[time-nuts] Re: GPS antenna locations

[Bob kb8tq]

Hi



> On Apr 17, 2022, at 12:58 PM, N1BUG <paul at n1bug.com> wrote:
> 
> Thanks Bob.
> 
> I have no idea what makes that gap to the south. There is nothing to block that direction above 15 degrees.
> 
> I am not concerned about cable losses. I have LMR-400 on this roof mounted antenna because it is only a 50 foot run, but the smallest cable I have to either tower is 1/2". The antenna spec sheet says 30 dB gain.

The TBolt needs about 16 db in front of it to do a reasonable job. With a 
30 db gain antenna and no splitters, that leaves you with about 14 db or 
so for cable and connector loss. Most of us accumulate multiple GPS
gizmos. A 4 way splitter takes your 14 down to 8 db. That puts you “at
the limit” with a bit over 100’ of LMR-400.


> 
> The data sheet on this antenna shows it being down >60 dB at +/- 50 MHz. With a noise figure spec of 2.2 dB I suspect that filter is before the amplifier, but I wish it actually said that. Add several tens of dB for separation of antennas and I think it would be OK.

The filter is after the amplifier. However the antenna itself is pretty narrowband. 
If the filter was in front, getting a 2 db noise figure would be exciting. 

Bob

> 
> My only concern is with overload of the onboard amplifier in the antenna. It is filtered again by a GPS Networking splitter (-60 dB at +/- 60 MHz) before going to the Thunderbolt.
> 
> Side mounted at 80 feet on the VHF tower (60 feet higher than where the one on the roof is), it would have a much better sky view, neglecting any blockage to the north from the tower itself and at high elevation angles from yagis on top of the tower).
> 
> I am leaning toward getting another of these antennas and side mounting it on the tower, then doing a comparison of the signal strength vs az/el plot against this roof mounted one. It is an extra expense I will have to find a way to budget for, but certainly has educational value if nothing else.
> 
> Paul
> 
> 
> 
> 
> 
> On 4/17/22 09:04, Bob kb8tq wrote:
>> Hi
>> Not knowing everything about the local environment there’s
>> not much way to guess exactly what this or that location
>> will do. The biggest thing I see on your plot is something
>> due south of the current antenna location.
>> In a “typical” setup, anything within 20 degrees of the horizon
>> gets tossed out for timing. The paths are long and with normal
>> clutter multi path is likely at low angles.
>> The filters in the typical “telecom” antennas are set up to block
>> cell phone transmitters. The GPS and cell antennas are co-located
>> on the same tower so they can get hit pretty hard. That said, the
>> cell site isn’t running an ERP in the many hundreds of watts range.
>> The longer your cable, the more likely you are to need a booster
>> amp. The telecom antennas typically don’t have a lot of gain. Yes,
>> fancy cable can help with this. RG-58 is a bad idea :) ….
>> If you have a better sky view at 60 to 80’ on the tower, then a side
>> mount in that range would be my vote. Lightning would be my
>> biggest concern as you go higher. Assuming all the heights are
>> to the same reference, moving the antenna up 40 to 60’ should
>> do the trick.
>> Bob
>>> On Apr 16, 2022, at 2:05 PM, N1BUG <paul at n1bug.com> wrote:
>>> 
>>> Hello time nuts,
>>> 
>>> I finally have my long awaited Trimble Thunderbolt up and running. I am not thrilled with the coverage using a Symmetricom 58532A antenna on the roof about 20 feet above ground level. Here is what I get after ~24 hours:
>>> 
>>> http://n1bug.com/gpssig.png
>>> 
>>> Sometimes I have 8 satellites with usable signal, sometimes as few as 5. The problem to the west is trees. I believe the chaotic signal strength in the east is due to reflections from a metal roof.
>>> 
>>> I have three options:
>>> 
>>> 1. Leave the antenna where it is.
>>> 
>>> 2. Side mount it at 80 to 90 feet on a radio tower that has yagis for 50/432/222/144 MHz at 105/110/115/120 feet. These antennas are used for high power transmitting. Potential interference to GPS reception? I don't know if the filter in the 58532A is before or after the amplifier. Blockage from the tower and/or yagis? I assume mounting a few feet off the south tower face would be best.
>>> 
>>> 3. Mount at the top of a mast on another radio tower, at 110 feet. This would have a completely unobstructed sky view but would have antennas for 7/10 MHz about 3 feet below and 14/18/21/24/28 MHz about 13 feet below. Those antennas are used for high power transmitting. There will at some point be a 10 GHz dish about 8 feet below the top of that mast.
>>> 
>>> Any comments on these options? Is it good enough where it is? I am only using it as a 10 MHz reference now, but I may care about the 1 PPS later.
>>> 
>>> Paul N1BUG
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