[time-nuts] GPS antenna selection

Thu Aug 4 19:46:57 EDT 2016

Hi

> On Aug 4, 2016, at 7:26 PM, Herbert Poetzl <herbert at 13thfloor.at> wrote:
> 
> 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?

It threads on rather than latches on like a BNC. That makes it 
more water tight in the outdoor environment. 

> 
>>> 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?

You still have fog / condensation / humidity and the other sources
of moisture. So not quite so important, but we rule out the balcony below. 

> 
> 
>> 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.

It needs to face south and have a clear view over a 270 degree arc. 
If it faces north …. not going to work very well at all. 

> 
>> 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?

The more money you pay for the cable the lower it’s loss. For $10 a meter you
get a lower loss cable than $1 a meter or $0.10 a meter. Figure that anything over 
about a 30 meter run will require either a high gain antenna or some money invested
in LMR-400 cable. 

> 
>> 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?

There are a *lot* of them on eBay. Many of them have N connectors on them.

> 
>> 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).

It is a good bet that the antenna will be outside. I’d plan it that way.

Bob

> 
> 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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