[time-nuts] GPS antenna selection

Thu Aug 4 23:26:52 UTC 2016

On Thu, Aug 04, 2016 at 06:26:28PM -0400, Bob Camp wrote:
> Hi

Hey Bob!

>> 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). 

Understood!

>> 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

Hmm, I had to google TNC (Threaded Neill-Concelman).
Is it worth the trouble in the < 2GHz range?

>> 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. 

Even under somewhat protected conditions like on the
covered balcony?

> These days, I’d get one that does both GPS and GLONASS

Makes sense.

>> - 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.

Okay, thanks!

>> - 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). 

That would opt for the balcony, as it faces north
and extends the slanted roof, so basically clear
view from NE to NW down to the horizon.

> 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.

Understood! Is there some rule of thumb at what
cable lengths which amplifier gain is best suited?

> 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 did a quick search for SMA/BNC/TNC based surge
protectors and not much did come up, any suggestions
what to use there?

> I don’t know about Austria, but here in the US, 
> both are required. 

Outside definitely, "inside" I'm not sure, but it
won't hurt to have additional protection for the
receiver(s).

Thanks a bunch,
Herbert

> Bob

>> 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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