[time-nuts] GPS ANTENNA

Bob kb8tq kb8tq at n1k.org
Sun Apr 1 20:16:57 EDT 2018


Hi

They are indeed a bit unusual. Generally you do a DC block and bias tee ahead of the 
attenuator. For a “self contained” approach, a second block and bias tee pulls the DC
of the GPSDO side of the attenuator.  My preference has been to just run the bias tee
with an external DC source. 

Bob

> On Apr 1, 2018, at 6:29 PM, Azelio Boriani <azelio.boriani at gmail.com> wrote:
> 
> An unusual attenuator with a DC pass.
> 
> On Sun, Apr 1, 2018 at 10:21 PM, David C. Partridge
> <david.partridge at perdrix.co.uk> wrote:
>> 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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