[time-nuts] GPS antenna selection

Thu Aug 4 20:37:51 EDT 2016

What matters more than anything else is how good is the view of the sky.

Next You want one that will last basically "forever" outdoors.  The
best kind have a plastic radome over a metal base.  The base has pipe
threads for mounting on a standard 3/4" galvanized plumbing pipe.  The
coax wires goes down this pie and never sees the light of day or rain
water either

The shape of the dome does matter, you want the pointed kind that
birds can not perch on.  If the top is flat od even hemispherical it
will get covered with bird poop.

When I put mine up on the roof most of the work was done in the attic.
I drilled a 1 inch hole and pushed the mast (1" pipe" up through the
hole then secured it with U-straps to a rafter and joist and tugged on
it hard from all directions to make sure it would not move.  Put a
level on it first and check for vertical.   It is no more work to use
a longer pipe than a short one so go up 3 feet above the roof line.

Then while ion top of the roof just flash the hole to prevent a leak
and drop the coax cable down the pip and screw on the antenna to the
mast.

You local building code likely wants you to ground the antenna mast
lust like you would with an old TV antenna.  It is a good idea to
prevent lightening damage to the house.

With the radome typically sealed by o-ring to the base plate and the
lead wire hidden inside a galvanized pipe there is nothing to degrade
over time.  It should be maintenance free for decades or until hit by
lightening

Al the other stuff about what kind of coax to buy and what connectors
are best and so on are "in the noise" the important stuff is (1) view
of the sky and (2) build so you will never in your lifetime have to
fix it

If you must place it near a balcony or window make sure it faces south
as that is where the satellites are, they don't orbit over the poles.

> On Aug 4, 2016, at 2:29 PM, Herbert Poetzl <herbert at 13thfloor.at> wrote:
>
>
> Dear fellow time-nuts!
>
> I'm currently investigating my options regarding
> GPS antennae (of course for time related purposes)
> and I'm really confused by the variety they come
> in ... (my apologies in advance for the long post).
>
>
> Setting:
>
> I'm living in a three storey house with a sloped
> roof, a covered balcony and a larger garden with
> huge trees on the Austrian countryside (Europe).
>
> I've walked around with my smartphone (older one)
> and I get a GPS position fix within 35s in the
> garden (nine satellites shown), within 100s on
> the balcony (also nine satellites), and not a
> single satellite can be seen indoors.
>
> The obvious choice would be to put the antenna on
> top in the middle of the slanted roof for a perfect
> sky view, but this brings a number of problems as
> the roof is very hard to reach and quite high.
>
> I have my 'lab' at the floor where the balcony is,
> so I'm considering putting an antenna there and
> run about 5-15m of coax cable to the GPS receiver.
> The advantage there is that the antenna would be
> somewhat protected (it still gets very hot during
> summer and very cold during winter, but no rain
> and no snow) and easy to reach for maintenance.
>
> The third alternative would be to put the antenna
> somewhere in the garden and have a rather long
> cable running to the house and up to my lab.
>
>
> Antennae:
>
> Looking on eBay and Amazon shows a huge pricerange
> for active GPS antennae with and without cable.
>
> It seems to start at about 10 bucks with rather
> small black boxes [1] designed for cars, probably
> containing a 25x25 ceramic GPS antenna and an
> amplifier, progresses over very interesting out-
> door constructions for boats and whatnot [2] in
> the 20-100 bucks range and finally tops with high
> end devices [3] way above 100 bucks.
>
> The information about the cheap devices is usually
> very scarce, but typically boils down to:
>
> 1575.42 +/- 5MHz
> 24-28dB LNA Gain with 10-25mA at (3-5V)
>
> 7dB f0 +/- 20MHz
> 20dB f0 +/- 50MHz
> 30dB f0 +/- 100MHz
>
> They seem to use RG174 and come with SMA as well
> as BNC connectors (and a number of others as well).
>
> The mid range devices seem to use larger antennae
> with smaller tolerances (+/- 1MHz) and larger
> voltage ranges for the amplifier (3-13V).
>
>
> Questions:
>
> - What are the key specifications which need to
>   be verified before buying a GPS antenna?
>
> - How can they be compared based on incomplete
>   specifications?
>
> - Is a place on the roof or in the garden worth
>   the trouble over the covered balcony?
>
> - Are there any typical pit-falls or general
>   tips and tricks regarding mounting and cable
>   connection to the receiver?
>
> Many thanks in advance and my apologies again for
> the rather lengthy post. Please feel free to point
> me to previous discussion regarding this topic.
>
> All the best,
> Herbert
>
>
> [1] http://www.ebay.com/itm/99-Good-GPS-Antenna-SMA-Screw-Needle-10m-Super-Signal-Navigation-DVD-Antenna-/171802461614
>    https://www.amazon.com/Waterproof-Active-Antenna-28dB-Gain/dp/B00LXRQY9A
>
> [2] http://www.ebay.com/itm/Standard-Horizon-XUCMP0014-GPS-Antenna-f-CP150-CP160-CP170/331364914004
>    https://www.amazon.com/Garmin-010-12017-00-GPS-GLONASS-Antenna/dp/B00EVT2HSE
>    https://www.amazon.com/SUNDELY®-External-Marine-Antenna-connector/dp/B00D8WAVTC
>
> [3] http://www.ebay.com/itm/NEW-FURUNO-GPA018-Gps-dgps-Antenna-/182223355414
>    https://www.amazon.com/Garmin-nmea-2000-orders-over/dp/B0089DU96A
>
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