TELE-audiovision - The World’s Largest Digital TV Trade Magazine - page 45

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1 -12/2013 —
TELE-audiovision International —
全球发行量最大的数字电视杂志
DIY DVB-T Antenna
17. DVB-T Signal of our external
antenna. Reception is quite
good.
18. A good indicator for
DVB-T reception quality is the
constellation diagram. As long
as all dots are closely within
the centre of each square,
everything is fine.
19. The Deviser S7000 allows to
set the resolution bandwidth in
spectrum mode. This is unique
among the meters I have tested
so far. A smaller bandwidth will
feature higher resolution, while
a bigger bandwidth will result
in a slower spectrum refresh
rate. Because we are looking at
the specific DVB-T transponder
at 754 MHz, a span of 16 MHz
is sufficient and thus even the
lowest resolution bandwidth
setting will produce a real time
refresh rate. The resolution
is so good, that you can
practically see the carriers.
20. The same signal, but in
combined spectrum and
waterfall diagram view. This
allows to monitor the spectrum
over a period of time, while
simultaneously monitoring the
spectrum in high detail.
21. A generic rod antenna
provided with a USB DVB-T
receiver: this antenna was not
designed specifically for a
certain frequency; its length
does not match the SDN
frequency of 754 MHz as used
in Portugal.
22. As a result, the signal
power is only 41.7 dbµV and the
CBER is rather low, though still
correctable since the VBER is
under 1.0E-7.
23. Looking at the constellation
diagram using the generic
rod antenna, it is obvious that
the signal quality is far from
perfect.
24. First step of the DIY
antenna: strip the rubber off
the cable, maintaining the
length calculated for the SFN
frequency.
25. Flip the mesh back and
leave the inner wire with the
correct length: you just got
yourself the “poor-man’s”
DVB-T antenna.
26. Amazingly, the signal power
is on average 5 dBµV higher
than with the generic antenna.
27. Again, using the waterfall
diagram it is easy to adjust
the antenna so that reception
is maximized. It is strange
to think on how it has been
possible in the past to fine tune
the alignment of the antenna
without this functionality. One
thing is for sure: the waterfall
diagram is an outstanding aid
for installers.
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