900 ohm line reason? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2025, 05:09:48 am
content: So if you do the equation for parallel wires you will see that it gets absurd above 1k ohm.Old telephone lines were 900 ohms on the pole. Is there a reason why they chose this impedance, or was it just a consequence of practical mechanical design of telephone line feeders?

author: Berni
author_rank: Super Contributor
timestamp: April 18, 2025, 05:41:44 am
content: It is sometimes specified 600 Ohm too.Basically this mostly comes from the early days of telecommunication technology. The around 600 or 900 Ohm is what you get for the common dimensions of wires strung on utility poles from the day. This goes even back to the days of the telegraph.These days we don't string separate conductors onto poles anymore. Instead we use conductors jacketed in outdoor rated insulation and twisted together into twister pair cable. This allows for many twisted pairs to be ran along just a single cable bundle and strung onto a utility pole as just one cable. Much less labor intensive than hailing up a separate wire for each line one by one. As a result the impedance is also no longer 900 Ohm because the dimensions of a 900 Ohm twisted pair cable would be impractical. So modern lines are now somewhere around 100 Ohm. Don't count on it too much since telephone twisted pair is the crappiest twisted pair you can buy, it is only made to carry a few KHz of voice after all.

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2025, 01:17:14 pm
content: Hmm I was just curious if there was some electrical benefit to running at 900 ohms. Like something with sizing the transformers/reactors/etc. It sounds like there is not

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 02:29:22 pm
content: Long “trunk” lines often inserted discrete inductors periodically in series to make a transmission line out of two wires. The spacing between inductors had to be shorter than the signal wavelength (audio or frequency multiplexed audio) at the speed of the line. This was invented by Heaviside.A Wikipedia article on “loading coils” shows an assembly with multiple 175 mH coils (on a common core) from an AT&T trunk line, used every 6000 feet from New York to Chicago in the 1920s.The loading coils and existing capacitances resulted in a characteristic impedance in the 600 to 900 ohm ballpark.I like tha story of Heaviside’s invention: everyone knew that shunt capacitance or series inductance would slow down telegraph signals: he combined the two to solve that problem. Apparently, AT&T was quick to adopt this to “long lines” in the US.

author: mawyatt
author_rank: Super Contributor
timestamp: April 18, 2025, 03:18:09 pm
content: Heaviside gets little recognition for his work. Recall reading long ago he created algebraic solutions to Differential Equations to help solve transmission line problems, because they were too hard. He got the correct answers but couldn't "prove" his algebraic methods to the "purest", so no credit!!Now credit goes to LaPlace who apparently had nothing to do with these algebraic DE solutions we call Laplace Transforms!! We still refer to the Step Function as the Heaviside Function just to honor his remarkable achievements!!Best

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2025, 03:33:59 pm
content: did anyone ever see a feeder that tries to beat 1kohm? Playing with the equation kind of lead me to a asymptote. Those lines need to get very very thin to have higher impedanceIf you use a dielectric you can. But for air line it really hits a limitNone the less, even on a special structure, are there any applications to high impedance twin leads?Like a pipe filled with Calcium Copper Titanate.I.e. lets say you took mineral insulated cable, and replaced the MgO with Calcium Copper Titanate. Then it should be like a shielded twin lead with a high impedance. Just MgO has a dielectric constant of approximately 10, making this interesting.

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 03:55:08 pm
content: In a transmission line, for given dimensions, increasing the dielectric constant of the insulationdecreasesthe characteristic impedance.There are coax cables where the center conductor is a helix (possibly loaded with a magnetic material) to increase theinductanceper meter. They are used for high characteristic impedance and slow time per meter for delay lines.“Ladder” twin-wire transmission lines (used for RF antenna feed) with two parallel wires spaced by discrete insulators and relatively wide spacing are typically 600 ohms. A different construction has large “windows” cut into a flat plastic-insulated twin-lead cable to increase its characteristic impedance from 300 to 450 ohms.Above, I explained the historical reasons for 600 and 900 ohms in balanced audio lines.

