C Band Declination Angle

[tmsani]

Can some one point me in the right direction where should i place the inclinometer to measure declination angle in the attached picture of the dish?

[gulliver]

I usually put a straight edge across the dish/side and the inclinometer on top, which is very close,then do the fine tuning with the meter.
If you got enough room,on top of the bracket.

[Thala Dan]

A bit hard to describe based on your photo.

Hopefully these links might help.

[tmsani]

Sorry. I have uploaded better picture of the dish.

[beer4life]

,,

[tristen]

In the image above, the declination offset angle, which is the angle between the rotational axis A and the surface of the dish B (and also at D), can be measured and adjusted in the following manner. (Note: the surfaces B and D are parallel).

Firstly, rotate the dish on its polar mount (through its "rotational axis") so that the dish is at maximum elevation.

(Quick method: stand behind the dish and rotate it until the outer edges of brackets E and F are parallel).

Secondly, place the inclinometer on the lower (or upper - depending on available clearance) surface of metal piece A and note the angle (See Note*).

Thirdly, place the inclinometer on the rear surface of the metal ring at B and note the angle.

The angle measured on surface A will (should) be larger than the angle measured on surface B.

The difference between the two angles is the declination offset angle.

Adjust the declination offset mechanism (bolt/nut) at C until the desired declination offset angle for your latitudinal position (location) is obtained.

*Note: the surface at E (labelled "Motor Latitude Adjuster" (more correctly referred to as "Elevation Adjustment") on my borrowed illustration) can be used instead of the surface at A, but bear in mind that it is at ninety degrees to surface A.

Important: The foregoing description assumes that you are standing behind the dish.

[tmsani]

In the image above, the declination offset angle, which is the angle between the rotational axis A and the surface of the dish B (and also at D), can be measured and adjusted in the following manner. (Note: the surfaces B and D are parallel).

Firstly, rotate the dish on its polar mount (through its "rotational axis") so that the dish is at maximum elevation.

(Quick method: stand behind the dish and rotate it until the outer edges of brackets E and F are parallel).

Secondly, place the inclinometer on the lower (or upper - depending on available clearance) surface of metal piece A and note the angle (See Note*).

Thirdly, place the inclinometer on the rear surface of the metal ring at B and note the angle.

The angle measured on surface A will (should) be larger than the angle measured on surface B.

The difference between the two angles is the declination offset angle.

Adjust the declination offset mechanism (bolt/nut) at C until the desired declination offset angle for your latitudinal position (location) is obtained.

*Note: the surface at E (labelled "Motor Latitude Adjuster" (more correctly referred to as "Elevation Adjustment") on my borrowed illustration) can be used instead of the surface at A, but bear in mind that it is at ninety degrees to surface A.

Important: The foregoing description assumes that you are standing behind the dish.

Thanks a lot for excellent explanation. Angle B seems to be on the circular mount.How do i measure on that can i place the inclinometer anywhere in the metal ring or should that be in the middle?

I measured by leaving the dish in Palapa D position placing the inclinometer in the middle of A as per pic i get 32 & in B i get 37.As per your explanation angle A should be greater than B but in my case it is other way round..What is the expected values for Perth?

[Thala Dan]

The angle measured on surface A will (should) be larger than the angle measured on surface B.

Tristen,
I'm confused.
Shouldn't the angle on Surface B be larger than the angle on surface A, relative to the Horizontal axis?
I thought the angle on Surface B would be the sum of the Latitude angle (Surface A) plus the Declination angle?
That's a nice diagram, by the way - I tried to add figures to the one I had, but couldn't figure how to do it.
Your diagram has got it all.
Cheers.

[tristen]

Thanks a lot for excellent explanation. Angle B seems to be on the circular mount.How do i measure on that can i place the inclinometer anywhere in the metal ring or should that be in the middle?

I usually place the inclinometer on the polar mount dish support ring at something approximating the position as shown in the photographs below, i.e. on a section of the ring that is at a right angle to the ground with the dish and polar mount each facing due north.

Note: I was lazy and did not drive the dish depicted in the photographs to due north.

