How large a dish do I need?

Practicalities

Before you determine how large a dish must be before you can receive a specific channel, there are a number of operational factors that need to be taken into account:

1. The make and efficiency of the dish – a solid dish will outperform a mesh dish of the same size. Similarly, a one-piece solid dish will be better than a segmented solid dish.
2. The quality of the LNB – the lower the noise figure (dB) and the smaller the local oscillator drift (KHz), the better the performance of the LNB. Note that some manufacturers only quote the best noise figures for their LNBs, rather than the average figure across the frequency band.
3. The design of the feedhorn – a mismatched feedhorn (or LNBF) and dish will impact detrimentally on performance.

Mismatched offset feedhorn with prime focus dish


Matched prime feedhorn with prime focus dish

Further archived information from Swedish Microwave.

4. The sensitivity of the receiver tuner – there can be a wide variation in the sensitivity of satellite receiver tuners. The tuners in PCI DVB-S cards, for example, are generally not very sensitive.
5. Weather – a Ku-band signal will be seriously attenuated by moisture. This is less important for lower frequency C-band transmissions.
6. Error correction – for digital transmissions, the lower the error correction, the larger the dish required to maintain a stable picture. For example, if you were able to receive signals with an FEC of 1/2 without any problems, signals with an FEC of 2/3 would require an antenna that is 1.3 times larger than the original antenna. Signals with an FEC of 3/4 means an antenna 1.4 times larger and with an FEC of 5/6 a factor of 1.5 is required.
7. Cable quality and length - poor quality cable or long runs of coax will impact on performance. A detailed analysis is provided here.


Theory

With the above points in mind, we can now look at interpreting footprint charts. These charts can be split into 3 types:

Global beam – 40% coverage of the earth’s surface. The signal strength degrades gradually as you move away from the focal point i.e. fringes.
Hemi beam – 20% coverage or one half of the hemisphere.
Zone/Spot beam – confined to a small area, but usually with high power output. The signal strength falls abruptly as you move away from the target zone.

Footprints for Intelsat 705 50W


The charts generally show a number of lines enclosing areas of the world that are marked with numbers indicating either signal strength or the size of dish required for reception.

The signal strength is measured in EIRP (Equivalent Isotropic Radiated Power) in dBW (Decibels in relation to 1 Watt). The EIRP figure indicates how much of the energy transmitted by the satellite actually reaches the surface of the earth at a given location. A look at a typical footprint map shows the EIRP values from the target zone (where the signal is strongest) outwards and appear to decrease gradually towards the footprint fringes. Note that a 3dBW decrease actually means that the signal strength has been reduced by 50%!

Ku Spot Beam for Intelsat 705


Used in conjunction with the following chart, we can now determine what is the minimum diameter dish required to receive a specific channel. Remember, however, that these values are only a guide and will vary depending on your actual system.

Ku-band EIRP values vs. dish diameter (LNB noise figure of 0.7dB)

EIRP	Size	EIRP	Size
55dBW	40cm	43dBW	100cm
54dBW	45cm	42dBW	110cm
53dBW	50cm	41dBW	120cm
52dBW	50cm	40dBW	120cm
51dBW	55cm	39dBW	135cm
50dBW	60cm	38dBW	150cm
49dBW	65cm	37dBW	180cm
48dBW	70cm	36dBW	240cm
47dBW	75cm	35dBW	300cm
46dBW	80cm	34dBW	355cm
45dBW	90cm	33dBW	400cm
44dBW	90cm	32dBW	450cm

Direct links to the satellite operators' EIRP charts can be found at Lyngsat. Satcodx also provide EIRP charts in a single, standardised format.


More Caveats

EIRP charts are often drawn up even before a satellite has been launched! The actual signal strength on the ground may differ significantly from that in the chart. Other obstacles to reception include temporary fluctuations (deliberate or otherwise) in signal strength output on individual transponders or ‘wobbles’ in a satellite's orbit – this will have a large impact on those with fringe reception. For these reasons, reception reports from other users can often be a better guide than EIRP charts. Kingofsat is a useful resource in this regard.


Finally

A common question asked is how can I improve the performance of my system in order to receive a certain channel. Invariably, increasing the size of the dish is the most effective option as the surface area increases with the square of the radius. For example, a 90cm dish with 60% efficiency will have an antenna gain of 38.1dB at 11.000GHz. A corresponding 130cm dish will have gain of 41.3 dB, representing an increase of over 3dB i.e. a doubling in the effectiveness of the dish at gathering a signal.

With satellites, as with most things in life, size really does matter!