tgg@otter.hpl.hp.com (Tom Gardner) (08/22/90)
Mini quiz: Most coax cables have a characteristic impedance of 50 ohms or 75 ohms. Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic impedance of 61.8 ohms? I'll give a piece of wedding cake to the first person with the correct answer.
dg9g@maxwell.acc.Virginia.EDU (David Guercio) (08/23/90)
>Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic >impedance of 61.8 ohms? Because if it was 0 ohms (i.e. short circuit) the signal would not cross the ocean. David Guercio
mark@mips.COM (Mark G. Johnson) (08/23/90)
>>Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have >>a characteristic impedance of 61.8 ohms? > Because its geometry (R2/R1) and its dielectric material (Er) [along with God and James Clerk Maxwell] dictate it. mj -- -- Mark Johnson MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086 (408) 524-8308 mark@mips.com {or ...!decwrl!mips!mark}
larry@kitty.UUCP (Larry Lippman) (08/23/90)
In article <1770009@otter.hpl.hp.com>, tgg@otter.hpl.hp.com (Tom Gardner) writes: > Most coax cables have a characteristic impedance of 50 ohms or 75 ohms. > > Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic > impedance of 61.8 ohms? Because it's just the way the *physical* design of the cable works out. The actual characteristic impedance of such a transmission line is immaterial, provided that suitable matching is achieved in connections to any active or passive circuitry. 50, 75 and 93 ohm coaxial cable are merely common values that have been standardized upon for many commercial and military applications. They are not "magic" numbers for any particular reason. Almost all "real" coaxial cables employing a polyethylene (or similar) dielectric result in some characteristic impedance between 25 and 125 ohms. Unless one creates a bizarre design - like 30 awg inner conductor surrounded by 1 inch radius of dielectric :-) - any *practicable* coaxial cable will develop a characteristic impedance in the above range. Bear in mind that characteristic impedance is solely determined by series resistance, dielectric conductance (i.e,. leakage resistance), capacitance and inductance, with the latter two being measured at a given reference frequency. The geometric *proportion* between inner conductor diameter, dielectric radius and shield diameter remain within a rather narrow range in *real* coaxial cables, hence the comparatively narrow range of possible characteristic impedance values. The 61.8 ohm value is pretty much middle-of-the-road, and should not be a surprise to anyone. I suspect that this impedance was the result of other design considerations, that the value was "reasonable", and that no effort was made to make it conform to standard values. Such "other" design considerations probably include: DC resistance (because of series repeater power considerations); dielectric breakdown (DC repeater power voltage to ground is probably 3 to 5 kV); shunt capacitance at working RF frequencies; a minimum inner conductor diameter chosen for mechanical strength considerations; a minimum dieletric cross section chosen to provide mechanical flexibility; etc. > I'll give a piece of wedding cake to the first person with the correct > answer. If you determine that I am the first person with the correct answer, I will settle for a bag of Tender Vittles sent to my cats. :-) 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 {utzoo, uunet}!/ \aerion!larry
corey@verdix.com (Corey Ashford) (08/23/90)
In article <1770009@otter.hpl.hp.com> tgg@otter.hpl.hp.com (Tom Gardner) writes: >Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic >impedance of 61.8 ohms? > It was a compromise, U.S. wanted a 50 Ohm Cable and Europe wanted 75. So they setteled on something that was non-standard for both so neither would get a better deal. 61.8 is about halfway inbetween and corresponded to a cable impedance that was manufacturable with available materials and tools. How's that for B.S.? - Corey "where's my cake" Ashford
tgg@otter.hpl.hp.com (Tom Gardner) (08/23/90)
|mark@mips.COM Mark G. Johnson |Because its geometry (R2/R1) and its dielectric material (Er) |[along with God and James Clerk Maxwell] dictate it. Yes, of course. So why not choose a different geometry? (oops, this could be construed as help :-)
ISW@cup.portal.com (Isaac S Wingfield) (08/24/90)
Larry Lippman writes: >50, 75 and 93 ohm coaxial cable are merely common values that have >been standardized upon for many commercial and military applications >They are not "magic" numbers for any particular reason. ^^^^^^^^^^^^^^ According to some things I read a few years ago, the two values 50 and 75 ohms are, in fact "magic". 75 Ohms answers the question "For a given outer diameter, what impedance provides the lowest attenuation per unit length"? This is good to know if you are going to distribute video or cable TV, for example. The fact that dipole antennas are also 75 ohms is coincidental. 50 (or 51, or 51.5) Ohms answers the question "For a given outer diameter, what impedance cable can handle the maximum amount of power?" The answer is really around 35 ohms, but the minimum is quite broad; 50 ohms is only fractionally poorer, and also matches vertical quarter-wave antennas, so that's why it was chosen. Reference: Schaum's Outline Series volume on transmission lines. There's also some fascinating information in there on frequency dispersion in ordinary wires at audio frequencies which should be of interest to people who believe that such effects are non-existent in speaker cable, for example. BTW, I'll bet that the Trans-Atlantic cable impedance was carefully chosen to provide an optimum set of parameters such as minimizing loss while providing sufficient breakdown to allow the voltages necessary to series feed the repeaters, or some such. Ma Bell doesn't work by guess or by golly. At least one of the TA cables used about 15KV at each end...(And you think YOU have ground bounce problems.) Isaac isw@cup.portal.com
