[TheList] RFI

Mon Aug 27 20:54:22 AEST 2012

Hi John,

They are loading coils. There is a formula RC=LG, the point when loss in 
a cable is minimum, the inductance bit is hard to achieve so they add 
coils at something like 1.5km points along the cable. Sometimes the 
point might have been too close to and end terminal and they'd add 
capacitors across the pairs to achieve the correct impedance. From 
memory 44mH was for subscribers on smaller gauge cable, the 88mH for 
trunk cables between exchanges with larger gauge cables.

Trouble with loading cables is they form a low pass filter, 3.5 to 6 kHz 
cutoff from memory.

I still recall the arguments with cable jointers over the first PCM 
(2.048 Megabits) systems going up SH36 (Pyes Pa Rd). The DC feed to the 
first regenerator was intact but the loss was huge and we couldn't get 
the systems to work. They said we've removed the loading coils and 
eventually an echoflex revealed a lovely big reflection, at the distance 
of the reflection they found loading coils that weren't on the plan.

Made earlier problems with the PUE2 Negistors make sense, but that's a 
whole new topic.

Regards,

Neill ZL1TAJ.

On 27/08/2012 5:25 p.m., John Barnhill wrote:
>
> Anyone know what these were used for? 44milli henry (44mH) Inductances 
> made by Hatfield Industries Ltd. 1981 NZPO order number H8959. They 
> are in a plastic sleeve with between 17 – 20 per sleeve. Something to 
> do with NZPO Telephony back in that era? Are they any use to anyone 
> (free to a good home!)
>
> John B
>
>
>
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