Re: Differential mode transmission lines with no groundplane

Jul 07, 2003 3 Replies
J


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 ?
>

Please answer one question - if, as you claim, you are a manager why do you get involved in the sharp end? Alternatively if you are an engineer, do you have some aversion to reading up on these subjects?


I believe what you are asking about is similar to the "phantom" circuits that used to be used in line communications.

Jim Backus OS/2 user bona fide replies to jimb-thecirclethingy-jita-dp-demon-dp-co-dp-uk or remove "NOT" from address remove dashes and make the obvious substitutions for valid email address
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T

Because I am the team leader and the technical expert at the same time. I work for an organisation that allows engineers to take management positions rather than relies on humanities types.

Information on differential mode circuits at RF frequencies is very sketchy and thinly spread across lots of publications. Differential mode is normally used for data communications or audio. There isn't even a standard characteristic impedance for differential mode circuits at RF let alone any specifically designed cables or connectors.

Telephone local loops operate in differential mode but are at audio frequencies.

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J

Nice work if you can get it - I've always avoided the cost account side of being a manager, which seems to make up a significant part of a manager's role in the bigger companies in Britain.

Jim Backus OS/2 user bona fide replies to jimb-thecirclethingy-jita-dp-demon-dp-co-dp-uk or remove "NOT" from address remove dashes and make the obvious substitutions for valid email address
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A

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:

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