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

Greg Troxel gdt at ir.bbn.com
Fri Jan 3 13:10:26 UTC 2014

Ralph Aichinger <ralph at pangea.at> writes:

> 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.

Think about lightning protection.  Code in the US requires an arrestor
at the cable entrance to the house with a run to a ground rod, and that
all ground rods be bonded.  I have no clue about code.at, but I would
guess it's similar because your lightning is much like ours.  Besides
protecting the structure, if you can unplug the receiver, then it's only
the antenna that will get broken.

You may want to use different receivers with the same antenna.

Almost all GPS antennas have preamps, typically powered by 5V DC on the
coax (look for "Bias T" in the ham radio literature).   These preamps
have enough gain to overcome the coax loss, so the system noise figure
will be dominated by the preamp, not the coax loss or the GPSr noise
figure.  Beware that not all receivers and preamps agree on bias voltage
and desired signal levels.

You may want to get a geodetic grade antenna ("choke ring", typically)
that has good low-angle rejection to avoid terrestrial multipath, so
that when you later use it as a double-difference carrier-phase base
station it will work better.

In addition to it being useful to have antenna cable, you may find other
cables useful, and you can't predict what you'll want in 10 years.  So
consider running a 3" PVC pipe (or whatever the handy metric equivalent
is, of course) from attic to the radio/timing room and to the basement.
I did this when my house was built and have added and removed various
cables over the years.
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