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?”

 

Scenario 1

You already have English TV by satellite and French analogue TV  via a standard roof mounted antenna.

 

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.

Multiple LNB’s for English and French TV

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.

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Scenario 2

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.

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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 southern France.

 

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.

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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.

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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).

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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).

 

Settings

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 here.

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Click image to enlarge

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