Elecraft KX3 the Magic Radio

Software Defined, User Assembled, an Adventure

The KX3 shown here in position when used as my base station rig.  

Just as the SDR design has changed the way the rig works, the new compact form factor has changed the way the operating position looks.  This ain't your Grandpa's Collins S line any more.  The little rig is attached to the following station components a Sennheiser E845 mic on a boom with Behringer 502 mixer (also used for Echolink and Skype, and computer recording), PTT switch installed in a wooden Easter egg, Vibroplex Bug, 100 Watt RM Italy amplifier, Station Computer (home built and under the desk running XP) and 30 Amp Astron power supply.

I chose to build (assemble), save $100, and have fun.  I have built just about everything Elecraft has except the K3 kits, and they are all absolutely first class.  Since all the boards are pre-built and tested, all that is left is assembly and a little alignment.  It took me about 3 hours, not counting the time spent on my hands and knees looking for a spacer and screw.  There are several videos on YouTube showing construction.  Of course, it's software defined, and in constant flux.  After just one month of building the rig, it have had its firmware updated 3 times.  With an innovative and successful company behind it, it should continue to advance and improve.  

To try to describe all the features and quality if this radio is beyond the scope of this webpage, but I will hit a few high spots.  CW and digital auto spot takes control of the VFO and automatically tunes the signal to the middle of your pass band.  It boasts the following: spatial stereo, a temperature mapped oscillator, current, PA temp, OSC temp, supply and battery voltage read outs, RX signal strength in dBV, and of course a built in 2 tone test. I would not have it any other way.  The crystal and DSP filtering work so well that the rig is at the top of the Sherwood Engineering receiver test specs, yep, unless you have a Hilberling PT 8000A, the KX3 is better than what you're using. See for yourself at   http://www.sherweng.com/table.html


Connections  LTR: Power, USB, Bug, Station Speakers, Mic and PTT, RF in/out is on the other end.

The rig also has internal batteries and an automatic antenna tuner, neither of which are used in base station operation.


The step from 12 to 100 Watts.

The government and the CBers pretty much eliminated manufacture and sale of amplifiers for QRP radios 30 years ago.  However the law did not stop the proliferation of high power CB amps, but ended commercial QRP amps for Hams .  This has recently changed and Ten Tec is making a 100 Watt QRP amp now for its new Argonaut rig.  I tried one and was not happy with it.  The HLA 150 cost less than 1/2 as much as the Ten Tec 418, and works much better.

Since the KX3's max power is 12 Watts and the radio gods are mistreating us,  I use an RM Italy HLA (Ham Linear Amplifier) 150, Plus (fan cooled), sitting on an MFJ tuner sitting on a shelf over the computer screen.  Both have been modified, the tuner for quality and use and the amp for use (it's quality is very good).  The amp has had a switch added to turn off the cooling fans, which are normally on low setting all the time.  In SSB and CW modes just using 100 Watts, the amp doesn't need the fans at all (you might consider the cheaper fan less model). However if you want to run RTTY or any other full duty cycle mode, the fan is needed.  I like the silence when possible...a signal to noise ratio improvement for the shack. The thermometer was added to see what the limits on the internal fan controller were. Besides it's a good place to keep it.  I can't say it makes it look cool, because the amp is butt ugly, so much for Italian radio design.  They do much better with cars and actresses. 

This amp has a big problem. First of all, it is not FCC type certified, probably because it is over rated.  They claim it to be 150 Watts (which is possible, even with just 2 to 5 Watts drive), but it's IMD output is unacceptable at this level.  The low pass filters work very well, and the amp puts out a good  clean signal with a 13 to 15V power supply at the 100 Watts or less.  I have added a 4.4 Farad capacitor (thank you mobile audio nuts) to the power input to further guarantee the voltage stability.  It can be seen reading 13.6V in the background of the pix.  The MFJ tuner has had a bypass switch added and some basic internal repair and soldering like most of their products need to produce a competent piece of equipment.  It is usually in bypass mode.  I must limit my power out put to 100 Watts so I don't over drive my SGC 230 auto tuners.

Many Hams are leery of using an amplifier that is not FCC certified, however, certification is not a problem for the Ham operator, its a problem for the importer and retailer.  Since Hams are licensed to build and use their own amplifiers, they can certainly use one that has not been certified.  The Ham is responsible for the transmitted signal that radiates from his antenna.  I have tested my radiated signal with this amp at the 100 Watt level and find that the first order IMD is down over 40dB, noticeably better than my certified Yaesu FT 857.  The HLA-150 can be purchased here: http://www.dnjradio.com/DNJRadio_Catalog_2011.pdf


The KX3 display showing direct copy of digital modes.  This one was a PSK31 QSO scrolling by.

It is possible to send and receive the digital modes, RTTY, PSK31, and CW with the built in decoder.  For transmit you can use the attached CW paddles for all modes.  In RTTY, and PSK, you send with the paddles also. When transmitting, you hear the CW side tone and the RTTY or PSK tones together.  It makes it possible to use these modes in the field with nothing except the rig, it's internal batteries and tuner, and some wire.  No computer needed!!

KX3 Utility program in terminal mode

For computer use, this terminal window gives full control with just a USB connection. No sound card needed.  Many programmable memories are available as well as keyboard use.  The same utility program is used for updating the firmware, testing, configuring, and calibrating.


Two modifications.

The angle of the rig as presented by the rear feet was a bit of a problem.  This was realized early on and the feet were lengthened to increase the angle to better see the display, however, that put the attached CW paddles too close to the table.  I shortened them to make the paddle height right for field use, and added "feet extensions made from 1/2 in. automotive heater hose for the right presentation angle in the base station mode.  These are easily removed by squeezing them and sliding them off.



The heat sink as originally set up was not good enough for full duty cycle modes like RTTY and PSK31 and AM.  It was gel coated and no heat compound was uses.  The key down performance was very poor at less than 2 minutes, and it recovered slowly (the other duty cycle).  With the addition of a 1/8th in. transfer bar, heat compound, and removal of the gel coat, the performance was improved to about 5 min.  When the full size 3/32 heat sink was added, it would "steady state" at 10 watts.  The shape of the transfer bar made the cooling uniform on both transistors, and allowed air to circulate on both sides of the sink.  Don't worry about voiding your warranty making this mod.  See Wayne's response to the problem below:



Recordings and spectrograms with the KX3

The KX3 is a great platform for recording off the air audio and making spectrograms of other Ham's transmitted audio.  The pass band of the Kx3 is perfectly flat as seen below.

The trace with the pass band set from 0 to 3.5 kc. reading white noise off of a dead band (very easy to do these days).  The radio will transmit and receive up to 4 kc band width.

The audio from the KX3 is fed into my computer which is running Hamalyzer and Audacity.  They have had the inputs balanced, and a spectrogram and a audio recording are made at the same time.  The trace is saved as a jpeg and the audio is saved as a MP3 file.  The trace can be attached to an email and sent to the Ham along with the MP3 of their audio.  The MP3 can also be re-transmitted on the spot back through the KX3 very faithfully.  The above trace is off the air on 17 meters.  The high and low frequencies are marked.  The part of the trace above 2.6 kc is band noise since the signal was only about 15 dB above the noise level.




The KX3 has been reviewed in the November issue of CQ and the December 2012 QST magazine.

Disclaimer: This web page has been formatted so it looks great on Sam's computer.