[TowerTalk] Total station grounding and ethernet

[Michael Tope]

Pete,

Do you have an antenna impedance analyzer? If so, measure the
reactance of the MOV. This will give you a feel for whether or not
it will load the Ethernet line significantly. I know from previous
experience that MOVs are not used across VHF circuits (some of the
CATV amps I have seen use gas discharge tubes across the RF
lines), but I am not sure how significant the loading is down at
10 MHz.

Mike, W4EF.............................................

----- Original Message -----
From: "Pete Soper" <psoper@pjs.East.Sun.COM>
To: <towertalk@contesting.com>
Sent: Wednesday, May 31, 2000 9:29 AM
Subject: [TowerTalk] Total station grounding and ethernet


>
> In arranging for all conductors coming into my station building to pass
> through a single point ground I've run into a snag. I've dealt with or
have a
> sure plan for the feedlines, rotor and relay control, AC power and
telephone
> but I'm not sure what to do about an ethernet line.
>
> Can 10mbs ethernet performance be maintained with MOVs hanging off the
twisted
> pairs of an ethernet cable? How about with fuses inline?
>
> (By MOV I mean "Metal Oxide Varistor", a device that present an open
circuit
> until the potential difference across it reaches a certain value,
typically a
> few tens or hundreds of volts, at which point it becomes a short circuit.
> These are very common in power outlet boxes, surge suppressors for modems,
> etc. CAT5 is short for "category 5", the common name for twisted pair
cable
> used for network connections)
>
> My plan is to strip the outer sheath off the CAT5 cable such that it can
sit on
> the the single point ground window copper sheet with copper straps holding
it
> down and the twisted pairs flattened out. I'll then separate the two wires
of
> each pair and attach MOVs to each, keeping the lead length short. The
other
> lead of each MOV is connected to ground:
>
>         (CAT5 CABLE from house)
>         /\   /\
>  .......\/...\/.................
>  .      /\   /\                .
>  .gnd   \/   \/     gnd (strap).
>  .|...../\.../\.....|...........
>   |     \/   \/     |
>   |     /\   /\     |
>   -MOV_/  \__\/__MOV-
>        \  /  /\
>         \/   \/     (copper sheet well
>         /\   /\       connected to earth gnd)
>   -MOV__\/__/  \_MOV-
>   |     /\  \  /    |
>   |     \/   \/     |
>   |     /\   /\     |
>  .|.....\/...\/.....|.....
>  .|     /\   /\     |    .
>  .gnd   \/   \/     gnd  .
>  ......./\.../\...........
>         \/   \/
>         (CAT5 CABLE into station)
>
>
> After looking at the punchdown connector of a typical RJ45 jack designed
for
> ethernet it's my belief that ethernet (at 10mbs at least) can tolerate an
> impedance "bump" like this. But you know how optimism goes when you're
> operating outside your area of expertise :-)
>
> But if this is complete overkill with respect to controlling the impedance
can
> I also put fuses into the path? I'd like to have fuses in each lead on the
> house side of the MOV connection to arrange an air gap to be present if
the
> MOV is destroyed by the surge from an initial lightning strike.
>
> What do you think?
>
> Regards,
> Pete
> KS4XG
>
> PS I'm also arranging to be able to do a total disconnect of the station
> wiring. The ground system is really just a backup (so don't preach at me
> about this please, I'm in the choir!)
>
> --
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>


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