[TowerTalk] 600 Ohm Line

[Jim Lux]

At 09:46 AM 7/29/2004 -0500, Jim Brown wrote:
> >Pretty much the same here. 600 ohm balanced pair to the house.
>
>What is coming into the house on a 600 ohm balanced pair?  An RF transmission
>line I would believe. A telco line or audio line I would not. The military 
>surplus field
>telephone transmission line being discussed in conjunction with the 
>Beverage thread
>on the Top Band reflector was cited as being 150 ohms, and someone confirmed
>that with measurements.
>
>I'm a fairly old guy, and I have never seen a transmission line having a 
>Zo much
>greater than about 150 ohms that wasn't some form of either ladder line or 
>open wire
>line. Virtually all cable used for telco and audio has a Zo on the order 
>of 50-100
>ohms.  Doing the arithmetic for Zo and cable construction shows that it's 
>pretty
>difficult to get a Zo greater than about 200 ohms using anything but open 
>wire line.  A
>careful study of a cable catalog (Belden, for example) will confirm this.
>
>73,
>
>Jim Brown  K9YC

600 ohms is pretty much a standard for audio balanced pairs. Think about 
it.. 3 kHz, the wavelength is around 100 km... I think you can consider a 
phone line (or audio cable) as a lumped component, not a transmission line.


The nominal terminating impedance for phone connections IS 600 ohms (i.e. 
that's the impedance used in things like transformer hybrids). It's also 
the standard impedance used for specifying things like signal and power 
levels. Since the frequencies are low, the fact that the actual line 
impedance is much less (probably around 100 ohms, like most twisted pairs) 
isn't a problem, although there is the interesting fact of "loading coils" 
on voice grade lines (used to compensate for the capacitance of the pair).


Recall that originally, all phone lines WERE open wire pairs, and fairly 
far apart to boot.  They may well have been 600 ohm characteristic 
impedance (but I doubt it).