K3 transceiver, available as either a kit or factory-built, has received some very favourable reviews
for its receiver performance particularly its close-in IMD and dynamic
range. Its functionality is largely controlled by firmware which
is being updated on a regular basis in response to user suggestions.
This service is both very responsive and free. Additional
hardware options are also available - a 6m preamp, a 2m
transverter that fits inside the K3 enclosure, and a
separate panoramic adaptor.
Construction of the kit
K3 serial 1198 kit was received on 4 July 2008, and was built 7 July,
with firmware version 2.10. It was straightforward. The stages of construction can
be seen in the photos.
| 1 - RF board||2 - Filters and panel fitted|
| 3 - Rear and side panels fitted||4 - Rear panel view|
|5 - Front panel fitted||6 - Transceiver completed|
|7 - K3 with 4m transverter decoding the locator of the GB3ANG beacon sent on cw|
Modifications since first build:-
Jan 2009 - KRX3 second receiver, KDVR3 digital voice recorder, AFMODKT,
6kHz filter on main rx, plus treatments for birdies, encoder noise, and
RFI. 14 Oct 2010 - VFO noise mod (to cure encoder noise - see
K3_vfo_noise_mod_Rev_C.pdf and MCU3.25 notes); Speaker
output mod to protect audio chip (see
AF_Output_Mod_Rev_C.pdf); K3 IF o/p mod to increase output for P3.
The P3 panadaptor
Ordered 5 Sep 2010, delivered 13 Oct 2010 (delayed at my request), built 14 Oct 2010.
[The UK Border agency charged me import duty. It took over 2 months to get a refund, and still waiting for explanation.]
My P3 is serial no 666 (my lucky number!!)
the P3, it is easy to spot activity away from the tuned channel, even
at very low signal levels (but need to reduce span and scale to get the
I have also used the P3 to make some
measurements (P3 displays signal level to <1dB, although accuracy is
only about +/-0.5dB). Measured the height gain achieved from 31
to 46 feet AGL at 3.5dB on 70MHz; and antenna polar diagram.NEW! Here are some measurements
of the transmit spectrum of the K3 using the P3.
| The display board|| ||A mostly empty box!|
Likes, dislikes, problems and solutions
I really do like:-
second receiver - for diversity reception using two antennas on the
same frequency. Can also be used to listen to two frequencies at
once on the same antenna - useful when listening to a split frequency
QSO, or in a contest when waiting for two stations to go QRZ, and you
want to call the first one. Good isolation between the
receivers - I measured best figure of 94dB (7MHz, ant unterminated) and
worst figure of 64dB (50MHz, terminated).
- The noise blanker which almost completely eliminates my power line noise.
retuning when switching from ssb to cw to maintain the same pitch and
tuning direction - this is magic on vhf! And hitting the keyer paddle
when on ssb sends cw immediately.
- Operation with transverters allows for power setting, frequency display, frequency offset adjustment.
- Synchronous AM reception.
(Audio peaking filter) - this applies a narrow band (~30Hz at -6dB)
audio filter and
increases the gain by ~9dB. This is very effective at making weak
signals easier to copy, but don't expect it to perform miracles! Others
have commented on its performance seeming to change with band
conditions - this is likely due to ringing of the filter caused by high
transient band noise.
Here's an example of signal processing in the K3. In this mp3 recording
(1.3MB,1min 24sec, recorded at 1739utc on 28 Feb 2011), you'll hear
first the GB3ANG beacon on 70MHz (tropo, 607km) in ssb bandwidth
(2.3kHz) with no noise blanking. There's bad power line noise. At
6.7seconds in, the noise blanker is switched on (set at dsp t2-6, IF
narrow 3). At 21.4secs in, the mode is switched to cw - the audio tone
remains the same, and the bandwidth changes to 900Hz. At 30.5secs
in, the APF is switched on until 56.4secs. At 61.5secs we revert
to usb mode and at 68.3secs, the noise blanker is turned off.
|Issue / dislike||Resolved by|
|Sprogs / birdies - especially on 28 and 50MHz||Using firmware 3.10 approach for fast tuning sprogs - mostly successful|
|Encoder noise / tuning hash / VFO noise - esp on 28.006 and 28.047||Check
tightness of all screws and cables. Problem reduced once 2nd rx
installed. VFO noise mod done, changed Config:VCO MD to SPI 2 to
change bus speed|
|Deaf rx on 6m - noise figure is 13dB. You can measure noise figure with a signal generator and this spreadsheet.|
|Using a preamp - I have my own but Elecraft sell one also|
|Unable to simply scan through all frequencies in memory||Can set memories to "channel hopping" mode, but it's rather clumsy|
"noisy" receiver problem - in my case a reduction in S/N on 6m with
slight reduction in RF gain control - problem evident with firmware
3.63, not there with 3.27. ||S/N resolved by performing RF gain
calibration (using K3 utility) and using factory defaults rather than
an external signal generator. I have also reduced the receiver
gain by 3dB by adjusting RX equalisation values.|
|Also noted with f/w 3.63 weird behaviour of S meter when RF gain backed off - it increases to S9+25 then falls back to S7.||The above gain calibration resolved the weird behaviour. |
reads up to 20dB too high at signal levels
above S9+10dB. Was ok with f/w 3.27, but now wrong again with
can't be cured by the s meter calibration procedure. Problem
outstanding (at 21 Dec 09). And it's still outstanding at 2 Jan 2011
with f/w 4.22|
receiver on 6m has spurious responses of the form nLO +/- IF. Only affects the sub receiver - the main has adequate filtering.||Problem
requires a band pass filter. Details
below. The subreceiver seems to have a reasonable noise
figure (I measured 6.4dB) and doesn't usually need a preamp.|
Sub-receiver spurious responses
to 50.150MHz, the local oscillator is on (50.15-8.215) ie 41.935MHz.
The image is at 33.72. Harmonics of the local oscillator produce
spurious responses at 75.655, 92.085, 117.59, 134.02 etc. My
measurements of these are:
|Frequency MHz||Relative response dB|
I used Elsie
to design a Cauer bandpass filter with parameters as follows:-
|Centre frequency||50.5MHz|| ||Frequency MHz||Response dB|
The circuit is as follows:
Measured results are as follows
|Original subrx response dB||BPF response dB||Subrx response with BPF dB|