[time-nuts] Re: GPS antenna locations

[N1BUG]

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 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.

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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