Source: Real-World Technology Ltd
Charts of TVRO Dish Size vs. EIRP:
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: |
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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. Allowance is made
for a reducing value of antenna efficiency above 3m aperture.
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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.
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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
requirement
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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.
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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.
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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.
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