Among the most requested items in RWT's systems consultancy days was the
dish size versus EIRP chart. We have now updated this in six flavours, which
we present for the use of our readers.
For each transmission type the required grade of service is selected from one
of 6 or 7 bands:
DTH is the
central band and shows the range of recommended dish size for reliably high
quality individual Direct-To-Home reception, based on current standard to good
performance systems under moderate conditions of climate and elevation angle.
The higher bands, SMA,
CAB, and
RBR, include the additional performance
and fade margins required for SMATV, Cable Head and Re-Broadcast applications,
respectively.
Of the lower bands, MAR
indicates Marginal: pictures are clear of sparklies under clear-sky
conditions with zero pointing error, but will be rapidly impaired by any signal loss.
In digital terms this corresponds to just meeting the QEF (quasi error-free) condition.
THR indicates operation at
threshold: expect some threshold artefacts to be visible most of the time. And
SUB-threshold reception is for
anoraks only: the enthusiasts and DX-ers will enjoy the exotic shimmer their
receivers impart to the most ordinary sets of colour bars. With digital transmissions
there is no sub-threshold; this region is equivalent to loss of signal.
Where the bands terminate in vertical lines this indicates some external
constraint on any further reduction in dish size -- typically the risk of
adjacent-satellite interference. But in all cases a larger dish than indicated
may be used, so allowing DTH to overlap with SMATV, SMATV with Cable,
and so on.
The six transmission "flavours", each with its own chart, are:
PAL 27
Covers the Astra type signal, assuming a PAL (or SECAM or NTSC) transmission
of some 16 MHz/V (peak to peak deviation) at Ku-Band in a 27 MHz receiver
noise bandwidth. An 8dB extended dynamic threshold is assumed -- see
Threshold. Allowance is made for a reducing
value of antenna efficiency above 3m aperture.
PAL 36
A Eutelsat or similar full-transponder analogue transmission, at 25 MHz/V in a
36 MHz Ku-Band transponder. The closer spacing of the curves is typical of
wider deviations, with a steeper decline below a higher threshold, but also an
improved signal to noise ratio once clear of that threshold.
MAC 27
B-MAC, D-MAC or D2-MAC modulation in an Astra or similar transponder. The
threshold region is narrowed by the steep failure mode of the audio/data channel,
and above threshold the enhanced colour SNR reduces the high end dish size
requirements.
C-Band 36
A full-transponder (36 MHz) PAL, SECAM or NTSC transmission of 21 to 25 MHz/V,
downlinking in the 4 GHz band, with a 25K LNB. The steepening of the curves below
1m antenna size is due to feed blockage and diffraction causing low illumination
efficiency. Atmospheric influences are much reduced at these frequencies, resulting
in smaller margins being required. Note the range of EIRPs covered is 10dB below
that of the Ku-Band charts, as encountered with typical satellites.
Euro DVB 33
This assumes an Astra or Eutelsat full-transponder multiplex, 55 Mbits/s (27.5 MS/s)
QPSK with 3/4 rate FEC. Moderate assumptions are made on satellite loop
degradation and modem implementation margin.
US DSS 24
Here the transmission is at 40 Mbit/s (20 MS/s) in a 24 MHz channel (e.g DirecTv),
with 6/7 rate convolutional coding. This lower level of redundancy means about a
1dB higher G/T (larger dish) is required for a given EIRP, relative to the European
example, despite the lower symbol rate.
Copyright © 1996 Real-World Technology Ltd.
Threshold, common misconceptions addressed.
Specification features of RWT compact tuners.
Existing tuner designs.
The RWT story so far.
News from RWT.
Links to interesting (this isn't?) Web sites.