I measured by leaving the dish in Palapa D position placing the inclinometer in the middle of A as per pic i get 32 & in B i get 37.As per your explanation angle A should be greater than B but in my case it is other way round..What is the expected values for Perth?

1/. You cannot set the declination angle correctly unless the dish is pointing due-North.

WhY? Because as the dish swings to the east or west of North, its elevation angle changes and therefore the relationship between the elevation angles of (1) the dish and (2) the polar mount, also alters accordingly.

2/. Read the instructions for a polar mount dish alignment at . It is an excellent tutorial and might serve to "flesh out" both this and the previous explanation a little.

Note: the description refers to installation in the northern hemisphere so you will need to read north for south and vice versa.

Details for your location versus Palapa D appears in the graphic below. The declination angle for Perth's latitude (31degrees 58 minutes) is a little under 5 degrees (5 degrees is near enough for practical purposes). You can see a table of declination angles versus latitude at .



NB. Setting a declination angle is necessary ONLY for dishes employing a polar mount and actuator (for tracking the Clarke Belt). It is NOT necessary to set the declination angle on a fixed dish, even if it using a polar mount fitted with a solid locking bar (to lock the dish in position).

Hopefully, the foregoing will also answer the questions posed by Thala Dan.

[weirdo]

nice leaf catcher tristen

[Thala Dan]

Hopefully, the foregoing will also answer the questions posed by Thala Dan.

I'm afraid not.

tmsani,


I have just positioned my dish at true north.

With a digital clinometer, the angle at Surface A is 37.2 degrees.

The angle at Surface B is 42 degrees, which is the angle at Surface A (Latitude angle) PLUS the Declination Offset angle (approx 5 degrees).

As Tristen has indicated, the dish must be pointing true north when these measurements are taken.

Based on your general location in Perth, Surface A should be around 32 degrees, and Surface B should be around 37 degrees.

I would suggest either place a flat piece of metal or wood across the back of the dish mounting ring in the 12 o’clock – 6 o’clock position, and place your clinometer against that, or alternatively, place a smaller piece of wood or metal across the exposed ribs of the dish (where the gaping hole is in the centre) and measure against that.

Either way, the angle on Surface B should be about 5 degrees greater than the angle at Surface A.

[tmsani]

Thanks Tristen & Thala Dan for your explanation, it was quite useful. I will try it out tomorrow @ my friends place.

[cmangle]

As said above a fixed dish does not need to have a declination angle SET or ADJUSTED! Setting declination on a polar mount dish with actuator improves/corrects the tracking of the Clarke orbit belt(aka Geosynchronous orbit 22,236mi or 35,786km out in space) so you don't miss the exact/precise locations of the arced satellites when moving from one to the other! 1 degree off down here misses the satellite by 500mi or 805km.
I believe this is correct, too little declination and you will get only the middle satellites in your arc and miss the end ones, too much declination and you get the end satellites and miss the middle ones AS YOU TRAVERSE THE ARC!!

[tristen]

Tristen, I'm confused.

O.K. Let me try to explain.

I believe that some of the discrepancy lies in the way that inclinometer readings can be/are interpreted.

Let me give you a graphic example in order to illustrate what I mean.

In the picture below, I have placed two different inclinometers side-by-side on the same steel section of a polar mount.



Looking closely, you will notice that each inclinometer indicates a completely different angle!

"Why the difference"?, you ask.

Well, there's a fairly simple explanation. It's because the scale on each inclinometer is drawn in relation to a different reference point, i.e. they have different points of reference.

Here's a longer shot to enable you to see exactly where the inclinometers are sitting on the polar mount of a C-band dish.



Note that the angle shown by the meter on the right indicates 55 degrees (and No, I haven't fiddled with it to support my argument!).

Looking again at the first photograph, you should also notice that for the scale of the inclinometer on the right to indicate zero (0) degrees, its base would have to be attached to a vertical surface, while the one on the left is set up for its base to be horizontal in order to indicate zero (0) degrees. Clearly, the two inclinometers have scales which have different reference points - one vertical and the other horizontal.

(The inclinometer on the right is in fact an adjustable type, which can be set up for either vertical, horizontal or "any-angle-in-between" use).

