[time-nuts] GPS antenna in snowy environment

Bob Camp kb8tq at n1k.org
Fri Aug 29 14:38:54 EDT 2014


Hi

Typo, FLX should be FXL helix. Sorry about that.

Bob

On Aug 29, 2014, at 2:36 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
> 
> Well if you want to run 600’ you either will need some fancy coax or a couple inline amps. 
> 
> LMR400 is roughly 5 db per 100’ when new. At 300’ that’s 15 db plus connectors (maybe another db) brand new. Figure that it will degrade another 3 or 4 db before it dies. Net is about a 20 db loss. That’s certainly more than a 26 db antenna preamp will handle and still deliver a 15 db net gain. 
> 
> You could go to FLX-1480 and drop the attenuation to the point that a 600’ run would not matter. It’s debatable if you can save any money on a smaller diameter / custom order helix even on a 600’ order. You local cable TV outfit might have a half mile spare spool sitting around gathering dust ….
> 
> Bob
> 
> On Aug 29, 2014, at 2:00 PM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
> 
>> Hi Bob,
>> 
>> House is tucked in against a tree line to the south of my field.
>> Lab/shop is on the north side of the house. I need to go north and east
>> into the yard/field to get a good view of the sky east-south-west. (In
>> the shop, the southern half of the view is blocked by trees, and I get
>> no birds just south of straight up...)
>> 
>> I can run a coax across the yard (towards alternate GPS antenna location
>> show in image, north is up in image). I can run right out the shop wall
>> as far north and east as practical. At 150 feet or more, I get most of
>> the sky. At 600 Feet, I get pretty much get everything above ~10 degrees
>> elevation for 360 degrees around. And I'm further away from any noise
>> sources in the house also.
>> 
>> There is lots of room, 660Ft by 1200Ft field. Right on top of the hill).
>> I have lots of sky without anything else around (if I run cable). Is
>> there a practical reason not to take advantage of it?
>> 
>> This is as much a mental exercise, as a practical problem, so any input
>> is most welcome! :)
>> 
>> See image.
>> 
>> Dan
>> 
>> On 8/29/2014 1:28 PM, Bob Camp wrote:
>>> Hi
>>> 
>>> Most GPS antennas have a preamp in them. All of the common Time Nut antennas have one. Gain varies from the mid twenties to over 40db between models. You really do not want much more gain than you need, so more is not generally better. 
>>> 
>>> Satellite TV coax is the material of choice for GPS antennas. It’s cheap and low loss. If you need to run 150’, that’s quite a castle you live in. I typically find that adding another 50’ gets me just about anywhere I need to go. I might have ten or twenty feet of coax already involved in getting to the nearest window. That still nets out well below 100’. 
>>> 
>>> Bob
>>> 
>>> 
>>> On Aug 29, 2014, at 11:55 AM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
>>> 
>>>> Björn and Tom,
>>>> 
>>>> Thanks for the links. It helps visualize things a lot! Our snow was
>>>> unusually weird last year. It stuck on everything, any stick larger than
>>>> a pencil had at least basket ball sized hunks of snow on it. That's
>>>> probably a worst case scenario, tho.
>>>> 
>>>> Joe,
>>>> 
>>>> OK on the study of snow. It's good to know that it doesn't attenuate the
>>>> GPS a lot. That's good information to have in the back of my head!
>>>> 
>>>> We're just east of Minnesota (Upper Michigan).  The air coming over the
>>>> big lake warms up, picks up water and dumps it on us all winter long.
>>>> Because of the lake effect we're lots warmer than Minnesota, but a lot
>>>> whiter too! :)
>>>> 
>>>> 
>>>> The next question that comes to mind, is how much cable is too much
>>>> cable from the antenna to GPS? Granted every environment is different,
>>>> so lets assume you add 150 ft of cable to gain 30% to 40% more sky view
>>>> to the south, is the trade off worth while? Pick a coax, say something
>>>> like RG-6 (mismatch and all) or something like LMR-400. Is there a
>>>> practical limit? Does temperature changing the length of the cable make
>>>> any noticeable difference for a Timing GPS?
>>>> 
>>>> Dang it! I'm getting bit this time-nuts bug now!
>>>> 
>>>> Dan
>>>> 
>>>> On 8/29/2014 10:34 AM, time-nuts-request at febo.com wrote:
>>>>> I had to study this issue once.  The question was if we needed to 
>>>>> provide antenna heaters in the far North.  Like Minnesota and Alaska.
>>>>> 
>>>>> Turns out that snow and ice are almost transparent to 1.5 GHz, while a 
>>>>> fat seagull perching on the antenna was a problem, so we did the tall 
>>>>> cone and let it go at that.
>>>>> 
>>>>> The only exception to the transparency is salty sea ice, which can 
>>>>> accumulate on shipboard equipment.
>>>>> 
>>>>> Joe Gwinn
>>>> _______________________________________________
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>>>> and follow the instructions there.
>>> 
>>> 
>>> 
>> <Tapiola.jpg>
> 




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