A bigger dish, or a better lnb?

Walking through any residential area you see lots of small, cheap and effective satellite dishes. Thanks to the latest generations of high power satellites just a small dish will get you perfect reception of DirecTV in the USA, or SkyDigital in the UK, or Viasat in Scandinavia.

But move outside these tightly focussed satellite footprints where all the satellite transponder power is concentrated, and  minidishes just can't hack it any more: there just isn't enough signal for them to work. Endless discussions on Usenet and many satellite forums recycle the same question time and time again - "Will a low-noise lnb help me receive more satellite channels without buying a bigger dish"?

There never seems to be a really good, believable, answer to this problem - perhaps because it is phrased in such woolly terms. Now the professor does not have an vast antenna farm at his disposal, or a bucketload of lnbs to test, but with the aid of that wonderful computer program, 'Satmaster Pro' available from http://www.arrowe.com, perhaps some kind of answer is possible. You can download a demonstration version of the program yourself, free of charge, from this website, and use it to get a feel for how satellite reception really works.

(All the calculations used in this article were based on using Satmaster Pro Mk6.2. The latest version available from the website is now Satmaster Pro Mk6.4.)

As a worked example, let us consider the enthusiast in the U.K who wants to watch the fabled 'nordic beams' of the Sirius satellite at 5°E. He already has an 80cm dish and a 0.7db lnb which gives him almost perfect reception from the Astra and Hotbird satellites.

Swinging the antenna round to 5°E however sees a different state of affairs. Most of the horizontally polarised channels on the European beam come in quite well, but there is barely a sniff of signal from the mainly vertically polarised 'nordic beams'. In the evening, when the signal from this satellite drops even lower, there seems to be no chance at all of ever seeing a picture from the nordic beam.

Let's look at the footprints.

The satellite operator's map of the nordic beam indicates that a dish of greater than 1.2 metres is necessary in the UK.

My experience indicates that the footprint map is a little on the conservative side, and that it is possible to receive the channel on smaller dishes than that, on the eastern side of the U.K. The further west you go, the weaker the signal.

More useful to us is a map that shows the EIRP - the equivalent isotropically radiated power like this:

 now this map gives us some real numbers to work with and plug into the program!

The European beam

or, more usefully, in terms of EIRP

So, it would appear that we have about 51 dBW on the European Beam, but only about 46 dBW on the nordic beam.

Starting with the European beam at 51dBW, using an 80cm dish with 65% efficiency, a 0.7db lnb with a gain of 55db, and assuming an antenna noise temperature of 43°K the end result is an Eb/no of  12.32 which is more than enough to guarantee superb reception, we are only looking for a value of about 9!

For those mystified by Eb/no, it's a way of measuring the noise on a digital signal in terms of Energy per bit, the Eb part, and no which is a measurement of the noise. When we also factor in the FEC ratio, the forward error correction that the broadcaster uses to further reduce the effect of noise on the signal, we can derive a bit error ratio, BER which is a good measure of the amount of picture disturbance we will see.

Basically a bit error rate of one in a thousand is useless, one in a million is poor, one in a billion is pretty good, one in a trillion is near perfect, one in a quadrillion is called quasi error free - or less than one uncorrected error a day!

We can see the nordic beam results by plugging exactly the same values in to Satmaster Pro, except for the much lower EIRP value of 46dBW.  We now get an Eb/no of  only 7.68. The reception is bound to be a little patchy, as that equates to a bit error rate, the BER, of about one error in every million bits.

So what happens if we plug in a magic 0.4 db lnb!

Keeping everything else the same, but substituting a 0.4dBlnb this time, gives an Eb/no ratio of  9.64, which is above our target value of 9. This translates to a final bit error ratio BER of one error in every 100 million bits - not brilliant, but quite acceptable. So, theoretically at least, this solution could work!

But let's consider keeping the old 0.7db lnb, and increasing the dish size instead. How big a dish would we need to get an Eb/no ratio of more than 9?

The somewhat staggering answer is that a dish of only 84cm will give us an Eb/no ratio of  9.19!

A dish of 90cm diameter, the next practicable size up from 80cm., gives us an Eb/no ratio of 9.75.

The moral is, that for 'real world' size dishes, less than 1.2 metres in diameter, stepping up to a bigger dish is probably going to be much cheaper than buying a magic lnb, and gives a better overall result too!

This has all been very interesting, but the results are all very theoretical. To rush out and spend money on the basis of this analysis alone might be a trifle foolish. Some points to consider are:

Is the noise figure of your present lnb known accurately? Just because it says 0.7dB in the specification can you be sure that it really is a 0.7dB lnb?

Similarly is the 0.4dB lnb truly a 0.4dB lnb? Any measurements made at this level may have a considerable margin of error, in fact we cannot even be sure that the measurement techniques used by different manufacturers would return the same results on identical lnbs. 

My calculations assume some parameters are stable when they are unlikely to be so in practice. For example, I assume that the antenna efficiency remains constant at 65% and that the coupling losses between the dish and the lnb remain constant at 0.4db. This is unlikely to be true in reality, different lnbs may match the dish more or less accurately, and larger dishes are, in general more efficient than smaller ones.

Tweaking what you have already got may well give you a similar gain in performance to fitting a lnb with a 0.3 dB lower noise. After all 0.3 dB is only another way of describing a 7% improvement.

Although this analysis is fairly thorough, it does not consider everything that may have a bearing on the final result, such as local oscillator phase noise in the lnb, or the behaviour of your receiver when presented with spurious signal outputs from your lnb.

Here are the parameters I used.

Why not download the program and have a play yourself - there's a lot more in the program than I've shown you here. There are plenty of example files and help texts. The only disadvantage of the demo version is that it is not possible to save any of your results, for that you have to pay!

The Professor