Posted by Joris Everaert on
URL: https://www.es-forum.com/Intermodul-harmonics-mixing-signals-at-GSM-base-stations-Aaronia-Spectran-tp1543519.html

Hello,

I have a portable spectrum analyzer from Aaronia (Spectran HF-6080).
The measurements in the GSM900 & 1800, and UMTS bands are (seem to
be) good and normal.

But at most GSM/UMTS base stations, I found that for example in the
5000-6000 MHz range and at 3000 MHz, there are several signals which
are higher in intensity than the actual GSM or UMTS frequency band
signals.
The signals are certainly coming from the base stations. The high
values between 2099 - 3011 Mhz (and further) are only detected at
base stations where GSM900/1800 and UMTS antennes are installed,
not at those without UMTS. The 3000 MHz frequency is normally
military. The high signals between 5000-6000 Mhz are almost always
found at all base stations, and seem not to be WLAN (also not the
typical sound from WLAN/WIFI).

There might be some (small) errors in the measurements of the
Spectran
in these higher bands (?), and some "mirror frequencies" (not real)
might exist in the range that I am measuring (at pulse mode), but
I measure several ranges separately (for example the 2099-3011 MHz,
5470-5725 MHz, 5725-5825MHz ranges) and found that the high signals
are very clear and also hearable (audio analysis), in many cases at
around 200 meters from the base ststions ca. 2-6 V/m, in contrast to
the for example 0,2 - 1,2 V/m values in the GSM / UMTS bands !

I think these high signals can be real, possibly 'intermodulation
interference signals' (and out of band). I read a lot on the
internet
about measured signals and that all spectrum analysers have the
problem
of second and third (...) "harmonic frequencies", "intermodulation
interference" from several different base station antennas, "mixing
products from harmonics", etc, and that the problem is becoming more
important, as more different antennas are being installed at each
base station.

See for example
http://www.tek.com/Measurement/App_Notes/2G_14758/eng/2GW_14758_0.pdf
and
http://www.summitekinstruments.com/oasis/documentation.asp
click on "White Paper".

The 900 (gsm) and 2100MHz (umts) signals from base stations could
indeed also mix and give something around 3000MHz, and the readings
between 5-6 GHz could also be explained by (third order) harmonics
and mixing ?
The not-real 'mirror' signals ("in" the measured range) are normally
weaker, but can the real (really in the air) signals
like "intermodulation/mixing/harmonics" signals be more stronger
than
the original signals separately ?

At 2400MHz, 3000MHz, 3500MHz, 5000-6000MHz, the sound (audio
analysis
of pulses/modulations) with Resolution Band Width (RBW) at 3 MHz and
1 MHz is a loud constant noice, almost the same (?) like at
GSM1800/UMTS
(base control carrier) but it seems a little different and in most
cases
more intense. At RBW 300 and 100 kHz, the sound is clearly with
small
hard pulses (around 10 Hz ?, like woodpecker). Can anyone explain
this ?

I also noticed that when I put my GSM on (in speaking mode), high
signals
in the 3000 MHz and 5000-6000 MHz range are also found (and in audio
analysis also clearly the sound of a GSM). This can also be
explainedby "intermodulation/harmonics/mixing" signals ?

Best regards,
Joris Everaert
Biologist
Belgium