[Elecraft] K2 cellphone interference

[David Woolley (E.L)]

Don Wilhelm wrote:
> 
> I have heard another report of this noise, but no analysis of the 
> effect.  Yes, the fact that the level does not depend on your audio gain 
> setting indicates that it is confined to the AF Amplifier.

Yes, the K2 does have rather poor uHF EMC properties.  You can slightly 
pull one of the VFO or BFO with a PMR 446 (500mW handheld) close to the 
left of the case.  It also generates clicks on keying, especially close 
to the speaker grill.  (The same rig crashed my PC held at similar 
distances from the keyboard!) Similar powers on 70cm also have an effect.

Part of the problem is that the seams on the case are not continuous. 
This was compounded on #6123 because not all the screw holes for the 2D 
connectors had masking tape.  That meant that the paint overspray 
results in quite a high resistance between the blocks and the panels.  I 
fixed this by sanding off the paint around all the blocks.  Actually I 
can still pull the frequency (BFO v VFO on the 7MHz birdy) by about 5Hz, 
but I need to hold the transmitter in front of front panel, with the 
antenna parallel and horizontal.

Improving those connections means I can't, now, reproduce any frequency 
pulling, but there is still some audio pickup, which does seem to be 
post volume control.  (The failure to reproduce the frequency shift may 
be partly due to having a less sensitive test configuration, though.

Real high quality EMC design would use some form of conductive gasket, 
or at least finger stock, but they are expensive in small quantities, 
difficult to add and, in some cases, compromise the ability to remove 
covers (e.g. conductive tape).  They would also need more extensive 
removal of overspray.

Incidentally, this thread has shades of the pin 1 problem thread, 
because the K2 case violates the Faraday cage principle at UHF.


> source of the interference.  Of course, the real problem is the phone - 
> if it radiates that much, I wonder just how well it meets other 
> radiation requirements.

GSM interference is generally an unintentional receiver issue, not a 
spurious transmissions one.  The TDM envelope gets AM detected by some 
non-linear element in the circuit (often a semiconductor junction).
-- 
David Woolley
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