Have you spotted the relationship between the two readings yet?

Going on...

...Shouldn't the angle on Surface B be larger than the angle on surface A, relative to the Horizontal axis?

Logically, in order to focus the dish on a distant satellite, the elevation angle of the dish must be identical to that of the target satellite from that same location. The point of reference for determining the angle is the earth's surface at the location where the dish is installed, i.e. the location's latitudinal position. Obviously, when installing a (motorised) dish, it is also necessary to know the desired satellite's elevation angle (again, in reference to the (horizontal) surface of the earth) at that same (latitudinal) position. The illustration below shows what I mean.



Tables derived from satellite dish aiming programs are provided so that we don't have to do the complicated mathematics involved. Such programs and tables are readily available via the internet e.g. and .

In order to accurately track the Clark Belt, the location immediately above the equator where geosynchronous satellites (of necessity) are located, a "modified" polar mount is used. The term "modified" refers to an added adjustment referred to as declination (adjustment). The declination adjustment compensates for the curvature of the surface of the earth, which is spherical in shape. The value of the declination angle is a function of latitude.

Logically, because the elevation angle of the dish MUST be set so that it is equal to the elevation angle of the satellite of interest, the elevation angle of the (modified) polar mount on which the dish is fixed, of necessity, must be higher by an amount referred to as declination (sometimes called declination offset). Declination or "downward tilt" is applied to the dish and not to the polar mount on which the dish is mounted. See also .

Back to the pictures...

In the picture above), when using the inclinometer pictured to the right, I am able to simply read "straight off the scale, (55 degrees), i.e. with no correction whatsoever. However, if I apply the same technique to reading the inclinometer shown on the left, I read 35 degrees. Instead, I must count the graduations on the scale backwards (from left to right) from the ninety degree mark (90-35). This then gives me the same figure indicated by the inclinometer on the right, which is 55 degrees (90-35=55).

...tmsani, ...Based on your general location in Perth, Surface A should be around 32 degrees, and Surface B should be around 37 degrees...Either way, the angle on Surface B should be about 5 degrees greater than the angle at -Surface A.

I have calculated the elevation angle for Perth to be 52.8 degrees (Data obtained from ).

OR, if using your method, it will be 37.2 (90-52.8) degrees (close to the figure you have suggested).

But to avoid unnecessary confusion, let's get used to using the appropriate and universally accepted terminology and to measure from the correct point of reference.

In practice, I would settle for 52 or 53 degrees in round figures (depending exactly where in Perth the dish is located).

Incidentally, in practice, elevation and other readings can be a little different to expected values for other reasons, e.g. a dish focal point which has moved due to incorrectly installed and/or adjusted LNB feed/scalar rings. I have found this particular part of dish assembly/installation to be a very common failing, even with some so-called "professional" installers.
But that is another story for another time.

[Thala Dan]

Tristen,

A few preliminary comments before I launch into my response.

First, let me thank you for the effort you have put into explaining your position.

Second, I acknowledge and respect you experience in this field – I think you referred to “hundreds of C-band installations” in another thread.

Third, I would like you to know that, whilst I am new to the field of satellite reception, I have spent in excess of 50 years in the field of electronics, including radio and cable communications technology throughout the world, and was an active “home-brew” Ham radio operator for over 30 of those years. I mention this solely so that you can appreciate that this subject is not “black magic” to me, and that we can converse in the same language.

Now……I understand everything that you have just posted, including how you arrived at your advice to tmsani about the angles at Surface A and B.
We are both obviously talking about the same thing…..the only point of contention seems to be how to present it to the newcomer.

You are probably aware that I recently set up my own C-band system, and posted the results in this forum.

I did so after months of research on the ‘net as to how to go about it, and without doubt, the most confusing aspect of everything I read was the constant reference to the “elevation” of the mount of around 37 degrees, when I knew damn well that the satellites were up to 60 degrees overhead.
Finally, the penny dropped….I realised that the “elevation” often referenced in respect of the mount was indeed the “latitude angle”. From there, everything else fell into place….Latitude angle, Declination offset angle, Declination angle.
It became clear that setting up C-band was pretty much all about the mount….the dish, LNB, Scalar rings, etc. were just the support act.