tgg@otter.hpl.hp.com (Tom Gardner) (08/24/90)
|Larry Lippman at Recognition Research Corp., Clarence, NY | I suspect that this impedance was the result |of other design considerations, that the value was "reasonable", and that |no effort was made to make it conform to standard values. Yup. |Such "other" |design considerations probably include: DC resistance (because of series |repeater power considerations); dielectric breakdown (DC repeater power |voltage to ground is probably 3 to 5 kV); I _think_ they put +11kV on the centre conductor at one end and -11kV at the other. The shield was, of course, grounded. |shunt capacitance at working RF |frequencies; a minimum inner conductor diameter chosen for mechanical |strength considerations; Bingo. This thing has to support it's own weight as it's dropped to the bottom of the ocean. That's part of the answer settled. Incidentally, the core conductor was made of steel with a thin skin of copper. Total core diameter approx 1.25cm (hey, it's a decade since I last saw the thing!). |a minimum dieletric cross section chosen to |provide mechanical flexibility; etc. | |If you determine that I am the first person with the correct |answer, I will settle for a bag of Tender Vittles sent to my cats. :-) What are they? Sounds like a drink that you attach to the back of a steam locomotive (?!). Or some chocolate that is spread over a document defining how much you'll charge for doing soem work.
myers@hpfcdj.HP.COM (Bob Myers) (08/25/90)
> 50, 75 and 93 ohm coaxial cable are merely common values that have >been standardized upon for many commercial and military applications. They >are not "magic" numbers for any particular reason. Hmmmmmm. It always struck me as odd that certain of these numbers work very nicely in the good ol' quarter-wave matching section formula, when dealing with some very basic antenna types. Remember, the formula is Zm = SQRT (Zload x Zline) where Zload is the feedpoint impedance of the antenna, Zline is the char. impedance of the feedline to which you are matching, and Zm is the impedance of the matching section (one-quarter wave long). Now, a simple dipole has a feedpoint impedance of around 72-73 ohms, and - surprise! - RG-59 or any other "75 ohm" cable provides and excellent match. A quarter-wave vertical has a feedpoint impedance half that of a dipole - about 37 ohms - and applying the above formula says that to match it to 75-ohm line would require a 53-ohm matching section. (And does anyone remember when we called RG-8 fifty-TWO ohm line?) I don't know that any of this had anything to do with the selection of standard values - as Larry said, the characteristic impedance of any reasonable coax is going to be about in this range. But perhaps the selection wasn't *completely* random. As far as the 61.8 ohm question goes: I also note that the geometric mean of 75 and 50 is close to this number - it's 61.2 - but that's on the wrong side of the formula, as it would be the impedance required of the matching section, not the main line. (In other words, a 61.2 ohm section would match a 50 ohm load to a 75 ohm line, and vice versa.) Does this have anything at all to do with it? Did somebody just happen to have a few thousand miles of 61.8 ohm cable that they wanted to get rid of? :-) 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) (08/25/90)
In article <33145@cup.portal.com>, ISW@cup.portal.com (Isaac S Wingfield) writes: > >50, 75 and 93 ohm coaxial cable are merely common values that have > >been standardized upon for many commercial and military applications > >They are not "magic" numbers for any particular reason. > ^^^^^^^^^^^^^^ > According to some things I read a few years ago, the two values 50 and > 75 ohms are, in fact "magic". That was probably a misleading choice of words on my part, since it requires further explanation. However, this article should fill in a few details. > 75 Ohms answers the question "For a given outer diameter, what impedance > provides the lowest attenuation per unit length"? This is good to know if > you are going to distribute video or cable TV, for example. The fact > that dipole antennas are also 75 ohms is coincidental. Actually, the answer is 77 ohms (close enough to 75 ohms, though), but it applies *only* to a coaxial transmission line having AIR as the dielectric (permittivity of unity). However, polyethylene, as a typical dielectric, has a permittivity of around 2.4. Since characteristic impedance of a coaxial transmission line at a permittivity of unity is reduced by the reciprocal of the square root of permittivity, the characteristic impedance of a coaxial cable having a polyethylene dielectric and having minimum attenuation per unit length is much *less* than 77 ohms. > 50 (or 51, or 51.5) Ohms answers the question "For a given outer > diameter, what impedance cable can handle the maximum amount of power?" > The answer is really around 35 ohms, but the minimum is quite broad; > 50 ohms is only fractionally poorer, and also matches vertical > quarter-wave antennas, so that's why it was chosen. Maximum power transmission with an