Public broadcasting satellites orbit Earth high above the Equator once a day in the same direction as it
spins, so appearing geo-stationary, motionless viewed from Earth. Thus they lie in a
circular arc across the sky reaching zenith over the meridian, for northern latitudes in the southerly sky
stretching between south west and south east, and vice versa for southern.
Satellites orbit either individually or more usually in clusters, and henceforth here, 'satellite' should
be taken to be interchangeable with 'cluster' unless context dictates otherwise. Clusters are spaced
approximately evenly around the Equator about 2-3° apart.
There are two main types of domestic installation, a fixed dish, the commonest, and a rotor
(aka H-H Mount,
Equatorial Mount or Polar Mount). As implied, a fixed dish is permanently pointed
at, aligned to in sat
speak, a single satellite, while a rotor is aligned to the meridian and under the viewer's control can
swing a dish as far as 40-70° either side to receive from a surprisingly large number of satellites.
(Additionally, a fixed dish can be converted to cover neighbouring satellites by mounting multiple
receiving components, but this is rather too specialised to be covered here.)
Just as a concave mirror focuses light, the dish's concave reflector focuses the incoming satellite signal
into the receiving component, a Low Noise Block (LNB).
This converts the signal into an electrical form in the downlead suitable for the satellite tuner at the
other end to interpret. Note that, unlike terrestrial TV where
one aerial can serve multiple devices, each satellite tuner requires its own LNB, and to allow recording of
one or more programmes while watching another, PVRs often
have multiple tuner inputs, requiring an LNB with as many outputs, connected by as many downleads.
This should not be confused with loopthrough on some STBs,
where the LNB signal is routed out via a second connector, to which can be chained a second STB, which can
use the signal either when the first is in standby, &/or when both are tuned to the same transponder on
the satellite.
There are three main dish types, hopefully having scales on the elevation adjustment:
Axi-Symmetric Dish
Round, LNB on the centre line, scale reading 0° when the dish points at the horizon;
Offset, Or Asymmetric Dish
Slightly taller than wide, LNB beneath the centre line, scale reading
20-30° when the dish points horizontally.
Sky Dish aka Mini-Dish
Offset dish wider than tall, requiring a special LNB, externally similar but internally shaped to match
the dish.
Initial alignment must be accurate enough to receive the target cluster's signal, signal level itself being
used for fine alignment. This can be read using either a satellite meter (aka satellite finder),
and/or the receiver's own signal data. Clusters being so close, the likeliest error, and source
of consequent confusion, is to align on the wrong one. To avoid this, initial alignment must be
accurate to within about 1°, but to realise the full range of a rotor, final accuracy will need to be a
fraction of that.
To obtain alignment settings, directly or indirectly you will need to know some or all of the following:
Site Longitude & Latitude - the
Calculator
page (next) has other ways to specify mostly UK locations.
Target Satellite's Longitude - in the UK, for a fixed dish receiving
Freesat/Sky this is
<a href="http://www.ses.com/4628803/astra-2d" target="_top" title="www.ses.com">Astra 2</a>
(link now dead) at 28.2°E, or for setting up a satellite rotor the meridian satellite will usually be
Thor
at 0.8/1°W.
Offset - only required for an offset dish with no scale, or one that does
not compensate for the offset; choose How to find the dish elevation, Other
in the Calculator, and if available, enter the offset from the dish specifications, otherwise make the
measurements for one of the two methods explained here, and enter those:
Offset Formulae.
Rotor Crank Angle - many rotor instructions are incomplete, misleading,
or obscure, so while it's easily accessible before mounting, measure this as accurately as possible
with a protractor. This is particularly important if your model of rotor is not pre-programmed
into the Calculator. See also
Crank Formulae.
