[time-nuts] GPS antenna selection

Bob Camp kb8tq at n1k.org
Thu Aug 4 18:26:28 EDT 2016


> On Aug 4, 2016, at 5:29 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

That’s the spec on the interference  rejection filter. Tighter is better 
as long as it still passes the desired signal(s). 

> They seem to use RG174 and come with SMA as well
> as BNC connectors (and a number of others as well).

The better ones will have a TNC connector on them

> 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?

You want one that is designed for permanent outdoor use. That eliminates the $10 
car mounts. These days, I’d get one that does both GPS and GLONASS

> - How can they be compared based on incomplete
>   specifications?

They can’t. It’s just luck. The ones you see for about $40 and up that are designed
for mast mounting are usually pretty good. 

> - Is a place on the roof or in the garden worth
>   the trouble over the covered balcony?

The real question is how much of a sky view you get. Ideally you would like a clear view
of the sky from about NE clear around to NW (270 degrees). You also would like to be
able to “see” down to within 10 degrees of the horizon over that range. The segment from 
E to W (180 degrees) is pretty important. Being able to see to within 30 degrees of the 
horizon is also pretty important. 

> - Are there any typical pit-falls or general
>   tips and tricks regarding mounting and cable
>   connection to the receiver?

Some receivers put out +12V, most antennas don’t like +12 and want +5. Some modern
antennas will only handle +3.3V. 

If you have a long run to the antenna, feed line loss is what matters. To some degree
you can cope with this by buying an antenna that has a higher gain amp in it. They 
range from about 21 db to about 50 db. You also don’t want to over drive your receiver
so just getting the 50 db version is not a perfect solution. 

Grounding the antenna is always a good idea. A surge suppressor  in the line could save
you some real cost if there is a lightning strike. I don’t know about Austria, but here in 
the US, both are required. 


> 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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