With the digital switch over well under way here in Europe I thought I’d try pick
out a few salient points that may make life easier for those grappling with what
to do next.
A typical question for English speaking ex-pats here in France is : “ How can I get
English and French digital TV without the house looking like Jodrell bank?”
You already have English TV by satellite and French analogue TV via a standard roof
When the digital switch over arrives you just need a set-top-box decoder, normally
around 30Euros from the hyper-markets or Brico Depot. If you already have a French
digital TV (after March 2008), it has the TNT decoder built in and you should just
need to do a new channel search to find the new channels. If you have an HD digital
TV then it should automatically detect and display the HD transmissions.
In addition to the above error, be very wary of all purpose, multiple LNB brackets
available on the Internet, many of them just won’t work.
Although there is no quick fix or widget to buy, it is possible to work out what
you need to do. The first thing is to understand what look angles are all about.
Basically the ‘look angles’ are the Azimuth and Elevation angles to the satellite
with respect to your location on the Earth. The AZ and EL angles for ASTRA2 relative
to our location in Ambert are illustrated in the diagram below.
Note : Digital TV does not degrade gracefully when the signal is poor, you’ll either
get it or you won’t. Digital decoders require a signal level of around -70dBm, although
tests with mine showed good operation down to -80dBm. Basically if you’ve got good
clear analogue reception you should be OK for the digital signal.
You already have English TV by satellite and would like to receive French TV off
the same dish (often because the terrestrial signal is poor or unavailable).
The first mistake people usually make is to go and buy a dual head LNB for ASTRA1
(19.2E ) and HOTBIRD (13.0E) reception. These mono-block heads are designed for
receiving 2 satellites with 6.2deg orbital separation . The satellites you require
are ASTRA2 (28.2E) for the English and ASTRA1 (19.2E) for the French, these satellites
have 9deg orbital separation.
The ‘look angles’ for a satellite can be calculated given the satellite’s orbital
location and your location (Lat, Long). An excel spread sheet tabulating the satellites
most commonly used in France can be down loaded from here, just enter your Lat, Long
at the top of the sheet.
The AZ values calculated are with respect to true Geometric North. This is different
to the magnetic North found by your compass and would usually have to be corrected
for. Fortunately the current difference (declination) here in central France is only
0deg 14sec E (as of 2011) and not worth worrying about. The declination for your
particular location can be found here (follow links to declination calculators).
From the spread sheet the ‘look angles’ for ASTRA1 and ASTRA2 at our location here
in Ambert are :
ASTRA1 (19.2E) AZ=158.82 EL=35.42
ASTRA2 (28.2E) AZ=147.48 EL=32.30
Difference AZ=+11.34 EL=+3.12
Because ASTRA2 is nominally targeted at the UK, the signal we get here in France
is more by accident than design. As a result the signal is the weaker of the 2 satellites
and should be the primary alignment for the dish. Despite improvements in LNB noise
figures (0.2 dB typ) an 80cm dish is still preferable for reception in central and
Having aligned the dish with a single LNB on the ASTRA2 satellite we need to add
a second LNB for the ASTRA1 satellite. This needs to be position such that the dish
with this second LNB ‘looks’ 11.34 degrees West and elevated 3.12 degrees with respect
to the primary alignment.
Knowing some basic dish parameters it is possible to calculate how the second LNB
needs to be displaced from the first. For small angles the direction of the second
beam can be assumed to work in the same way as the reflection off a mirror (this
is a very simplistic view but adequate for our purposes). That is the angle of incidence
equals the angle of reflection. For an 80cm dish with a focal length of approx 500mm
(most 80cm offset dishes have focal lengths between 480 and 520mm, the focal length
is the important value).
9mm of lateral LNB movement = 1 degree change in look angle .
So, for our dish and location the second LNB needs to be displaced 9x11.34=102mm
horizontally and 9x3.13=28mm vertically. The sense of displacement is like the reflection
off a mirror, so looking at the front of the dish the second LNB should be to the
right and lower with respect the the primary LNB.
Note these are only theoretical values. In practice the exact values for best reception
will depend on the primary alignment on ASTRA2. The important thing to observe
is that the displacement is both horizontal and vertical, something that many commercially
available multi-LNB mountings do not cater for! The diagram below shows the geometry
we require for reception at our location, Hotbird has been included for completeness.
Schematics are all well and good but ultimately we need some sort of practical solution.
Although I have an adequate terrestrial signal I thought I should offer some sort
of practical solution for the satellite option. The pictures below show the results
of my experiments using some M4 x 40mm nuts and bolts and some 25mm x 2mm flat Aluminium
bar. The arrangement uses the bottom half of the existing plastic clamp for LNB #1
and the top half of the original clamp as the lower lower half for LNB #2.
Last but not least you will need to buy a TNT satellite decoder. Despite being free-to-air
a standard free-to-air box will not work; so no, your existing UK box will not work.
For ASTRA1 TNT reception you will need a TNTsat decoder. (For ATLANTIC3(5W) TNT reception
you would need a FRANsat TNT decoder. Most French viewers go with FRANsat because
they were already lined up on ATLANTIC3 for the analogue satellite tv signal).
Connections wise, the easiest thing to do is run 2 cables :
LNB#1 to decoder #1 (UK tv)
LNB#2 to decoder #2 (TNT sat)
You can download this whole page in .pdf format here.
As a final homage to analogue tv transmissions I thought I’d add this photo of our
local tv spectrum. It shows the analogue (spikes) and the digital (square blocks)
co-existing between 540MHz and 740Mhz. Rather fittingly they are displayed on an
old analogue Spectrum Analyser (HP 8558B).
Centre Freq : 640 Mhz
Span/Div : 20 Mhz
Res BW : 300Khz
Ref Lev : -10dB
Scale : 10dB/div
If all this has piqued your interest in Antennas the really techie stuff can be found