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A few days ago, Geoff, GM4ESD, provided spectrum analyzer
data showing the short wave broadcast signal levels observed in the 7 MHz band
from his location in Scotland. Typical signal levels run 0 dBm into his antenna,
described as a 40m dipole at 70 ft AGL, fed with open wire.
This raises the question as to what signal levels are seen on
the east coast of the US. I've done a cursory survey over the 19-21 October 2008
period using an Advantest R3463 spectrum analyzer.
I've run the spectrum analyzer in "peak hold" mode which
means that the display shows the strongest signal received during the time since
peak hold was engaged. I've connected the spectrum analyzer to my main antenna,
which is an M2 skip-band log periodic, 7-7.3 MHz and 10-30 MHz, at 100 ft above
ground level, pointing north east. The frequency span is 0 to 20 MHz.
Extensive comment on these plots is not necessary. Each
plot has the date and time it was taken and I've placed a marker at the
strongest signal. Its frequency and amplitude are displayed at the top of the
image.
The strongest signal seen is -2.2 dBm, at 9.62 MHz.
Signals in the -10 dBm or stronger range are also seen in the 11.7 MHz and 6.9
MHz range.
Since the antenna I used rolls off sharply below 7 MHz,
signal levels below that frequency are significantly reduced from the values
seen with an optimized antenna.
Note that signal levels in the 7 MHz range are often in
the -10 dBm or stronger range. This places a major stress on a receiver's front
end stages, as practical input bandpass filters are generally not sharp enough
to provide significant attenuation to these very strong signals between 6.5 and
7.5 MHz. Indeed, many strong signals are actually within the 7.0 - 7.3 MHz band.
Of course, a general coverage receiver, even with
sub-octave preselectors, will have even wider front end selectivity.
0-20 MHz captures |
