely shows up some spurs which are not otherwise audible. Mostly these are at low level, but there are on some bands digital artifacts from the other K2. Also there is a very low level signal caused by low level leakage . This signal appears on or close to the frequency when both K2s are set to the same frequency. Steps are underway to design a better isolation circuit between the two radios.

Frequency accuracy.

The software assumes that the frequency information will set K2T and K2R to the same frequency. This will depend on the individual calibration accuracy. The procedure I use to minimise frequency differences is :-

- align K2T so that it is exactly as youu like it.

- set the 4MHz osc in K2R to be the samee as K2T (it will in any case be very close)

- run CAL PLL on K2R.

- copy down all the filter settings fromm K2T

- set K2R filter settings to the same. At this stage when you do a 'copy' you will find there will be a small offset between the two K2s typically between 50 - 250 Hz. In my case it increased a bit with frequency. Rather than just mess with CAL FIL and CAL PLL anymore I just used the IRT of K2R to take out the error - in fact I set it at about 200 Hz and that was fine.

If anyone can get things closer then please let me know.

Muting of K2R.

There is no provision for full muting of K2R. This was thought to be a disadvantage but in practice is not too bad. A mute could be arranged by putting K2R into transmit when K2T transmits. A transistor switch driven by K2R 8R (available on KIO2) could be used for this. However the present arrangement has the benefit that the out going signal can be monitored. In this case a little more attenuation in the off state would be useful.


Although it would be possible to copy more information between the K2s such as filter info. This has not found to be very useful. The use of the A VFOs only is quite neat in that it leaves a second memory free for other uses on each K2.

Other second receivers.

As structured this could probably work with any computer enabled receiver if the software commands were revised. It should work with Kenwood directly. None of this has been tried.

Antenna Switch

As there is enough spare capacity in the PIC to decode K2T frequency information into a 1 in 9 format which is used to drive an 8 bit open collector relay driver to switch antennas directly.. This could then drive external filters as well. I have a separate antenna switch which sits on the RS232 line to K2T and switches antennas as required.

Limitations or this is the fine print

All this information is given in good faith and no liability will be accepted by G3PJT in respect of any damage however caused. Except as noted this page has copyright G3PJT(2006). Last update 15 Nov 2006.