Author Topic: Installing an external timing GPS antenna  (Read 438 times)

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Offline jpb

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Installing an external timing GPS antenna
« on: November 14, 2017, 03:31:16 AM »
I want to install an external GPS antenna for connection to GPSDOs and other timing purposes. But I want to do this in as aesthetically least intrusive fashion as possible. In my old house I installed it on a fence and ran a thick coax cable underground to a type N connector on a box just outside a french window. In my new house my study/electronics lab is on the north side of the house but I want the GPS antenna to be on the south side (I'm in the UK) on a fairly well hidden wall where there are currently two (ugly) satellite dishes so I reckon I won't be adding much to the ugliness. I also have a lightening arrester with type N connectors.

I thought that this would be straight forward but I have a number of questions.

1.) Is being on the south side worth the extra effort/signal loss of running an extra 20m of cable through the loft? (I could add an in-line amplifier in the loft but that is more expense.)

2.) Do I need to install an earth rod for the lightening arrester or can I use my house wiring earth?

3.) Can I use the lightening arrester just indoors (to save having external connectors subject to the weather)?- I realise that the path to ground would then pass through the house earth wiring or out through a separate earth cable but a direct hit by lightening is very unlikely given that the house has an existing TV aerial stuck on the chimney at great height and some even taller trees just beyond the garden fence.

4.) Do people have good ideas about hiding GPS antenna whilst giving them a clear view of the sky? James Miller told me that he installed his in a disused chimney pot - unfortunately I hope to install a wood burning stove at some point so my only chimney is not disused.
 

Offline Gyro

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Re: Installing an external timing GPS antenna
« Reply #1 on: November 14, 2017, 06:09:48 AM »
You might want to evaluate its actual performance loss if you mount it in the loft (assuming that it's of normal UK construction). You might find it a better compromise than the loss due to extra cable length, additional connectors. It's certainly worth trying before thinking about additional amplification.
Chris

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Offline jpb

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Re: Installing an external timing GPS antenna
« Reply #2 on: November 14, 2017, 07:50:46 AM »
You might want to evaluate its actual performance loss if you mount it in the loft (assuming that it's of normal UK construction). You might find it a better compromise than the loss due to extra cable length, additional connectors. It's certainly worth trying before thinking about additional amplification.
Thanks, that will be worth experimenting with. I used to have a tv aerial in the loft which worked well though I needed a much bigger one than would be required externally, but I thought at GPS frequency that the tiles would give too much attenuation. The issue might be the attenuation when the roof is wet (which where I live is almost all the time!) I've ordered myself a USB GPS receiver so I can combine it with a laptop to use as a signal strength/satellites viewable meter.
 

Online cdev

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Re: Installing an external timing GPS antenna
« Reply #3 on: November 14, 2017, 08:07:51 AM »
A timing antenna is a specific kind of GPS antenna that picks up satellites from higher angles better and rejects reflections and low angle signals. I have one, it works well but its not the antenna I am using on my GPSDO right now. (And my GPSDO works fine with a non-timing antenna).

My timing antenna is made by Maxrad, its the same one that is sold under the Lucent name. Its a "GPS-TMG26N"  Internally its a quadrifilar helix antenna. It actually works best without a ground plane. just stuck in the air. You can mount it on a metal or PVC pole, but if its high up you likely want to put a good ground on it and also a lightning arrester. Which is kind of a pain in the neck.

How old is your GPSDO? Newer GPSs would probably do fine with an (active) ceramic patch antenna mounted on a saucer-sized piece of metal (say twice the size of the the antenna itself) on a windowsill or put on a piece of metal sheeting in your loft or like Gyro does on top of a bookshelf or similar on an upper floor. (or even not, my home network's NTP server's antenna is on the ground floor on the table my little used TV set sits on, it works fine. That GPS antenna is passive and it uses a CD/DVD as its ground plane).

Unless you are also planning on also using it for geodesy or similar, pretty much any flat conductive object should do as a ground plane. Size is not critical but having it there to make the antenna resonate at the right frequency is. So, a ceramic patch GPS antenna doesn't just work best with a ground plane - they often perform quite inadequately without one.

