Your first assumption is incorrect: the transmission line only operates as a transmission line if it has a source at one end and a load at the other end. If the source, line and load are all the same impedance, maximum energy is transferred to the load.
Assuming all impedances are matched, the load is near the source and some unknown length of line is also connected to the load, that line could act in a number of ways: depending on its length, the frequency of the applied signal and whether or not the line is open or shorted, it would act as a series or parallel LC circuit connected to the load. If this extra line is a single line and not paired with a return, it will likely operate as an antenna. If that line were the same impedance as the load, half the energy from the source would go to the "antenna" and half to the load... Mike Hopkins [email protected] > -----Original Message----- > From: Allen Tudor [SMTP:[email protected]] > Sent: Wednesday, May 12, 1999 5:31 PM > To: [email protected] > Subject: Transmission Line Theory > > > Greetings group, > > I am trying to draw a parallel between transmission line theory and > radiated emissions. > > From what I understand, a transmission line can be terminated at the > source or the load with an impedance that equals the characteristic > impedance of the transmission line itself. > > With this in mind, consider this scenario. A printed circuit card drives > a clock signal down a trace on a backplane. The length of the backplane > trace is long enough to be considered a transmission line. The driver on > the printed circuit card is located within ½ inch of the edge connector > (mating with the backplane) and is terminated with an impedance equal to > the characteristic impedance of the backplane trace. However, the > backplane trace is open ended (there is nothing connected to the end of > the trace). Transmission line theory says the signal integrity will be > maintained in this case. > > Now for the questions: > > (1) How much, if any, of the energy will be radiated into free space when > it gets to the end of the open transmission line? To me, this looks like > a monopole antenna. I don't have a very good understanding of antenna > theory, so this could very well be an invalid assumption. > > (2) If radiation does take place as stated above in question (1), which is > better for reducing the radiation, termination at the source or > termination at the load of the transmission line, or does it matter? > > (3) If the characteristic impedance of the trace on the printed circuit > card differs from the characteristic impedance of the trace on the > backplane, how is this handled? Is a termination needed at each end in > this case? > > I look forward to your responses. > > Thanks. > > Allen Tudor, Compliance Engineer > PairGain Technologies tel: (919)875-3382 > 2431-153 Spring Forest Rd. fax: (919)876-1817 > Raleigh, NC 27615 email: > [email protected] > > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