author: coppercone2
author_rank: Super Contributor
timestamp: April 18, 2025, 04:03:52 pm
content: I see the calculator I was using is skewing the numbers and shifting the decimal point in a small box because its programmed poorly to make it look like the inverse is trueStill interesting that you can lower the impedance so much

author: Berni
author_rank: Super Contributor
timestamp: April 18, 2025, 04:16:34 pm
content: The desired line impedance is typically based upon what is going to be connected to it.If you can sensibly pick a transmission line that matches what it is connected to, then you don't need any impedance conversion, just hook it up directly. These 600 to 900 Ohms matched pretty well to the impedance of the telephone connected to the end of it, including what a carbon microphone likes to deal with along with a sensible current at the 48V line voltage. Also the amplifiers for longer distance calls were vacuum tube based, those like to drive higher impedance loads.The main rationale for going higher impedance is the generally lower losses, but as you have found out, there is a sensible limit to how high you can sensibly go, all due to the values of universal constants like the dielectric constant of empty space. But at the same time higher impedance also mean a higher required voltage to transmit the same amount of power. Old broadcast transmitters used 300 or 600 Ohm or similar since it matched well to the tubes.But as broadcast transmitters got more powerful the voltages involved to send 100s of kW of power into the antenna started becoming ridiculous, so lots of insulation needed to prevent an arc over, so they started moving back down to lower impedance, even as low as 35 Ohm. Tho for really high power they just sidestepped the problem by going to waveguides.Today most things on the end of transmission lines (be it RF or high speed data links) are semiconductor based, those have no problems driving low impedance lines (or can't even work at very high voltages), so we just stick to impedance that are easy to manufacture, that way the cables are as cheap as possible. This happens to be around 100 Ohm for twisted pair or 50 Ohm for coax.If you want low loss for long distance transmission lines. we have something much better now. It is called fiber optic, way lower loss than any copper conductor based transmission line you can come up with for high bandwidth.

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 04:27:23 pm
content: A nice thing about audio (as in the original post) is that one can use transformers to deal with a wide range of impedances.For example, dynamic microphones can have source impedances in the range of 50 to 250 ohms. Vacuum tube amplifiers can have best SNR at high impedances (> 50 k\$\Omega\$), while BJT amps might prefer < 1 k\$\Omega\$. “Microphone” transformers are available for this purpose.For audio outputs, balanced 600 ohms became popular because of telephone standards based on the transmission line conditions I described above.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 18, 2025, 04:28:42 pm
content: Quote from: coppercone2 on April 18, 2025, 03:33:59 pmThe standard terrestrial transmission line impedances in the old Australian PMG's Dept were 600& 1200.

author: TimFox
author_rank: Super Contributor
timestamp: April 18, 2025, 05:58:03 pm
content: Attached are my calculated graphs for air-insulated (maximum characteristic impedance) two-wire balance air lines, using the equationZo= (276\$\Omega\$) x log10(2D/d), whereD= center-center spacing andd= wire diameter(valid forD>>d).The two plots are forD= 100 mm (approximately 4 inches, typical for RF ladder lines) and 300 mm (approximately 1 foot, possible for telegraph poles).600 ohms is reasonable for either spacing, 1000 ohms would require loading coils to increase the impedance for reasonable wire diameter.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 19, 2025, 12:52:01 am
content: Quote from: TimFox on April 18, 2025, 05:58:03 pmAs, indeed, were used with terrestrial trunk systems.The PMG had a lot of literature about that application of open wire systems, as no doubt did Ma Bell & the other Telephone companies/authorities throughout the world.On simple pole routes, the PMG used about 4 foot spacing.Those sort of pole routes weren't often just carrying a single telephone or telegraph connection, the very long two-wire system into Wyndham Western Australia back in the 1960s carried a 12 channel carrier telephone system & a 24 channel carrier telegraph system.As a "Radio Tech", I was only peripherally connected to such systems so don't know much about them.Conventional open wire feeders for MF & HF radios used varying impedances from 200to 600.There was also, so-called "6 wire line", which was sort of "erzatz" coax, with two parallel conductors in the centre, & the other four in a square surrounding them.It was driven unbalanced in the same manner as coaxial cable.Sadly all those types of RF feeders are largely gone, replaced with boring old coax!

author: Andy Chee
author_rank: Super Contributor
timestamp: April 19, 2025, 02:49:06 am
content: Quote from: vk6zgo on April 19, 2025, 12:52:01 amI suppose if VK amateurs ever get 1.5kW privileges, parallel feedline may be a cheaper/easier than obtaining coax at that power rating?