You should only need one amplifier. Most GPS antennas contain low noise amplifiers which are powered via their coax cable, so its likely you can't add another one unless you figure out some way of passing additional power through it to the antenna. (could be done with a bias-tee)

But likely you wont need it.

Just try different locations and pick the best one.
« Last Edit: November 14, 2017, 02:08:05 PM by cdev »
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Offline Gyro

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Re: Installing an external timing GPS antenna
« Reply #4 on: November 14, 2017, 08:13:25 AM »
I've been running an LEA-6T with a separate active patch antenna mounted on a small groundplane for some time. The antenna sits on a high shelf, close to the ceiling in an upstairs 'bedroom', the loft isn't exactly empty either (:palm:). It acquires all the satellites that should be there according to the Almanac, in fact I have the receiver is configured to ignore low angle satellite that it still quite happily acquires. I wondered about wet tiles too but rain doesn't seem to have a significant effect in practice.

I'd say your chances are better than even.  ;)

P.S. Chronicled here: http://www.eevblog.com/forum/projects/ebay-u-blox-lea-6t-gps-module-teardown-and-initial-test/

EDIT: Yes, the groundplane certainly makes a noticable difference - mine is about 90 x 120mm steel sheet (magnetic attachment).
« Last Edit: November 14, 2017, 08:22:21 AM by Gyro »
Chris

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Offline medical-nerd

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Re: Installing an external timing GPS antenna
« Reply #5 on: November 14, 2017, 12:05:59 PM »
Hiya
 
I run my Z3801a in the attic where my 'lab' is based with just a cheap puck antenna with no problems as long as the puck is within 2 ft of a window. It almost never goes into holdover unless it is moved accidentally. Although it has undergone 'roll-over', its 10Mhz output is still stable and supplies my equipment. So an external antenna may be overkill.
You need to test in your reception area.

Cheers
'better to burn out than fade away'
 

Offline jpb

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Re: Installing an external timing GPS antenna
« Reply #6 on: November 15, 2017, 07:06:47 AM »
Thanks for the replies everyone.

I don't have a particular GPSDO in mind - my long term project is designing/building my own and I wanted the best possible signal for the various gps modules I experiment with.

The antenna I have has a built-in amp but even with this, if I use RG58 cable (as it is not too large for drilling holes for etc) then the limit from what I've read seems to be around 20m or so whilst I probably need more like 35 m. One solution is an additional in-line amplifier such as the Tallysman TW125B but these aren't cheap (£130 + VAT from Digikey) - being inline they pass the dc through to the active antenna. The alternative is to use lower loss cable but this is rather thick and unwieldy and wouldn't look good running down the side of the house.

I have a lightening arrester but the main pain is the earth connection, I don't want to go hammering copper rods into the ground (and probably holing a water pipe or damaging underground cables).

For the ground plane I'll probably use a square (or disk) of aluminium with a hole for the antenna (it is designed to bolt through a hole anyway.

My current thought is to mount it on the end of my garage which would give it a fairly good view of the sky from an angle of around 45 degrees up.
 

Online cdev

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Re: Installing an external timing GPS antenna
« Reply #7 on: November 15, 2017, 07:59:32 AM »
If the first LNA has an SAW filter, etc in it you could probably use a much cheaper non-GPS-specific broadband LNA and bias tee in the middle for the next leg, assuming you need it, (you may not) and have power there or can feed it to there alongside the coax.

In my experience, I've been able to successfully implement two stage LNAs only when I took a lot of care to isolate the two amplifiers inputs and outputs (that should be a no brainer) and also *power sources* from one another.

Also the LNA in the middle should be enclosed in a metal box with a good feed through cap and it also helps to put some ferrite beads around the lead inside it for the DC power.



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Offline jmelson

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Re: Installing an external timing GPS antenna
« Reply #8 on: November 15, 2017, 08:34:13 AM »
2.) Do I need to install an earth rod for the lightening arrester or can I use my house wiring earth?

You want to BRING lightning inside your house??!!??  Is your name Frankenstein, by any chance?  Definitely, drive a ground rod as close to the antenna as possible, and tie the antenna(s) to it with heavy wire.

