Thicknet/10base5 Test Segment: The Cable is In!

[Paul Koning]

> On Jun 26, 2018, at 7:20 PM, Eric Smith via cctalk <cctalk at classiccmp.org> wrote:
> 
> On Tue, Jun 26, 2018 at 4:37 PM, Chuck Guzis via cctalk <
> cctalk at classiccmp.org> wrote:
> 
>> On 06/26/2018 03:15 PM, Grant Taylor via cctalk wrote:
>> 
>>> I can only guess that having a terminator too close interferes with or
>>> weakens the signal too much in some way.
>> 
>> Exactly what would the effect be?  I recall putting terminators on
>> 10base2 coax just hanging off one leg of a BNC tee.  Really, no distance
>> at all.  Didn't seem to affect speed or distance.
> 
> 
> If the termination resistance matches the characteristic impedance of the
> cable, there should be no difference. When terminated properly, there is no
> reflection from the terminator, so it looks equivalent to an infinitely
> long cable, though in practical terms with less leakage than an "actual"
> infinitely long cable would have.
> 
> Of course, in reality it will never be terminated perfectly, so there will
> always be a small reflection, which can be seen with a TDR. If the
> termination resistance is pretty close, the reflection will be small enough
> not to matter at all for Ethernet.

Exactly.  And the specs for the Ethernet terminator are quite tight for that reason.  The connectors themselves have non-zero impact but very small; they are high quality microwave grade connectors.

You got the definition precisely correct: a terminator is a device that is electrically equivalent to an infinite length cable.  You can cut the unused part of a coax anywhere you want and put a terminator at that point instead, and as far as the rest of the cable is concerned nothing has changed (apart from very small effects because the components are not perfect).

There clearly is confusion about what terminators are and how they work.  It's all perfectly straightforward elementary classic E & M, and any halfway decent RF theory textbook will make things clear.  Even a source as elementary as the ARRL Radio Amateur Handbook will serve.

	paul