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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
