[HamWAN PSDR] Perhaps 900MHz isn't a complete write-off

Bart Kus me at bartk.us
Wed Oct 28 18:54:23 PDT 2015

Some of you may know I did a survey of 900MHz recently, along with the 
real-world modem tests.  The spectrum analyzer inside the modem showed a 
very dire picture of the band, with the noise floor averaging about 
-60dBm.  Today, in preparation for 1.2GHz antenna installs, I ran some 
coax to the roof and was able to see the 900MHz vertical there directly 
on a proper spectrum analyzer.  The spectrum story is quite different!  
Here's a view of a typical spectrum from 902-928MHz:


The reference line at the top is -20dBm and each vertical graticule is 
10dB.  The noise floor looks to be about 7 graticules below -20dBm, or 
-90dBm.  There is about 2dB loss in the 45ft feedline, so let's call the 
floor -88dBm.

There are however some sparse transmissions in this band.  I wish I 
could find a way to measure their duty cycle or plot their average 
power.  Sadly the best I can do is to record their maximum power using 
the peak-hold mode of the spectrum analyzer.  I captured about a 10 
minute sample of air time, and here are the results:


The bright dot is the peak marker, and it reads -42dBm, which is really 
-40dBm at the antenna after you account for the coax loss. The peaks are 
all suspiciously uniform in power, so this may be a single transmitter 
just jumping around the spectrum with each burst.

I can't explain why the Mikrotik 9HPn reports such a vastly different 
spectrum.  It may be mixing the 860-900MHz cell band into the 900MHz 
band?  I need to perform some further tests to figure this out.  There 
are definitely strong signals in the 860-900MHz range (not shown in 
these photos).


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