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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:<br>
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<img alt="floor" src="cid:part1.04060509.05070800@bartk.us"
height="449" width="800"><br>
<br>
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.<br>
<br>
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:<br>
<br>
<img alt="peaks" src="cid:part2.03050101.00080003@bartk.us"
height="449" width="800"><br>
<br>
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.<br>
<br>
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).<br>
<br>
--Bart<br>
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