Ed_Meyer@mindlink.UUCP (Ed Meyer) (01/25/91)
While in a meeting, one technical "guru" declared that my statement was wrong when I said that a telephone was a form of transmission line. The "guru" across the table from me declared that, not only was I totally wrong, but that he and a senior engineering in the company had "proven" that a telephone line was not a transmission line nor could it be modelled as such. So, is a telephone line a form of transmission line or am I totally wrong as this "guru" declare? Ed Meyer
mcovingt@athena.cs.uga.edu (Michael A. Covington) (01/27/91)
In article <4574@mindlink.UUCP> Ed_Meyer@mindlink.UUCP (Ed Meyer) writes: >While in a meeting, one technical "guru" declared that my statement was wrong >when I said that a telephone was a form of transmission line. The "guru" >across the table from me declared that, not only was I totally wrong, but that >he and a senior engineering in the company had "proven" that a telephone line >was not a transmission line nor could it be modelled as such. So, is a >telephone line a form of transmission line or am I totally wrong as this "guru" >declare? > Ed Meyer It certainly is a transmission line; in fact, long phone lines have 88-mH inductors inserted into them periodically to keep the transmission line impedance closer to 600 ohms (which is the source impedance of a telephone set).
myers@hpfcdj.HP.COM (Bob Myers) (01/29/91)
>While in a meeting, one technical "guru" declared that my statement was wrong >when I said that a telephone was a form of transmission line. The "guru" >across the table from me declared that, not only was I totally wrong, but that >he and a senior engineering in the company had "proven" that a telephone line >was not a transmission line nor could it be modelled as such. So, is a >telephone line a form of transmission line or am I totally wrong as this "guru" >declare? Well, of COURSE it's a transmission line; after all, it carries an electrical signal from over THERE to over HERE, doesn't it? But what I suspect is meant here is that telephone transmission cannot be accurately modelled completely using the simple transmission line theory we all get in first-year communications classes ("Here's a load impedance, here's the source impedance, and here's the characteristic impedance of the line, and that's it."), and I suspect they're right about that. For one thing, the line length is horrendous and there's probably a fair amount of losses via radiation and coupling to other structures, and a raft of other other effects more exotic than what's typically covered in the simpler models. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.
larry@kitty.UUCP (Larry Lippman) (01/29/91)
In article <1991Jan26.163014.1986@athena.cs.uga.edu>, mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: > >While in a meeting, one technical "guru" declared that my statement was wrong > >when I said that a telephone was a form of transmission line. The "guru" > >declared that, not only was I totally wrong, but that he and a senior > >engineer in the company had "proven" that a telephone line was not a > >transmission line nor could it be modelled as such. So, is a telephone line > > a form of transmission line or am I totally wrong as this "guru" declared? With respect to the original author's article, this alleged "guru" doesn't seem to know much about the definition of a "transmission line"! > It certainly is a transmission line; in fact, long phone lines have > 88-mH inductors inserted into them periodically to keep the transmission > line impedance closer to 600 ohms (which is the source impedance of a > telephone set). A non-loaded transmission line of sufficient length does a nice job of establishing a characteristic impedance (Zo) at a given frequency *without* the use of series inductors (loading coils). The purpose of loading coils is to "counteract" the effect of distributed capacitance and reduce the insertion loss at frequencies below 3,000 Hz. A loaded transmission line has a comparatively flat curve of insertion loss -vs- frequency up to a cutoff frequency determined by the loading design and cable characteristcs. Such a cutoff frequency is usually between 3,000 and 3,500 Hz. It is important to understand that Zo of a telephone transmission varies with the frequency. The impedance specification commonly used in communication apparatus generally refers to a frequency of 1 kHz. It is also important to realize that Zo from an equipment design standpoint is merely a *compromise*, represents an engineering guideline, and is not some magic absolute figure. Non-loaded telephone transmission lines are generally considered to have a Zo of 600 ohms @ 1 kHz. More specifcally, 22 AWG cable has a Zo of around 560 ohms, and 24 AWG cable has a Zo of around 760 ohms. The finer the gauge, the higher the Zo (which should suprise no one). Loaded telephone transmission lines for voice frequency service are generally considered to have a Zo of 900 ohms @ 1 kHz with H-88 loading which uses 88 mH loading coils space every 6,000 feet. More specifcally, 22 AWG cable with H-88 loading has a Zo of around 1,050 ohms, and 24 AWG cable with H-88 loading has a Zo of around 1,080 ohms. Note the small difference in Zo between the same wire gauges loaded versus the larger difference unloaded. 26 AWG cable with H-88 loading has a Zo of around 1,170 ohms, which is why some subscriber loop apparatus, such as 2W/4W terminating sets, is also available in a 1,200 ohm impedance. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry