†††††††††††††††† How to Align your Polar TVRO Dish

 

 

Due to the many requests on the internet for guidance with respect to

how to properly align a polar TVRO dish mount, I have put together the

following recommendations.There are other techniques which will

yield acceptable results, but if followed properly, this technique

will provide accuracy far better than is required to track the

Clarke belt.

††††† First a few comments and definitions.The geostationary TVRO

sats are located about 22236 miles above the surface of the earth

(ie about 26200 mile radius)in the plane of the equator. Some of

the TVRO sats are geosynchronous, but not geostationary, ie their

longitude is nearly constant, but their orbit is inclined so that

they drift north and south of the equator once per day.Since a

properly aligned TVRO dish only looks at the sats in the equatorial

plane, this means that without some form of elevation control, that

these will only be visible twice a day for about an hour each, as they

cross the equator.Most on the sats can be tracked without elevation

control.

†† If the sats were infinitely far away, a TVRO mount would be aligned

like an astronomical telescope, ie the rotation axis of the mount

would be aligned parallel to the earthís axis (ie pointing toward the

north star), and the dish would be aiming perpendicular to this axis,

ie parallel to the equatorial plane. Since the TVRO sats are not

infinitely far away, you cannot sight your dish parallel to the

equatorial plane, but you must tilt the dish down from this plane

slightly, by an angle referred to as the DECLINATION ANGLE.For a

sat due south of you, this declination angle can be estimated by the

arc tangent of the height of your position on earth above the equatorial

plane "h", divided by the distance parallel to the equatorial plane to

the sat , which is 22236 + (3963 - "a")where a is the distance from

your position on earth to the polar axis of the earth.

(See figure http://megalink.net/~wejones/satal.bmp for a description

of the various angles involved.†† This method (ie aligning your mount

rotation axis parralel to the earths axis, and dropping the aim angle

by this declination angle below the equatorial plane) is unfortunately

NOT satisfactory for tracking the Clarke belt. The reason for this is

that for sats to the east or west, the distance to the sats is greater

(ie 22236 + 3963), and thus the declination angle will be less.

Typically, for temperate latitudes, the declination of a sat to the

south might be around 6.4 degrees, and the declination of a sat to the

east or west might be around 5.7 degrees (assuming a latitude of 41

degrees).Sats in between are of course intermediate. One popular

technique is to use a declination in between these two extremes, ie

about 6 degrees in the above example. This will provide accuracy to

about 0.3 degrees or better in most cases, which is good enough to get

excellent results with C-band satellites, and good to fair results on

most KU sats, and is in fact one of the most popular installation

techniques. HOWEVER, if you want to track the belt more accurately, it is

necessary to compensate for the changing declination angle.The easiest

way to do this, is to set your declination angle to that of a westerly

sat (ie 5.7 degrees in the above example). If you do this, you will be

in alignment for westerly sats, but pointed too high for southerly sats.

To compensate for this, all you need to do is to tilt the rotation axis

of your mount down slightly towards the southern horizon, ie in the above

example, you would tilt the axis by about 0.7 degrees (see the figure for

clarification). This adjustment will bring the southerly sats into focus,

and since you are in effect rotating your mount axis around an axis

pointed east/west, it will NOT affect the alignment previously obtained

for westerly or easterly sats, and intermediate sats will be affected by

amounts between zero and the full 0.7 degree amount. It has been

demonstrated that using this technique properly should allow you to have

alignment errors of less than a few hundredths of a degree, ie much more

precision than you need to get excellent results on both C and KU bands.