The above completely determine the following setting angles, which must be calculated before work begins,
and a workable method chosen for setting each of them sufficiently accurately …
Fixed dish:
Azimuth, measured clockwise from True North, to align the dish
horizontally to the satellite - this is best set by using one of the methods
below (although none are entirely without complication) to find a distant landmark in the
required direction, and then pointing the dish at the landmark:
Using A Compass
A compass is simple and about sufficiently accurate. Nearby metal, such as
the dish itself, can deflect the needle, in which case walk off in the direction
of your sighting until it steadies. Allow for Magnetic Variance, the
difference between Magnetic and True North, which varies significantly over
place and time. In the UK, get your local current value from a recent
OS Map.
Less accurately, the Calculator can guesstimate it.
Using A Map (including an internet map)
For printed maps, note that their gridlines are usually laid out as if we lived
on a flat rather than a spherical surface. In the UK, this means that Grid
North varies significantly between about 3°W of
True North in the West and 3°E in the East.
Lesser maps such as A-Zs may not give grid data,
but the following Calculator page can work it out mathematically. For
elsewhere in the world, you can enter your own local value. The Calculator
will use the value to display a grid-corrected azimuth alongside the true
azimuth.
On a suitable local map, measure the Calculator's Grid Azimuth with a
protractor and follow its line outwards from the site looking for a landmark.
Alternatively, there is internet mapping technology, such as on the following
Calculator page, which allows you to position a pointer over a map and/or
satellite photo of the dish site. Although in principle a neat idea,
appealing in its simplicity, in practice note the following potential problems:
A system can only be as accurate as its underlying data (for which often
no figures are published);
Important parts of the map may not print properly in all browsers.
Using The Sun
By one of the following methods, find out for the location and date what time
the sun is in the desired direction, and at this time, mark the direction of
the shadow of something known to be
vertical.
O
←
↑
Using a spirit level with both vertical and horizontal levels, perhaps less accurately a
plumb-line, measure at least two points at right-angles to each other …
For a fixed dish, the Sun Azimuth time is required, for a rotor, the Sun Transit
time (NOT necessarily 12noon, see 'Common Errors Using The Sun'):
Sun Azimuth: Alt_at_Az, <location>,
The Sun, <azimuth>, <date>;
Sun Transit: Rise_Set, <location>,
The Sun, Transit, <date>.
If you have already personalised with your location a weather page like
Yahoo UK Weather
giving local times of sunrise and sunset, then the Calculator can use
these to calculate the transit time.
Common Errors Using The Sun:
Forgetting about Daylight Savings Time. British Winter Time
is the same as GMT
while British Summer Time is 1 hour ahead of it. Potential
error = 1 hour, or 15°.
Forgetting about the 'Equation Of Time'. Earth's orbital
speed and axial tilt with respect to the sun vary seasonally.
Consequently, our clocks do not keep time with the sun on a daily
basis, only on a yearly average. Thus on any given day at
Greenwich the sun will usually not be directly overhead at
12noon GMT. Potential error = up to 16 minutes, or 4°.
Forgetting to allow for longitude, which affects when the sun will be overhead. Potential
error: 4 minutes earlier/later for every degree of the site's longitude East/West.
Except at sunrise and sunset, the colours of which can change radically within a minute, we tend to
underestimate how fast the sun is moving - the sighting must be done promptly at the
designated transit time. Potential error = 1° for every 4 minutes error in timing.
Elevation, measured upwards from the horizontal, to align the dish
vertically to the satellite - this is set using a scale on the dish, unless:
The Scale Of An Offset Dish Reads Zero When The Dish Is Pointed At The Horizon
Unusually, the scale measures elevation of the actual dish, without compensating
for offset as is conventional. Therefore, it must be compensated for in
the Calculator by choosing How to find the dish elevation,
Other (so invoking the formulae from
Offset Formulae).
There Is No Scale On The Dish
There
are three possible solutions: a satellite meter, a template, and a
tiltmeter.
Satellite Meter - Set azimuth as accurately as possible, then
starting with the dish pointing horizontally, increase elevation steadily until
the meter indicates a signal is received.