If you used the inside grounds, and you ever DID have lightning, even just a nearby hit, it could fry everything inside the house, including the wiring in the walls.  If you have a direct hit, probably nothing will help, but good grounds MIGHT save the house from burning down.

Jon
 

Offline jpb

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Re: Installing an external timing GPS antenna
« Reply #9 on: November 15, 2017, 08:50:56 AM »
2.) Do I need to install an earth rod for the lightening arrester or can I use my house wiring earth?

You want to BRING lightning inside your house??!!??  Is your name Frankenstein, by any chance?  Definitely, drive a ground rod as close to the antenna as possible, and tie the antenna(s) to it with heavy wire.

If you used the inside grounds, and you ever DID have lightning, even just a nearby hit, it could fry everything inside the house, including the wiring in the walls.  If you have a direct hit, probably nothing will help, but good grounds MIGHT save the house from burning down.

Jon
I suspected that might be the case  :)- I was only thinking about surges rather than direct hits and the house supply enters just by my study - but you've confirmed my own uneasiness.
There is probably not much point in fitting the lightening arrester anyway unless I retrofit something to the existing TV aerial as well which is much higher - it was there when we bought the house but I don't think it is protected, relying on the nearby trees being even  higher.
 

Offline jmelson

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Re: Installing an external timing GPS antenna
« Reply #10 on: November 15, 2017, 09:04:36 AM »

I suspected that might be the case  :)- I was only thinking about surges rather than direct hits and the house supply enters just by my study - but you've confirmed my own uneasiness.
There is probably not much point in fitting the lightening arrester anyway unless I retrofit something to the existing TV aerial as well which is much higher - it was there when we bought the house but I don't think it is protected, relying on the nearby trees being even  higher.
In the UK, you may have somewhat less trouble with lightning than we do in the US.  It is nothing to be trifled with, I've heard plenty of horror stories, with practically every appliance in the house reduced to metal scrap.

I've had several incidents myself.  Both have been from lightning bolts that struck hundreds to thousands of meters away.  The inductive radiation from a lightning bolt will induce a large voltage across any longer wires, especially those that are roughly parallel to the path the bolt takes.  Inside the house, you have phone wires, water pipes and mains wires, and these often form loops, or near-loops, where the wires come together or nearly so to appliances.  These loops act as antennas to receive the magnetic energy, and can fry computer and video gear that are connected to multiple conductive paths.

Jon
 

Offline jkf1000

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Re: Installing an external timing GPS antenna
« Reply #11 on: November 16, 2017, 11:35:50 AM »
I run one of the little hockey puck style antennas to my GPSDO. it is only about 50mm square and I was able to slide it out under a tile in my loft. From the outside if you didn't know it was there you would never see it. It has about 15mtr of RG316 or similar running to the base unit and it has worked well for the last couple of years. I generally see between 5 to 11 birds and have never lost signal according to the logs. The system has never been turned off in the two years I have had it installed as I just switch in the distribution unit when needed. Karl, M0KRL
 

Offline jpb

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Re: Installing an external timing GPS antenna
« Reply #12 on: November 22, 2017, 08:35:17 AM »
I run one of the little hockey puck style antennas to my GPSDO. it is only about 50mm square and I was able to slide it out under a tile in my loft. From the outside if you didn't know it was there you would never see it. It has about 15mtr of RG316 or similar running to the base unit and it has worked well for the last couple of years. I generally see between 5 to 11 birds and have never lost signal according to the logs. The system has never been turned off in the two years I have had it installed as I just switch in the distribution unit when needed. Karl, M0KRL
Thanks for the information.
Last weekend I attached a GPS dongle (mouse thing) to my daughter's laptop and checked reception all round the outside (and inside) of my house. As I expected the best reception was on the south side but reception elsewhere was ok.
The tiles idea is interesting, though we (like most UK houses) have roofing felt in the way. I also have an existing good timing antenna so I'm quite keen to use it.

There seems to be a trade off, if I go for good reception then I must have a long (30m perhaps) cable run and even with amplification it might introduce timing variations with temperature and extra noise.
I have an opportunity to set things up now as I'm having new sockets fitted in the study (for my electronics) so I'll need to redecorate anyway so drilling a hole in the wall for a cable will not be so bad.
 


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