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2025, 03:01:36 am
content: its substantially less lossy then coaxial but I don't know what happens if you shield it.I would love to hear about loss figures for shielded ladder line feeders.I mean the kind that its basically a ladder line wrapped in braid. Often made with foam.There is also another kind that says its parallel ladder coaxial cables, but that is not what I am inquiring about. Maybe someone with some ladder line can wrap it in foil and take a measurement ...This document claims that shielded ladder is still a bit better then foam coax. I don't know how it will stand up to good coaxial, and also what good twin lead might be, because that is a document detailing the battle for what appears to be home installers, so possibly cheaper materials... and I do wonder about some really high quality ladder line's performance ... say if you route it in the middle of a dryer duct as a shield and use true 'ladder' line with minimum dielectrics. Easy enough to build that with 3d printers and 'screw in' separator inserts (3d printed to screw into the duct). Hmm now I also wonder about making a gigantic coaxial cable with the same method. Pity its somewhat difficult to copper plate aluminum...https://schematicsforfree.com/files/Video/Circuits/TV-FM%20Antenna%20Lead%20In%20Wire.pdfBut have a look at their practical measurements, the immunity that shielded gets over regular lines in a haphazard installation makes it immensely less lossy... surprising

author: vk6zgo
author_rank: Super Contributor
timestamp: April 19, 2025, 08:45:55 am
content: Quote from: Andy Chee on April 19, 2025, 02:49:06 amSome hams in VK already use ladder line for its lower loss, whilst in the USA, many hams with their high power privileges still use coax.Of course, feeder lengths in Amateur Radio installations are rarely as great as those at Broadcast & HF comms sites.Another thing, in such places, 1.5 kW is hardly regarded as "high power". (the "six wire" feeder I referred to earlier was used at a 55kW MF Broadcast station).

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 19, 2025, 01:40:33 pm
content: In audio applications 600 is kinda the standard "line Impedance" used for microphones and other stage sound sources. Higher impedance means higher voltage which means less loss. Most mics today are about 600 ohms, the dynamic mics are that value normally and the electret mics have an amp in them to meet the line impedance. Many radios still use high impedance mics, several thousand ohms but there is not much use for 50 ohm carbon mics anymore.Any ladder or window line has less loss than coax. I recently read something about putting the window line in a tube, but now do not remember where I read it.Any window line has trouble with anything metal near it and also with concrete or brick. I think that is the main reason coax was used. Now it seems you cannot buy a radio with anything but a 50 ohm antenna connection.I think that 600 ohm ladder line was commonly made by early Hams and was made with popsickle sticks as spacers. Hence the 6 inch separation, less than the telegraph separation.Window line is about 450 ohms and VHF and UHF antenna line were smaller impedance.

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2025, 08:42:08 pm
content: Quote from: Wallace Gasiewicz on April 19, 2025, 01:40:33 pmRead the PDF, its most illuminating. I think its because coax is geometrically similar to electric wire. Shielded twin lead beats coax, and if you actually use a tube.... I think its a real fair assumption to make that it will substantially beat coax.I think maybe brain dead installers driving nails through the gap might be the biggest problem with these more complicated transmission lines. The article does state that with a conventional install, ladder line has huge losses due to the house/other wires.. so you need the shield for that.Like they would use plastic clips to hold it up. It would get beaten under a bent nail in typical 'home quality' work.So if it has a 'conformal' shield and it still beats coax, if it had a 'separated' shield it would beat coax more.It's like giving your electrons a rail road instead of making them schlep through the sewers

author: Andy Chee
author_rank: Super Contributor
timestamp: April 20, 2025, 03:04:47 am
content: Quote from: coppercone2 on April 19, 2025, 08:42:08 pmTBF, I doubt dodgy installations of consumer transmission line in the household, were the driver for the ladder line to coax transition.Use of coax cable probably increased in the post-war years, because the military manufacturing industry had made thousands of miles of the stuff for the war effort. When the war ended, what do you do with tonnes of surplus coax cable and coax cable making machines? Sell it to the consumer market!

