[Elecraft] Microham Microkeyer II, RFI in mike with my K3

[Jim Brown]

On Sun,2/14/2016 11:20 PM, Bernhard.Horst at bmw.de wrote:
> just an update..the Microham Microkeyer II has definitely a problem with RF!

Without diving into the details, I helped W6OAT chase down RFI issues in 
his station. He had a MicroHam box with an "interface" to the radio that 
was a jumble of wires. Virtually NO attention had been paid to the 
fundamental concept of a transmission line in the interface between it 
and the radio. While that is acceptable at  baseband (audio), it is 
monumentally foolish when RF is present (i.e. an antenna on the roof).

Current flows in loops, whether at DC or at RF, and the loops form both 
antennas and magnetic loops whose coupling is proportional to the loop 
area. In the near field, magnetic coupling dominates, so the loop area 
can be a VERY big deal.  EVERY signal path between devices must be 
treated as a transmission line of RFI is a potential issue. That means 
EVERY signal path should be a twisted pair or a coaxial pair, so that 
the path forms a transmission line. Any interface that does not do this 
is an invitation to RFI if the antenna is close enough, or if the power 
is high enough, or both.

This is NOT an indictment of Microham -- at a CQP expedition a few years 
ago, a team member showed up with an interface betweeen is radio and his 
amp to key the amp that was an RCA to RCA cable with a single wire, no 
shield. Chassis to chassis was depended on for the return. That works 
fine at DC to key the amp, but the resulting current loop area gives 
that circuit VERY STRONG coupling to an RF field, both as a magnetic 
circuit and as an antenna.

Noted RFI guru Henry Ott talks about understanding where ALL the current 
flows, which is revealed by "the hidden schematic lurking behind the 
ground symbol" that causes most issues with RFI and crosstalk.

73, Jim K9YC