[time-nuts] GPS ANTENNA

David C. Partridge david.partridge at perdrix.co.uk
Sun Apr 1 16:21:04 EDT 2018


Or use a choke ring survey antenna and an attenuator :)

Dave

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob kb8tq
Sent: 01 April 2018 14:43
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS ANTENNA

Hi

Indeed, it is *very* easy to put to much gain in front of a timing GNSS receiver. These beasts are trying to dig out a signal that you can’t even see with a spectrum analyzer.
It’s way to far below the noise floor to detect that way. They optimize things pretty tightly to get that done (and to hit a price target ….). Put to much gain in front of them and they get unhappy. 

Making this even more crazy, the survey industry standard antenna *does* have a lot of gain. Survey receivers need way more gain in front of them than timing receivers. Put a survey antenna directly on a timing device and trouble will likely be the outcome. Equally, a survey instrument probably will not be happy with a timing receiver. 

Why all this nonsense? As far as I can tell, it goes back to how the very early L1 / L2 survey boxes were designed back in the 1980’s and early 1990’s. They made a basic decision to put a lot of gain at the antenna. Motorola came along with their GPS modules later on. They made a *very* different decision about how to distribute the gain. There are very good arguments on both sides for why they did it this way. 
The bottom line is still - you need to match things up …

Bob

> On Apr 1, 2018, at 2:36 AM, cfo <xnews5 at luna.dyndns.dk> wrote:
> 
> On Sat, 31 Mar 2018 10:58:19 -0500, 
> donandarline-Re5JQEeQqe8AvxtiuMwx3w
> wrote:
> 
>> I found a supplier for high quality GPS antennas at a very reasonable 
>> price. PCTEL GPSL1-TMG-SPI-40NCB.
> 
> *** SNIP ***
> 
> I had one of those on 25m cable, and it worked fine on a Tbolt , until 
> i got an active antenna splitter that also had some gain.
> Then i had to replace it w. a 26dB version of same type, else the 
> "Jackson Lite" was loosing sync.
> 
> What i mean here, is that you can get too much gain too.
> 
> Btw: Good price.
> 
> CFO
> Denmark
> 
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