[ntp:questions] Roof antenna, which one, would you bother?

unruh unruh at invalid.ca
Fri Jan 3 17:39:34 UTC 2014

On 2014-01-03, Ralph Aichinger <ralph at pangea.at> wrote:
> I am currently in the process of remodeling my house
> and a dedicated outdoor/roof mounted GPS antenna
> would be possible to mount without excessive cost.
> I probably would not see a huge difference for
> timing purposes, but what would your choice of 

Probably not. As long as your gps has a view of the sky (southern if in
the northern hemisphere) your timing will be fine.

> an outdoor GPS antenna/receiver be? I am planning

Put almost anything into a plastic (not metal) can so the rain does not
soak it.
> to put the NTP server (small Raspberry Pi, nothing
> fancy) into a rack maybe 10 metres of cable length away 

10m=30ns delay, and since the Pi almost certainly cannot get the signal
into itself that fast ( more like us ) that 10m shold not be a problem. 
(actually since termination is liable to be a problem, the actual rise
time will be much longer than that. )

> from the roof. Or would you move the NTP host itself
> to the attic and run Ethernet up?
> I suppose something like the Garmin GPS 17x (mast mount
> GPS with serial output and PPS, intended for the marine
> market) would work fine, also be robust enough and 
> cheap on the used market.
Or the cheapest Sure. Antenna outside, gps receiver inside.
I had a roof mounted Garmin 18 (inside a plastic milk container) which
lasted about 2 yr.  
> Would mounting just an antenna (active or passive) also
> make sense, or would cable loss be too high? 

Sure. it would be fine.-- antenna outside with its 5m lead into the
attic where the receiver is located.

> What would *your* roof top setup be? Cost is of course
> an important consideration, on the other hand, 100
> or 200 Euros for the GPS, antenna and mounting hardware
> is probably OK. I would prefer to do cabling jobs now,
> while I am in the process of doing a lot of "dirty" things.

The mounting hardware could well be the most expensive thing. Mind you I
would not do anything very fancy. All it needs to do is see the sky.

> Or would you not bother at all, and just put some puck into
> the window (which probably works too)?

With a south facing window probably. Even with an east facing window
through trees it works for me. 

> /ralph

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