†††

The first step to aligning your dish is to determine the angles for your

latitude, ie the declination angles and the amount of tilt required to

align your mounts rotation axis with the earths polar axis, (which I call

the "latitude" angle, and the adjusted latitude angle). It is also

important to very accurately determine "TRUE SOUTH" at your site.True

south is NOT the same as magnetic south that is observed on a compass,

but rather that value plus or minus the magnetic deviation. You can

determine your magnetic deviation using pcgeomag3.exe, a dos program

available from the web page where you got this file

(ie http:megalink.net/~wejones) in the TVRO section, or by looking at most

USGS maps of your area. However the best way to find true south is to find

out the time of true NOON at your location, and look at the direction the

shadows from your pole are aiming. At true noon, the shadow should be

aiming north, so if you extend a line through this shadow through the pole

towards the sun, it will be aiming south. This is the direction that the

MOUNT should be aligned with (ie find a flat surface along the side of the

mounts rotation axis, and make sure that it aims exactly south. This is a

VERY critical step, but it will be tweaked later.).To find the time of

true noon, you caneither (a) bisect the time between sunrise and sunset

for your location (b) use the server at

http://riemann.usno.navy.mil/AA/data/docs/AltAz.html to find the time when

the sun is at an azimuth of 180 degrees, or (c) use the program TVCALC,

available at http://megalink.net/~`wejones/tvcalc43.zip , which will

calculate the azimuth/elevation of the sun as well as sunrise/sunset and

true noon for your latitude and longitude. The program TVCALC will also

determine the proper declination angles as well as the latitude angle and

adjusted latitude angle. . If you cannot run TVCALC, the following table

can be used to estimate your angles:

 

Latitude††† ††††† 27.530††† 32.535††† 37.540††† 42.545††† 47.550

90-latitude ††††† 62.560††† 57.555††† 52.550††† 47.545††† 42.540

Decl South††††† 4.6†† 5†††† 5.3†† 5.656†††† 6.3†† 6.6†† 6.8†† 7.1†† 7.3

Decl west†† ††††† 4†††† 4.364.7†† 5†††† 5.3†† 5.6†† 5.9†† 6.156.4†† 6.64

Delta Decl††††† 0.6†† 0.640.6†† 0.650.7†† 0.7†† 0.7†† 0.650.7†† 0.66

psuedo lat††††† 28.130.633.135.738.240.743.245.748.250.7

psuedo 90-lat†††† 61.959.456.954.351.849.346.844.341.839.3

 

 

(Note. The above table may not show up aligned properly, depending on

whether a proportional or non-proportional font is used, and upon the page

width used. If you canít read it, let me know.)

 

After your mount is aligned to the south, to get your mounts rotation axis

parallel to the earthís axis, you use the latitude adjustment on the mount

to tilt the rotation axis back from the vertical by an angle of 90-latitude

(or actually eventually 90- the adjusted latitude). If this is done

carefully (often before the mount is even on the pole), it may never have

to be touched again. If done with the mount on the pole, the best way is

to use an inclinometer mounted on some flat surface of the mount that

is parallel to the rotation axis. Some inclinometers measure angles vs

the vertical, some vs the horizontal, and some both, so make sure

whether you are measuringthe 90-lat angle or the latitude angle.

At this point, you should set the declination angle on the mount to

the "Decl west" value.If the dish is on the mount and pole, you can

often find a flat surface either parallel or perpendicular to the

surface of the dish, and use an inclinometer as above. What you are

looking for is an angle for the surface of the dishthat is less

than the90-lat value (that you just set forthe rotation axis ) by

a value equal to the declination west value.On some dish mounts, the

declination angle is set via an adjustable nut/bolt. On other dish

mounts, the declination angle is set via multiple locations of the

bolts that connect the dish to the mount. On the Orbitron "spinclination"

type mounts, the declination angle is adjusted by loosening the bolts

at the dish hub assembly, and rotating the dish aroundpointing axis.

This is possible because the pipe coming out of the dish hub is not

perpendicular to the dish surface, but rather tilted by about 8 degrees,

allowing the dish to aim anywhere from 8 degrees above the equatorial

plane to 8 degrees below the equatorial plane. Ie a 90 degree rotation

of the dish will provide 8 degrees of declination and 45 degrees

rotation gives 4 degrees, etc.

 

Once the declination is set, donít change it again. If it is pretty

close, it will be OK, especially on the spinclination type mounts that

allow very accurate declination measurement (but are less than desirable

in other aspects).At this point, you should aim your dish south via

the IRD motor controller, and scan around that area until you find the

sat which is closest to being south of yout location (ie the longitude

of the sat is close to your longitude, or the azimuth angle of the sat

is 180 deg).When you find such a sat, peak it as best you can with

the IRD, THEN perform final tweaking of the latitude adjustment on the

mount to get the best signal. This in effect will perform the operation

of tilting the rotation axis by an angle of about 0.6 degrees toward

the south horizon, to give the proper 90-adjusted latitude angle. Doing

this adjustment while looking at the TVRO signal is the most accurate

way to adjust this angle.

