28 June 2014

KX3 UHF Emissions

I've written his blog post primarily as a place to store my screen shots related to a topic that I initiated on the Elecraft mailing list about the UHF emissions I have noticed emanating from my KX3. The full topic thread can be seen here

These screen-shots have been taken using osmocom_fft, with my USRP-B100 and WBX daughterboard connected the discone antenna in my loft.  My preamp is set for 0dB gain, the USRP gain is set to a total of 43dB which is about as high as I tend to set it in order to avoid intermod.

The first few pictures are with my KX3 in its normal position with its normal cable connections, in its normal configuration, set to 7.080Mhz.  The cables are smothered in #61 ferrite.

Firstly a 8Mhz spectrum capture  (below) shows a fairly high noise floor due to the bandwidth and a faint trace of the sproggy (in the centre), but some other signals towards the upper end of the spectrum, probably TETRA stuff or similar.

Next, the same scenario, but with 125kHz sample-rate/bandwidth to drop the noise floor.

Note that averaging is enabled in all of these plots.  The next one had peak hold enabled so that we can see the effect of sweeping the VFO on the KX3.
Just a quick recap, the above pics shows the results of my attempts so far to suppress the UHF emissions externally.  Perhaps I could do better.

The next picture is with the KX3 disconnected from everything, but sat in the same location.

The next picture is with the KX3 relocated to the kitchen, about 30 ft away from it's operating position.  Both of these locations are roughly equidistant from the discone antenna in the loft, which is about 30ft away.

This one is with the KX3 relocated upstairs, to within 8ft of the discone antenna. There are still no cables attached.
I then moved the KX3 back to its normal operating position, the same scenario as the 4th picture.
I then insert the right angled 2.5mm to in-line 3.5mm stereo adapter cable, which is about 6 inches in length when straight.

This time, I have two turns (they just fit) of the 2.5mm to 3.5mm adapter cable  around the mix #61 ferrite toroid. The peak hold is left from the last shot, so we can see the improvement.
Finally, the KX3 is restored to its normal state of connectivity, with antenna,  power, CAT via Acc1, I/Q, audio out and in cables attached.  I have 3 toroidal chokes with various numbers of turns on the I/Q cable, and a bunch of snap-ons with a couple of turns and the remaining toroidal chokes smothering the other cables. This time, the sample-rate is increased to 1MHz, and I have enabled the 8kHz Rx Shift, which moves the sprog down 55*8kHz, to the left hand end of the spectrum.  An unrelated signal appears to the right of the relocated sprog and a peak trace of another unrelated signal appears near the right hand edge.


So far, I've found spurs on (some of) the 10th, 20th, 40th, 55th, 75th and 100th harmonics of various VFO frequencies. With my VFO on 21.25Mhz, I'm getting an S-band spur on the 100th harmonic at 2.125GHz.


M1KTA said...

Hi.... Own 2 early KX3 #1082 and #2334 (no BNC antenna mod) and wasn't even aware these were creating these UHF emissions. Please correct my misunderstanding if there is one... the 'cure' seems to be apply common mode chokes using -61 on all the cables leaving the KX3 if I read the blog correctly. Probably by accident I currently run with most cables running through TDK clip on using -61 already so maybe I have been lucky.

I briefly looked at a USB SDR RX on the c360-400MHz region but couldn't see the peaks being mentioned coming from either KX3. This effect them all?



g0hww said...

Hi Dom,

Wayne from Elecraft said that due to the design constraints and commercial imperatives "some radiation products may remain in the VHF through microwave region that cannot be easily eliminated." I had hoped that Elecraft might suggest that my KX3 is unusual in this respect and that it might have a problem that could be rectified, but they have never hinted at that possibility. It seems that the emissions are acceptable to Elecraft. I don't know if they are 'normal' for KX3s in general.

David, from Elecraft said this:
"- There will be some emissions when cables are connected to utilize the various signals available on the KX3. This is expected with the Elecraft cables and cabinet.

- We are investigating ways to reduce emissions but, at this time, there are no solutions available from Elecraft.

- As workarounds, here are some suggestions to mitigate the effects:
- Terminate cable jackets to the chassis. This will involve fabricating interconnect cables to do so.
- To further reduce emissions, filtering of the cables may be possible.
- Ensure that all metal contact points on the chassis are clean and well secured."

My main problem with these emissions (other than the simple fact that I wished the KX3 was cleaner in this regard) is that they cause problems for some spectrum sensing software that I occasionally run, scanning the UHF mil-air band. The spectrum sensing application has a really hard time anyway, due to the intermod problems I have but the KX3 emissions really bogged it down with false positives from those harmonic spurii when I was band hopping and QSYing on HF.

I managed to achieve about 20dB suppression for the spurii in the UHF mil-air band with the mix #61 ferrites. That seemed to mitigate the issues I had with UHF spectrum-sensing to a tolerable degree, but I'm not all that happy with the notion that these emissions should acceptable both to the manufacturer and the consumer.

Out of curiosity, I just decided to see if I could find a sprog from the KX3 related to whatever frequency it was left on. I turned it on and found it sat on the 20m JT-65 frequency, 14.076MHz. I tried looking for a spur on the 100th harmonic, at 1.4076GHz, with my FCD-Pro+ and found it immediately peaking 26dB above the noise floor, receiving it through a discone antenna in my loft, as in the blog post. It is nearly 50dB above the noise floor using the same receiver and a collapsed telescopic whip in the near field. This is with all the #61 ferrite still applied.

Thanks for your interest in this matter.
Darren, G0HWW