AIR dielectric is approximately 30 ohms. The same condition for a polyethylene dielectric is obviously less than 30 ohms. Maximum voltage transmission with an AIR dielectric is approximately 60 ohms. The same condition for a polyethylene dielectric is obviously less than 60 ohms. For this condition, I seem to recall that the ratio of coaxial outer diameter to inner diameter is e (2.718). > Reference: Schaum's Outline Series volume on transmission lines. A word of caution on the topic of coaxial transmission line calculations... Many reference books present explanations, equations and data based upon AIR as the coaxial dielectric. Such data may not be applicable when polyethylene or other plastic is used as the dielectric, as is the case for common coaxial cables. I have learned this, The Hard Way. :-) 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 {utzoo, uunet}!/ \aerion!larry
durham@w2xo.PGH.PA.US (Jim Durham) (08/27/90)
In article <1770009@otter.hpl.hp.com> tgg@otter.hpl.hp.com (Tom Gardner) writes: >Mini quiz: > >Most coax cables have a characteristic impedance of 50 ohms or 75 ohms. > >Why does TAT-7 (Trans-Atlantic Telephone cable number 7) have a characteristic >impedance of 61.8 ohms? > >I'll give a piece of wedding cake to the first person with the correct >answer. Well, I dunno about why, but 61.8 ohm cable would sure be handy. A 1/4 wave section of this would match 75 to 50 ohms very nicely for those of use with lots of 75 ohm aluminum hard line available and 50 ohm antennas. Where can we get some? -Jim -- Jim Durham internet: durham@w2xo.pgh.pa.us ham packet radio: jcd@cs.pitt.edu w2xo@w2xo.#wpa.pa.usa.na
del@thrush.mlb.semi.harris.com (Don Lewis) (09/12/90)
In article <4008@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: > Despite the existance and use of transistors during the 1950's, >the TAT-1 undersea repeaters all used vacuum tubes. If memory serves me >correctly, these vacuum tubes had flexible stranded wire leads which were >soldered directly into the circuit - with no connectors being used. How did they produce a vacuum tight seal with stranded leads, or were the leads connected to some sort of solid wire feedthrough? -- Don "Truck" Lewis Harris Semiconductor Internet: del@mlb.semi.harris.com PO Box 883 MS 62A-028 Phone: (407) 729-5205 Melbourne, FL 32901
larry@kitty.UUCP (Larry Lippman) (09/12/90)
In article <1990Sep12.035908.6083@mlb.semi.harris.com>, del@thrush.mlb.semi.harris.com (Don Lewis) writes: > > Despite the existance and use of transistors during the 1950's, > >the TAT-1 undersea repeaters all used vacuum tubes. If memory serves me > >correctly, these vacuum tubes had flexible stranded wire leads which were > >soldered directly into the circuit - with no connectors being used. > > How did they produce a vacuum tight seal with stranded leads, or were the > leads connected to some sort of solid wire feedthrough? I believe the stranded leads were welded to solid metal posts which actually penetrated the glass envelope. It is my understanding that stranded leads were used to isolate the vacuum tube, and in particular its glass-to-metal seal area, from external stress and vibration. Incidentally, the glass-to-metal seal area of a vacuum tube usually uses a glass of different composition than that of the envelope itself. Such glass, which is usually borosilicate in nature, is specially formulated to match the thermal expansion characteristics of the metal alloys used in the feedthrough. Such metal alloys are also of special composition, with two common tradenames being Dumet and Kovar. 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 {utzoo, uunet}!/ \aerion!larry
whit@milton.u.washington.edu (John Whitmore) (09/13/90)
In article <4022@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: >In article <1990Sep12.035908.6083@mlb.semi.harris.com>, del@thrush.mlb.semi.ha\ rris.com (Don Lewis) writes: >> > ... these vacuum tubes had flexible stranded wire leads which were >> >soldered directly into the circuit - with no connectors being used. >> >> How did they produce a vacuum tight seal with stranded leads > > I believe the stranded leads were welded to solid metal posts >which actually penetrated the glass envelope. > > Incidentally, the glass-to-metal seal area of a vacuum tube >usually uses a glass of different composition than that of the envelope >itself. Such glass, which is usually borosilicate in nature, is specially >formulated to match the thermal expansion characteristics of > [special metal alloys] with two common tradenames being Dumet and Kovar. I would like to add that the thermal stresses, while minimized by material selection, still often kill vacuum tubes. Some feedthroughs use a thin metal tube (instead of a rod) so that the glass/metal interface has less resistance to expansion/contraction. We use such feedthroughs with thermocouple wire (plugging one end with solder), so that our thermocouple alloy remains constant and our temperature measurements accurate across the vacuum boundary. By 1963, there were good quality ceramic feedthroughs (see, for instance, Nuvistor-type vacuum tubes). According to our glassblower (who still custom-builds the odd tube), these are superior to the soft-glass and hard-glass seals. Possibly the tubes for the transatlantic cable of 1963 weren't glass at all, but metal with ceramic/metal feedthroughs. I know Nuvistors were available in 1963; were the ceramic/metal tubes considered reliable then? I am known for my brilliance, John Whitmore by those who do not know me well.