[time-nuts] Difference in antennas

[John Ackermann N8UR]

Thanks to Bob, Achim, Greg and others for their input on this.

It seems the real challenge is that the outputs available from most GPS
don't provide the kind of info that would let you know if IMD or
overload are occurring.  In most cases we can only infer from
positioning performance, which is hard to quantify or correlate.

I like the idea of inserting attenuation until the SNR or Cn values
start to go down.  That may be the most practical solution.

John
----

On 11/21/19 4:05 PM, Greg Troxel wrote:
> John Ackermann N8UR <jra at febo.com> writes:
> 
>> One related question, especially with mixed systems -- how do you tell
>> if you have optimum signal level at the receiver?
>>
>> Most show some sort of SNR or Cn value.  What should we look for?  What
>> are the indication of *too much* signal?  One issue in particular is how
>> to handle a modern GPS that expects modest antenna gain when it's
>> plugged into a system with a 50dB gain antenna at the top.
> 
> Too much gain can manifest in at least two different ways:
> 
>   1) intermodulation distortion in the preamp
> 
>   2) distortion/overload in the GPS receiver
> 
> Adding an attenuator or cable as someone suggested can help you
> determine if the preamp gain is excessive *given your cabling and GPSr
> frontend*.  If you add 10 dB of loss, and the C/N0 doesn't change,
> arguably you have gain you didn't need, and which therefore has elevated
> risk of IMD.  If it goes up, you (mostly) know you are overdriving your
> receiver (which would be surprising to me).  If it drops, then you
> probably need most of the gain.
> 
> This is tricky, because a system with too much preamp gain will be prone
> to IMD if other signals appear but may operate just fine when they
> don't.
> 
> That said, I am unclear on:
> 
>   typical filtering before the antenna preamp (very little in a
>   dual-frequency antenna?)
> 
>   3rd-order IMD dynamic range in these preamps
> 
>   strength of non-GNSS signals that appear in the filter passband
>