[P20] Another hurdle to overcome!

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It often happens that an obstacle presents itself in front of our satellite dish. A roof, hedge, wall or a tree are the usual examples. The entire surface of the dish must see clear sky in the direction of the desired satellite. (assuming for the moment that the dish is fixed and only one LNB is fitted) Luckily each TV satellite sits almost perfectly still in the sky as seen from the ground. It doesn't matter if there are mountains or skyscrapers blocking your view in every other direction. All that matters is that the dish can see the required satellite without obstruction.  

By a happy coincidence Astra2 at 28E has a local altitude of almost exactly 25 degrees for my site. 25 degrees is a handy altitude angle to have. Because you can divide the distance to the object by two to get the allowed height of that obstacle. Well, near enough. Tan 25 = 0.46 rather than 0.5.  So halving the distance to check the allowable hurdle height is slightly too generous. Tan 27 degrees is nearer to a true half. So for 25 degrees satellite altitude I should really use 0.46 x distance to the potential signal blocker. Not quite so liberal as using a half x distance if it is really critical.

The simplest way to check whether the dish is seeing the satellite without any blockage is to draw a line on a piece of paper at the local altitude angle of your intended satellite.  Now mark centimetres along the bottom and draw vertical lines from your marks. Measuring up to the sloping line at any given distance point will give you the allowed height of any obstacle in the signal path to your dish. You can use the markings for any scale that suits you. Just remember to use exactly the same scale for height up to the sloping (altitude angle) line as your horizontal markers.

And, don't try and mix your units! Professionals have built bridges which did not meet in the middle because they mixed their metric units on one side with their imperial measurements on the other! No doubt the same problem has occurred with tunnels where two nation's contractors mixed their different units of measurement.

The bottom of the dish is the critical point from which to check that it can see the satellite without obstruction. It is here that any "shadow" on the signal path will be most likely to occur. Particularly with large dishes. Fortunately, many dishes are a mounted a certain distance above the ground. The height from the ground to the bottom rim can be added to the allowed height of the obstacle.

A simple example using my own site: Taking 25 degrees as the satellite's local  altitude I measure 5 metres to the hedge in front of my dish. Halving this figure gives me 2.5 metres allowed height. [Actually 2.4m] But the lower rim of my dish is a metre above level ground. So I can have a hedge 3.5 metres maximum height in front of my dish at 5 metres distance. Any higher than this and the hedge will block some signal arriving at my dish. Which would be very undesirable for fringe reception where every dB of clean signal is vital.

My own front hedge is 3 metres high and looks taller. From the dish it looks as if it might well block the signal from reaching the bottom of the dish. Those new spring shoots will have to be clipped back very soon! Raising the dish is another possibility but rarely practical.

Now I know that I must have the bottom of my 2.2m dish at least a metre from the ground too. Or I will need a lower hedge. Which would not offer the same degree of shelter from the prevailing wind.Though I suppose I could cut a lower step or notch out of the hedge in front of the dish in the direction of the satellite. Though this might make the dish far more obvious to observers beyond the hedge. Each situation is different, of course, but what the neighbours can't see won't hurt them.

Hedges can be measured with a long stick or pole and then the stick itself measured when it is lying flat. Insulating tape can be applied as a marker if the stick is much longer than the obstacle is tall. Then stand the stick up again and check from a reasonable distance that the tape coincides with the top of the obstacle. Now measure the stick or pole from the bottom up to the insulating tape to find the correct height. Much easier than trying to force a floppy tape measure to stand upright! Snake charmers excluded, of course.

Conversely, you can mark a pole at the correct allowable height for a known distance measured from the dish. Then stand the pole up using something like a stepladder or braces for support. Now use a small mirror at the bottom of the dish to check that the reflection of the hurdle beyond the pole marker is lower. Or you can grovel on the ground with your eye at the level of the bottom of the dish if you prefer.

If the hurdle is lower than your marker on your pole then the bottom of the dish is not in the signal shadow of the hurdle. If it is quite close to the marker and the hurdle can grow then clip the hurdle or move the dish. Hedges and trees have a nasty habit of growing faster than you would ever believe! So siting a dish with a fast growing hurdle in line with the satellite will almost certainly end in tears. With no useful signal sooner than you think! How much concrete did you put around your dish's support pipe? Well now you have to do it all over again with a new pipe. Or buy an expensive ground stand to allow some mobility.

If you can't use a measuring pole or stick because of the landscape in front of the dish or the dish is high on a building then life is rather more difficult. Taller and more distant objects are much more difficult to measure without a stick or measuring pole. Not everybody owns a clinometer but you can measure your own shadow on the ground. (or that of a stick) Then measure the length of the tree's shadow straight afterwards. Leave it too long and the sun's own altitude angle will have changed. Giving your calculation an inaccurate result.

Knowing the ratio of your own height to the length of your shadow you can use the same ratio to calculate the height of the tree (or other obstacle) from its shadow. The same goes for a building. This simple trick needs sunshine of course and is much easier with a reasonable solar altitude. It probably won't work well at noon at the equator nor at the Poles.

You can also use a known object in a photograph taken from some distance to measure an obstacle at the same distance in the same picture. Measure the object size in the image and then the obstacle itself. Use ratios again to determine the true height of the obstacle. Once you know the height of the object you can use a map to measure the distance between your dish and the distant object. There are measuring tools on Google Earth which can be very useful. Now add any extra height of the dish compared with the distant hurdle to your allowable hurdle height at that distance.

There are other methods to check whether obstacles will block your signal but I have already talked the subject to death. You could check which two days of the year the sun is at the satellite's altitude. Though it might be quite difficult to find this out in the exact direction of the satellite. There are plenty of free solar altitude calculators online. The problem is using them to find when the sun is at the correct altitude at the satellite's correct azimuth for your location.  

If an obstacle casts a shadow on your dish then something must be done about it. Owners of buildings, hedges and trees are highly unlikely to be sympathetic to your satellite reception needs. All you can do is raise the dish, move it back from the obstacle, remove the obstacle (if you can) or move to another location. All these possibilities have potential domestic, neighbourly, physical, visual and technical  limitations as well as almost inevitable expense.  

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