author: coppercone2
author_rank: Super Contributor
timestamp: April 20, 2025, 03:14:45 am
content: I mean as simple as routing a wire through a stud, small hole vs some complicated shape.But I did imagine some cool kit for making twin lead shaped holes in stuff, like multi geared drills and some kinda special chizzelWire = lashing = year 300000BCtape =? space age alien shit???

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 20, 2025, 03:07:37 pm
content: Coppercone: You have great points. I think that the losses in the twin lead in the paper you attached are practical expected losses in normal applications. The actual loss of twin lead is less and makes your point more valid.A classic paper by Wes Stewart, N7WS reports onloss measurements of 450Ω "Wireman" window-line. An impedance-matched, 100ft length of medium-grade type 552 window-line showed 0.24dB loss at 20MHz under dry conditions. In contrast, the ARRL Antenna Book reports for an impedance-matched 100ft length of RG-213 coaxial cable at the same frequency a loss of slightly more than 1.0dB.RG 213 is lower loss than the commonly used RG 58 and RG 8.which makes twin lead look even better.300 ohm twin lead has a little more loss than the 450 ohm Window line in the above.I know of at least one Ham who got the cable guys to drop off unusable lengths of RG 6, which is pretty good low loss stuff, and used it for his antennas.This is big commercial stuff and 75 ohm impedance.There is not a lot of 300 ohm twin lead available any more since it was used for TV antennas . You could also make folded dipole antennas out of the stuff.The shield adds another dimension to the installation and usefulness of the feedline.You could even use it next to your aluminum siding.

author: coppercone2
author_rank: Super Contributor
timestamp: April 20, 2025, 07:38:53 pm
content: I see the main thing they want to get rid of dielectric, meaning that the best twin leads are un insulated other then the separators. This makes it unusually sensitive to getting dirty.It's a pitty they don't specify how much losses are from leakage and how much are from AC effects. The big enemies are other wires, gutters, plumbing and then I guess certain materials after that.Also would be nice to separate the losses from metal objects and the other things.But I don't think anything in the manual is talking about bare ladder line without wire insulation of some kind. That would be too hard for a home owner to maintain.I wonder what the best separators would be. I think boron fiber is the strongest material, so it could theoretically be made physically smallest, but I wonder how it compares on a dielectric level to other choices.. and there is the choice of a knob and wire type construction too, where its held at high tension. Litz twin lead could exist too... for low frequencies ?I believe the USAF used a big twin lead cable to power their EMP in the wooden test building too (nuclear EM levels, I guess that losses really effect them)

author: TimFox
author_rank: Super Contributor
timestamp: April 20, 2025, 07:52:37 pm
content: Gore makes flexible coaxial cables with “foam” Teflon dielectric, effective dielectric constant 1.4 (almost not there) with very low loss, but they don’t have 50 ohm versions as standard product.They do have a 75 ohm RG-6/U replacement, but I can’t find 100 MHz data: 17.6 dB per 100 ft at 3 GHz.Similar cables from Andrew (50 ohm, nominal 1/4 inch od) are spec’ed at 0.8 dB per 100 ft at 20 MHz.Nom. 3/8 in with 0.53 dB per 100 ft at 20 MHz, nom 1/2 in with 0.45 dB per 100ft at 20 MHz.(Andrew makes different cables that are more or less flexible, up to 1-5/8 inch od.)Obviously, these are more expensive than standard RG cables with solid polyethylene dielectric.