[Elecraft] Twisted Pair RS232 leads, particularly [K2]

[Jim Brown]

On 6/1/2013 3:05 AM, David Woolley (E.L) wrote:
> However, how does twisted pair really help if the return in the 
> twisted pair is in parallel with that for the other signal lines and 
> any functional earth bonding strap between the two pieces of equipment? 

Twisting strongly rejects magnetic coupling from an interfering source 
to the differential circuit.  In the near field of a current source, the 
magnetic field is strongly dominant. Twisting also provides some 
rejection (at RF) of an electromagnetic field in the differential 
circuit.  Most end-fed antennas have a strong current peak at the 
feedpoint; if it's end fed wire that ends in the shack, there will be a 
very strong magnetic field.  I encountered this with a K2 feeding an end 
fed wire on 80 and 160 that shut down my computer at about 10 watts!  
Changing that cable to unshielded twisted pairs, and terminating the 
return of each pair to the shell of the DB9 at both ends, allowed me to 
run my Titan 425 power amp at 1kW. The antenna and the serial cable were 
within about 1.5 m of each other.  As part of my tests of that solution, 
I ran legal limit into that antenna on all bands up through 10M.  No 
cable shield was required through 17M, but shielding was needed at 15M 
and above. BTW -- as part of an EMC workshop I led for Audio and Video 
professionals, I demonstrated that unshielded twisted pair and shielded 
twisted pair were equivalent in their ability to reject RFI in balanced 
microphone circuits all the way up to the 900 MHz and 1.6 GHz bands used 
for cell phones in the US.  Neither cable was perfect, both had slight 
vulnerability, but neither was clearly superior. For the demonstration, 
I keyed a ham talkie on 2M, 220 MHz, and 440 MHz, a Nextel phone (TMDA 
900 MHz) and a borrowed 1.6 GHz phone that also ran TMDA, moving  the 
interference source near the vulnerable equipment and moving it along 
the cable for several wavelengths.

Does bonding help? Absolutely!  Sadly, proper bonding of equipment is 
rare -- hams are fearful of creating "ground loops," an entirely 
fictional boogie-man.  I have long advocated (written and lectured) for 
short fat copper bonding from chassis to chassis of every piece of gear 
in the shack, and from chassis to chassis of every piece of gear that 
has any interconnection between them, and from some common point of all 
of that to all other grounds in a premises.

73, Jim K9YC