Tests of 120cm antennas

[RimaNTSS]

when I set it up I set the elevation as per Dishpointer, 31.4º, with my electronic angle gauge, then found that I had to adjust it to receive more signal

Yes, and this is because dish producer made nice and durable dish with feedarms designed in such a bad way that they do not hold LNB in proper place. Or, that also could be question of purely designed plastic LNB holder, which is made "universal" for 1,2m and 1,8m dishes, just to make it easier to produce.

 

[Riverblue]

Yes, and this is because dish producer made nice and durable dish with feedarms designed in such a bad way that they do not hold LNB in proper place. Or, that also could be question of purely designed plastic LNB holder, which is made "universal" for 1,2m and 1,8m dishes, just to make it easier to produce.

We can't say it's poorly designed until we prove that it is, if it is!
There may still be some aspect of dish design/wave pattern acceptance/physics that we don't have enough knowledge or awareness of that affects the placement of the lnb!;)

 

[RimaNTSS]

I think, that 0,7* difference you got on your antenna already rings some bell. Of course we can and we will experiment and prove what we can prove. And we, hobbyists have lots of time and patience!

 

[battenfan]

It would be interesting to see a comparison with a prime focus dish. Maybe dishpointer doesn't take into account atmospheric refraction but if that were the case the error should be the other way. Puzzled.

 

[RimaNTSS]

It would be interesting to see a comparison with a prime focus dish.

Good suggestion. PFAs are known for pointing directly to satellite, and, usually, LNB is located in the very middle of it. I am pretty sure that if somebody point PFA towards satellite, at maximum signal, he will get precise elevation of satellite=leaning of the face of the dish. I did such checks on my PFAs long time ago and it worked perfectly.

 

[Riverblue]

Good suggestion. PFAs are known for pointing directly to satellite, and, usually, LNB is located in the very middle of it. I am pretty sure that if somebody point PFA towards satellite, at maximum signal, he will get precise elevation of satellite=leaning of the face of the dish. I did such checks on my PFAs long time ago and it worked perfectly.

Have you measured the angles on any of your offset dishes yet, what are your results?
How do prime focus dishes compare to offset with regard to the elevation figures stated on sites like 'Dishpointer'?
Does everyone have the stated elevation angles for the satellites they are aimed at for their location, or are the figures references to guide us to an approximate elevation to enable us to then tweak and peak our signal reception?

 

[RimaNTSS]

Fortunately Dishpointer shows exact elevation for satellites calculated for particular location and precision is up to 0,1*. PFA's face leaning shows exact elevation as stated on Dishpointer, and can be ,measured up to 0,1* as well, I've checked this long time ago and do not think something is dramatically changed over last couple of years. So, offset antenna should be leaning Elevation angle+offset angle (this is axiom of SAT technique). Antenna-designer (especially branded ones indicate offset angles with the precision up to 0,1*). Resuming- I do not think that 0,7* mistake is because of Dishpointer calculations or producer's wrongly stated antenna's offset angle. Mistake is definitely in wrong placement of LNB (not in antenna's focus but in multifeed position). We all know well that LNB placed in multifeed brings us worse results.
I did not make precise measurements of any of mine offset dishes, that is going to be done soon.
But, would be very interesting to hear other users thoughts about this issue and, maybe to see their measurement results as well.
CM 1,2m is quiet popular antenna, and we could share results on this particular dish.
I hope we will not have criticizers at the end of the process (whatever results will be). It is well known that some people like to do lots of critics without doing any practical things themselves (and, again, fortunately we do not have such a persons within this forum community ).

 

[RimaNTSS]

According to my calculations original LNB holder holds LNB little bit too low and points it to the wrong point. To fix that I just made new holder (yes, folks, not INOX ) , but, I promise, it is just for tests . This holder is adjustable. I only need now to install antenna and do some tests.

 

[purplemarina]

You can make a inox holder later if the results are good;) watching with Great interest!