Template - Place a straight edge across the dish (click diagram) and
with a protractor measure the Arm Angle between the underside of the arm and the
90° line. As shown (click again), accurately cut a triangle of stiff
cardboard with one angle equal to …
Arm Angle + Elevation Setting
Or:
Arm Angle + Required Elevation − Offset
Tape one adjoining edge along the top of a spirit-level, the other
along the arm. The level will indicate correct elevation.
Tiltmeter - Check that once mounted the back of the dish will have
an accessible flat surface to accept one. If so, and you are happy to
risk a protractor, check its baseline is parallel to its edge (some are
inaccurately stamped). At the intersection of the baseline and the
90° line, accurately start a small hole by twisting the point of a knife
blade until it will seat the point of a small drill bit, preventing skating,
then gently (excess force may break the plastic) hand drill
the rest. Thread strong thread through the hole and fasten it, leaving
enough hanging against the gradations to attach a plumb such as a large nut or
washer.
Skew (aka Polarisation Tilt), which aligns the LNB to the polarisation of
the incoming signal (measured clockwise from 12 o'clock viewed from behind the dish)
- set by eye or with a protractor.
Rotor and dish assembly:
Azimuth to align the assembly towards the Equator True North or South -
set as above;
Tilt to align the rotor axis (the line of its bearings) perpendicular, or
nearly so, to Earth's equatorial plane - set using a scale on the rotor mount
adjuster;
Elevation to align the dish to the satellite over the meridian -
set as above, correcting for the rotor;
Skew is set to zero by ensuring the receiving component is vertical -
usually set by eye.
Installation Requirements
To complete installation successfully, you will need the following:
Beforehand:
Alignment details - Azimuth, possibly Tilt, Elevation, Skew -
and a method of setting each;
A site for the dish with line of sight to the satellite free of obstruction by trees, buildings,
passing high vehicles, washing lines, etc, out of reach of vandals or burglars, solid enough
to hold it in a gale;
Satellite Dish, may be part of a kit - otherwise 50-60cm should be adequate for
Freesat in the UK;
An LNB, may be part of a kit - if not, specify the type of dish, normal or mini,
to ensure you buy to match; the number of outputs determines the number of tuners that can
be simultaneously connected, remembering that multi-tuner devices require an LNB with at
least as many outputs, each output requiring its own downlead;
Fixing bracket &/or mounting pole, may be part of a kit - if not, use industry
standard satellite brackets or I or T&K brackets for wall mount(s), at least 40mm steel poles;
Fixing bolts, may be part of a kit - avoid the plastic-sleeved coach bolts often
supplied, use something like anchor bolts having a wedge action to grip deep in solid masonry.
xx100 CAI Benchmarked satellite grade double-screened downlead cable, may be part of a kit;
Self-amalgamating tape for the cable join(s) to the LNB (and rotor);
Sealant for where the cable enters the house;
Cable clips for the downlead;
Optionally a double-screened F-Connector wall-socket and pattress;
F-Connectors for the cable ends;
Installation:
Ladders, if the dish is to be mounted on the side of a house;
Electric hammer drill and masonry bits (10mm?) for the fixing bolts and cable entry hole,
the latter long enough (400mm?) to drill through into the house, perhaps an extension
cable, an RCD is always a good idea.
Ring spanners, may need pairs, for the anchor/coach bolts (10-13mm?), mountings/adjusters (13mm?);
Screwdriver for the LNB holder;
Sharp knife or wire stripping tool;
Wire clippers or pliers;
Hammer;
Electrical screwdriver for the wall-socket;
Very nice to have:
Satellite meter - cheap ones squeal on signal and give an indication of its strength,
but even that is very useful; expensive ones can tell which satellite they are receiving, but
are probably overkill for DIY;
Spirit level with vertical as well as horizontal scales;
May be useful:
High scale map. In the UK, Ordnance Survey (OS)
Landranger 1:50000 or Explorer 1:25000 (Public Library);
Protractor;
Plumb Line;
Straight Edge.
Useful links (no endorsement of external sites intended nor responsibility taken for their content):
