Tech The virtual ground exists only for the differential mode, so the common mode needs a ground return path. Using multiple modes on transmission lines has been common practice on telephone circuits since about 1930, and the normal way to do it is to have two separate transmission lines, one circuit per line, and then to have another circuit which works between the common modes on each line. With good hybrid transformers, there is almost no crosstalk. This idea can be built up, so that with eight wires you can get seven circuits along them, and so on. Alan
The Technical Manager wrote:
> Is a two conductor transmission line such as the 300 ohm parallel flat
> type cable used for VHF antennas being used to convey a differential
> mode signal that is at a large (approximated as infinite) distance from
> a groundplane capable of conveying a common mode signal as well ? A
> common mode signal can only be conveyed if one or both of the current
> carrying conductors is in proximity to a grounded structure. A two
> conductor transmission line conveying a differential mode signal has a
> virtual groundplane between its two conductors but does a common mode
> signal view this virtual groundplane in the same way as it views a real
> groundplane and travels along the transmission line or is it suppressed
> from the lack of a real groundplane ?