 

[Trust]

With the Laminas OFC1200 the same story , solved it with this adjustible lnb holder

 

[jase1]

According to my calculations original LNB holder holds LNB little bit too low and points it to the wrong point. To fix that I just made new holder (yes, folks, not INOX ) , but, I promise, it is just for tests . This holder is adjustable. I only need now to install antenna and do some tests.

Are you going to set the dish up with the lnb in the normal position first? Otherwise how are you gonna know if you've improved the signal? You'll have nothing to compare it to.

Are you doing this test on a weaker sat?

 

[RimaNTSS]

Are you going to set the dish up with the lnb in the normal position first?

Of course, that was initial idea. I did not change feedarms. Will put on original holder tune to 7*W, scan all available transponders and record levels. Than change LNB golder, adjust elevation and scan all transponder again.

 

[jase1]

With the Laminas OFC1200 the same story , solved it with this adjustible lnb holder
View attachment 73473

Was it a particular sat you were after? What sort of improvement did you see and was it worth the effort.

 

[skomedal]

Are you doing this test on a weaker sat?

Only a fringe reception sat will make this test valid IMO

Waiting results and named fringe sat from the testers location with anticipation

 

[Riverblue]

Rima, would you mind running my measurements through your new Parabola program(as opposed to the old Russian one!) again for me?
large axis of dish = 1300mm, small axis of dish = 1200mm, depth of dish = 116mm
The only dimension of yours is the one underlined, (from your post55) I make that to be 650mm! Am I looking at the wrong dimension?

 

[RimaNTSS]

Is that your Channel Master or another antenna? If CM, than I am wondering about it's depth which is 116! I measured mine CM, it has 114,8mm depth. Anyway, there are your screenshots

 

[battenfan]

(Image source)

That site has copied text and images from a book, I haven't figured out which one but here is a pdf covering part of it. It's a little easier to follow when you see what the equations are supposed to look like when they've not been copy-pasted. More from the same book:
Within this context, we start by examining the influence of the feed pointing angle ф{ on the gain G, sidelobe level (SLL), and cross polarization of offset reflectors having...

...A classical design scenario has now emerged. The feed pointing angle ф{ is reduced until desirable cross-polarization performance is achieved or until the gain is reduced as far as can be accepted. If, on the other hand, the SLL is a critical parameter, ф{ can be optimized to yield nearly the lowest SLL over a practical range of angles, with only small reductions in G and the XPOL

Feed pointing angle (deg)

Figure 9. Gain, sidelobe level (SLL), and cross-polarization level (XPOL) as a function of the feed pointing angle фі for an offset parabolic reflector with a 85.5A diameter.

Now my head hurts.

Edit: The book is probably Wiley Encyclopedia of Electrical and Electronics Engineering

 

[humaxboy1]

With the Laminas OFC1200 the same story , solved it with this adjustible lnb holder

Trust1 - is that home made lnb holder ? The result of this mod would be very valuable for all Laminas users.Is that the Laminas from hvdh ?

Humaxboy

 

[RimaNTSS]

Made another holder today. So if somebody interested in old one (second picture attached), you are welcome to have it as it is for shipping price. First come first served.

 

[Trust]

Trust1 - is that home made lnb holder ? The result of this mod would be very valuable for all Laminas users.Is that the Laminas from hvdh ?

Humaxboy

It is indeed the OFC-1200P from HvdH and it did not make a lot difference .
I was surprised by the fact that the aiming point was above the center of that new type Laminas , it has 2 tubes and a plastic feedarm
Hans replaced the 2 tubes for 6 mm Inox treaded rods to lower the feedarm but by that , the lnb was pointing more up-high , therefore the lnb tilt-clamp
At the older version 1200 with the 3 tubes , that point was ± 10 cm lower .
And yes it was home-made by me .
@jase1 , cant remember which sat it was , but when it give improvement at one , it should be also for the other ones .