[TenTec] Trading radios

[Mike Schatzberg]

On Jun 2, 2014, at 12:28 AM, Jim Brown <k9yc at audiosystemsgroup.com> wrote:

> SO - for minimum crosstalk, a very robust shield is critically important, and this is just as true at HF as it is at UHF.

In theory, it is obvious that the more radiation you have from a cable, the stronger the EMF around the cable, and the potential would appear to be there for crosstalk from cable to cable.  This subject has been studied since about 1937 in Princeton, NJ, where ATT was concerned about crosstalk in long lengths of coax used in repeater applications at frequencies up to about 200 Khz.  When I was in university, those studies formed the sum total knowledge from research conducted for telecommunications.

Now, as you know, there are three three elements which must be considered in crosstalk measurements, near end crosstalk, far end crosstalk, and what you would call transfer crosstalk.  What I know is the length of the cables and their proximity to one another, is a major factor, as is the sheath (in telephone cable parlance) which covers them, and whether they are terminated with the cable characteristic impedance.  For purposes of research, RF signals were sent in opposite directions, to simulate the use of repeaters in the system.  

The simple cable is the twinax, that is, two coaxial cables side by side, carried within a larger cable.  We know from measurement that there is no far end crosstalk because of the opposing direction of travel of the waves.  The near end cross talk is is generally low with such a twinax, but the total crosstalk can be effected by the surrounding conductive sheath and its transfer crosstalk.

It is interesting to note, that after all these years, if there is a problem with crosstalk in high frequency coax, it is never measured directly that I am aware of.  The runs are not miles long, particularly those in high reliability applications like aircraft and spacecraft, despite the cable bundles being tightly grouped.  Crosstalk measurements are still required for toll and local area exchange cables which use twisted pairs, of varying lay lengths, to avoid crosstalk.  The military specifications do not require crosstalk measurements on coax, but rather the cables are swept through out their useful frequency range and checked for SRL.  This is a modern day addition to the military grade coax, which does find its way into amateur radio transmission quite often.

I had the task to design the first automated testing facility for toll and area exchange cables.  Up to that time in the late sixties, all these measurements where taken by hand, in a limited sampling plan.  Included in the primary transmission parameter testing was far and near end crosstalk measurement.  We terminated both ends of the cable with fixtures we had built by Clinton Instruments, the instruments were automatic leveling devices built by the Siemens Corporation in Germany, and we used a PDP 11 DEC computer to drive the system.  Being able to record so much data, would lead to much improved designs, and shorter development times for new cables.

Although our system took millions of measurements of crosstalk on twisted pairs, I must say I never ever took even one measurement of crosstalk on twinax, or quads.  That was a research activity way back when, and I have no experience with it.  

73,

Mike
W2AJI
Former US Diplomat to the ISO and Subcommittee Chairman