[Ground-station] UI for SDR

[Mike Parker]

> On Jun 30, 2018, at 1:23 PM, Phil Karn via Ground-Station <ground-station at lists.openresearch.institute> wrote:
> 
> 
> I recently bought a bunch of Ubiquiti access points that come with their
> own management package called "UniFi". I was reluctant at first since
> it's proprietary to them, but it looks fairly nice. You run it on a
> local system (including Linux) and talk to it with a web browser. Among
> its utilities is a way to command an access point to do an RF survey and
> plot local activity as measurements of signal strength and intensity (%
> of time busy) vs frequency.

First, I agree with most of Phil’s points.  I do have a comment to add about display of digital signals.

I put in a point-to-point 5GHz Ubiquiti link similar to Phil’s last year and agree that their analysis tool has some cool features.  It does have one important issue though that seems to be common to many digital systems.  That is, you don’t see a signal until the link amplitude breaks a certain threshold.
This makes it very difficult to align antenna pointing with the distant station until you get both antennas pointed close enough in azimuth to have the signal above this threshold amplitude.  Without visual line-of-sight it took a lot of trial and error with pointing of antennas on either end until I got both pointed close enough to get an initial signal strength reading.
If you have ever tried to align pointing of a digital TV's antenna with a weak station, you will recognize the problem.  Pointing the old AM video TV antennas was usually possible even if you couldn’t actually see much of a signal.  But you don’t see any signal on a digital TV until the demodulated signal is almost perfect.

The threshold effect of a digital point-to-point link is worse because the antenna’s 9 degree beam is much narrower than a TV antenna’s beamwidth.  Out of 360 degrees, there are 40 possible azimuths to point at at.  By knowing, sort of, the correct direction to point in, I reduced this to about 5 choices. But since there are two antennas to point, the number of pointings that you must try is squared (25 combinations) if the link is marginal to start with.  Again, you get no indication at all that there is anything working until you get the link above threshold.  Then adjusting pointing at each end to get the last 10 or 15  dB of link strength (in my case) is straightforward.

So I conclude that a needed display feature is to be able to detect a signal and make a rough signal strength measurement EVEN THOUGH THE SIGNAL IS MUCH TOO WEAK TO DEMODULATE.
Ideally, I would like to detect and display the presence of another station whose sidelobe level is 35 to 40 dB weaker than when its main-beam is pointed at me.  Then I could detect the presence of another ham's station (even if his antenna was pointed away from me) as my beam swings past his azimuth.  Once I am pointed at him, then he could hear me and swing his beam to point at me.

This might be achieved in a variety of ways, depending on the details of the digital signal.  A pilot tone, an unmodulated CW carrier, or even an unmodulated digital sequence that can be detected with conventional spectral analysis comes to mind.

Of course there would be some interesting scenarios if the strongest path at the start of this procedure were a multipath echo arriving at my station from a scattered path (say off the side of a nearby mountain).  But that’s what keeps being a ham interesting isn’t it?

73,
Mike Parker, KT7D