That is how I approached my installation – set up the mount very, very carefully first, fit the dish next – switch on – bingo!
The five satellites I wanted were there for the taking, one after the other. As I said in my posting on the subject “…much to my great delight, and even greater surprise….”

And this brings us to the point where we must agree to disagree.

Every single article I have read from the ‘net (and I’ve read plenty) treat the setting of the mount angles in the manner in which I posted in response to tmsani. That is, set the Latitude angle, then add the Declination offset angle to the dish ring, to end up with a dish mount (ring) angle equal to the sum of the Latitude angle and the Declination offset angle.



Some examples:

Using the declination adjustments on the mount, set the antenna for an elevation angle that is equal to the polar axis angle plus the number of degrees of declination for your area.
The dish should be looking down slightly from the angle of the mount.

Pacific Satellite International
Large dish installation



“Modified Polar Mount Tracking Angles

NOTE: Elevation Angle is the angle measured on (at) the pivot axis of the mount. To get the Total Declination Angle setting as measured on (at) the back of the dish ring (dish mount), add the Declination Offset Angle to the Elevation Angle.”

Geo-orbit.Org



“Example: Mackay 21 degrees latitude 149 longitude
Elevation =21.47 A
Declination =24.59 B

Tutorial produced by adp81, and frequently referenced to newcomers to this site



And, finally, Tristen, the very article, “Polar dish mount”, that you included in your posting:

“The easiest way to set the declination offset is with an inclinometer placed on the back surface that is parallel with the face of the dish. Or via a flat board placed in a vertical direction spanning the rims. This reading should be equal to the sum of the polar axis and the declination angles.”



(We’ll overlook the fact that their reference to “declination angle” at the end should in fact be “declination offset angle”.)

All of this is, of course, very confusing for the newcomer.
We have Polar axis angle, Latitude angle, and at times, erroneously, Elevation angle, all referring to the same thing.
Then we have reference to the Declination Offset Angle one minute, and then the same thing called the Declination Angle a few words later.

I completely understand where you are coming from on this matter, but my point has been that this is not the way dish mount installation angles are treated in the information available to the newcomer.

Everything I have read on the subject tackles the issue of angles as being:

Declination Angle = Polar Axis (Latitude) Angle + Declination Offset Angle.

Or, in terms of your excellent diagram….the angle on Surface B is greater than the angle on Surface A.

Personally, I see great merit in this approach. If the newcomer starts thinking in terms of elevation to the satellite whilst setting up the mount, he/she is likely to take their eye of the main game – which is setting up the mount correctly first.

You said:

“

Logically, because the elevation angle of the dish MUST be set so that it is equal to the elevation angle of the satellite of interest, the elevation angle of the (modified) polar mount on which the dish is fixed, of necessity, must be higher by an amount referred to as declination (sometimes called declination offset). Declination or "downward tilt" is applied to the dish and not to the polar mount on which the dish is mounted. See also Polar Dish Mount”

I respectfully disagree.

I do not believe the dish should be set to “…the satellite of interest…”.
In theory, it should be set to the zenith satellite at True North…but I don’t believe we have one of those.

A “higher” amount referred to as “declination”? A tad confusing for a newbie.

“

Declination or "downward tilt" is applied to the dish and not to the polar mount on which the dish is mounted

”

Sorry, but I have to disagree with that as well.

The dish ring is an integral part of the modified polar mount.
It is the ring that is set to the Declination angle…the dish simply happens to be mounted on that part of the structure.
An analogy is wheel alignment on a car.
The alignment adjustments are performed on the hub on the end of the axle, not the wheel itself.
It just happens that those adjustments are then passed on to the wheel when it is mounted on the hub.
So it is with a dish mounted on a polar mount.
Anyhow, been an interesting discussion.
I hope you’ve enjoyed it as much as I have.
Incidentally, much of the information that you have posted on this site was of great assistance to me in my caravan-based Ku adventures, and my now settled-down C-band endeavours.
I am very grateful for that assistance.


Cheers
Thala Dan