 

†† At this point, if you had accurately aligned your mount toward the

south in the very first step, you would be done, and would be tracking

all the sats in the arc, however, as mentioned above, the south pointing

alignment is VERY critical, and hard to get accurate using noon shadows

or a compass, so when you try to scan east or west via the IRD motor

control, you will probably find that the further east or west you scan,

the worse your reception will be. What you want to do, is scan to the

sat which is the most distant from your southerly sat, but which you can

still see with the current adjustment, ie it will be coming in with a

very poor signal. Loosen the bolts holding the mount to the pole, and

VERY carefully make VERY SMALL adjustments to the south alignment of

your mount (ie rotate the mount on the pole). This is a VERY fine

adjustment, usually about the thickness of a pencil line drawn on the

pole for alignment (this is recommended to help get back to your old

adjustment if necessary). After each small adjustment, re-tweak the

position along the arc with the IRD motor control. It would be best not

to save the sat positions yet. If they have already been saved, you can

usually "re-sync" the arc with most IRDís, which changes the positions

of all your sats by the same amount.This re-sync is necessary because

adjusting the south alignment of the mount will change the apparent

position of the sats in the arc.Once you have repeated these small

south alignment adjustments followed by arc re-syncing a couple times,

you will probably have adjusted the sat you are on so as to give very

good reception.At this point, continue on to the next, and next most

easterly or westerly sat (whichever is further away from south) until you

get good reception on ALL sats.

†††† In theory, at this point, if you have followed the instructions

properly, you should be able to go back to the southerly sat, and it

will still be aligned properly. If it is not, it is usually correctable

by re-saving its position in the arc at the IRD. If you cannot get

acceptable reception on the southerly sat, it either means that you

werenít following the instructions properly, or started too far off

in your initial estimates of the various angles. Check the various

angles on the mount with the inclinometer to make sure they are correct

to the best you can determine with the inclinometer. If they are

significantly different, make the appropriate adjustment (ie to the

latitude adjustment or declination angle), and start over from scratch.

Often, however, you can just make a minor adjustment to the latitude

adjustment, which will bring in the sourherly sats, then proceed directly

to the most east or west sat, and re-adjust the south mount alignment

(re-syncing along the arc each time) to bring in the east or west sats.

††† At this point, you should be done. Tighten up all the bolts on the

mount, and it is often advisable to drill a hole through the mount into

the pole, to permanently fix the south alignment of the mount on the

pole, but either leave some room for future adjustment, or be sure you

have it right before making this permanent.

††† Note, the above assumes that the orientation of the feedhorn has been

set properly so that the polarity of the sat signals is nearly correct.

If you have an Orbitron spinclination type mount, you should remember

that if you should ever re-adjust your declination angle (this really

shouldnít be necessary), that this adjustment will change all polarity

angles you may have set. Some people trying to make fine adjustment to

the declination angles end up making things worse because often they are

changing the polarity in addition to the declination. Also the

spinclination declination adjustment changes the apparent position of

the sats in the arc, so if you have saved sat positions before

performing the adjustments, you will have to re-sync the arc after

any change to the declination angle.

†† The above recommendations do not include any discussion of

positioning the feedhorn relative to focal length, etc. The program

TVCALC mentioned above does the simple calculations to determine what

the focal length should be, from measurements of the dish width and

depth, but it is best to follow the recommendations of the dish

manufacturer in this respect, as sometimes the dish shape may make

accurate determination of the parameters difficult. If the dish

manufacturers recommendations are not available, use the calculations

in TVCALC as a starting point, and make adjustments to the focal length

to get the best signal possible.

††† If you have any problems following these instructions, or have

recommendations for changes, email wejones@megalink.net.

 

William E Jones of Sweden, Maine retains copyright rights to this file.

You may download, re-distribute, copy and use this file in any way you

like, so long as it is not used for commercial purposes or changed in

any way prior to re-distribution.

 

Good Luck