[Elecraft] KX3 - UHF emissions

Tue Jun 24 18:57:32 EDT 2014

Hi Wayne and others,

Thanks for the response.

Yes, I've tried the KX3 on internal power, with no cables, of any kind,
connected, and the sproggy can be faintly heard on my AOR Rx in SSB mode just
above the noise but too weak to open the squelch on AM when not wide open.

The KX3 is in the living room on the ground floor, and the scanners are using a
discone that's about 30ft away in the loft, which is where the sproggy is
getting in.

At this signal level, I don't think there I would have any problems.  However if
I connect the RS-232/USB cable to the KX3, the sproggy goes up to around S2.  If
I instead connect the I/Q cable, it goes up to about S9.  Connecting an external
key lifts the sproggy a little above the noise, but only by less than an
S-point.  No other cables seem to have much influence, but I've not tried
anything in the Acc2 port.  The I/Q output seems to be the biggest culprit by far.

The sproggy is very strong in the near field (same scanner on batteries with
whip) of the KX3, but much less so via the discone, until the I/Q cable is
connected. I'm almost always using the I/Q output from my KX3, so leaving this
disconnected isn't really an option for me. My I/Q cable has an isolation
transformer about a foot away from the KX3 as the electrons fly and a ferrite
clamped to it already.

I took one of the scanners out to my HF antenna (after reconnecting it to the
KX3), and there was no sign of the sproggy coming out that way.

It's a hellish RF environment here, it seems.  The whole house is effectively a
distributed shack. The scanners cause noise in each other too, or in SDR's that
steal their antenna feeds.  I am trying to minimise the 'friendly fire', as
there's not much I can do about the 'incoming' :P

Cheers,

Darren, G0HWW

On 24/06/14 21:09, Wayne Burdick wrote:
> Darren,
> 
> Have you tried running the radio from its internal battery pack, with the external supply disconnected? If that were to cut the emissions, you might consider ferrite cores on the power-supply leads (or other cables).
> 
> UHF/microwave emissions are a common side-effect of using really high-frequency digital MCUs, DSPs, and the like, which is becoming the norm. We use state-of-the-art multilayer PC boards, and passed all required tests. But to completely eliminate emissions of the type that can activate a sensitive receiver would require a extra pound of sheet metal (shielding), bonding wires, clips, etc. That seemed out of the scope of the product, which is suppose to be small and lightweight. Mainstream desktop radios can afford fully bonded packaging, so they have fewer emissions. 
> 
> Fortunately we've had very few complaints about KX3 emissions, but clearly your shack is an exception.
> 
> Let me know if the power-supply leads are responsible for some of what you're observing.
> 
> 73,
> Wayne
> N6KR
> 
> 
> 
> 
> On Jun 24, 2014, at 12:00 PM, Darren Long <darren.long at mac.com> wrote:
> 
>> Hi,
>>
>> I've noticed that my pair of scanners often receive strong spurii from my KX3's
>> receiver, in the UHF mil air band.  Whilst the spurii can be detected faintly
>> with no cables attached to the KX3, when I attach certain cables, the strength
>> of the spurii increase, most notably the I/Q cable, which causes (what passes
>> for) the S-meter on the scanner to fully deflect. Connecting an antenna to the
>> KX3 doesn't seem to have any effect.
>>
>> The other day I noticed that my scanner had stopped on a locally used UHF
>> channel that was exactly 100 times the frequency in the 80m band that I was
>> listening to.  Today I have noticed that listening on 7.08Mhz SSB, my scanners
>> stop on 389.4Mhz, which seems to be the 55th harmonic of the KX3's dial frequency.
>>
>> This is affecting my AOR-8600mk2 and IC-R20. Unfortunately, I've also been
>> developing a hybrid spectrum sensing scanner (called Onsense), that uses a
>> HackRF to sample the mil air band and tune in my AOR-8600mk2 to the strongest
>> signal detected.  I find that I am constantly having to add frequencies to
>> Onsense's blacklist as I tune my KX3.  This isn't a good situation to be in.
>>
>> I already have a ferrite on the I/Q cable, but that doesn't help.  When I enable
>> the 8kHz shift on the KX3, the previously detected spurii goes away.  Whether it
>> goes entirely or not, I don't know, it may just shift to another frequency that
>> isn't being scanned.  The RX Isolation filter is enabled.
>>
>> Is anyone else having similar problems? Is this normal for a KX3?  Is there
>> anything I might try to mitigate this?
>>
>> Cheers,
>>
>> Darren, G0HWW
>>
>>
>>
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> 