[time-nuts] Difference in antennas

[Greg Troxel]

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