125W Bleeding into 123W (Ku)

[Cletus2k]

I just recently re-mounted my 90cm Fortec Offset Dish + SG2100 Motor on a fixed pole.. For the last few years it has lived on the side of my not to structurally sound shed attached with a J Pole, and needless to say over time it's moved some. I ended up having to replace all the bolts that came with my SG2100 after being exposed to the outside elements since 2002-03 (They were totally rusted).. I can't recall when I got this dish/motor to be honest with you but was with the recently released Fortec Lifetime if that gives some idea of it's age.. Enough of my ramble, and on to my question..

I've noticed that when blind scanning 123W I end up picking up some of the PBS transponders on 125W (12180 V 30000 for sure).. Is this typical because the power output of 125W (based on SatBeams.com for my location -- Central NC) is around 51 dBW and there isn't much in that frequency range / polarization on 123W? I have worked in RF for years (as a cellular wireless GSM/UMTS engineer) and have some background on RF, but I wasn't sure if this was pointing to something possibly being out of whack on my dish.. Since the 90cm Fortec has no support brackets for the LNB I wonder if it's gotten slightly bent.. For years I had a Invacom Circular/Linear LNB on it, and it also has been moved a few times over NC and re-located on various poles/mounts during that time as well..

As far as LNBs right now I am using a fortec universal LNB because it seemed to provide a better Q on most channels than the Invacom LNB as well as being much lighter..

If this is typical behavior for strong KU to bleed that badly when it's separated by 2 degrees that's cool.. Just wondering if there is something I could do to fix/tweak that away..

 

[singkwan2004]

Thats because your dish is not peak at the best position, with a 39" dish, its big enough to pick up neibour satellite if dish position is on the middle of two satellites. This result both satellite is in lower signal quality.

 

[Cletus2k]

How badly could it be off? The only transponder I can get from 125W bleeding over with any certainty is 12180 which there seems to be nothing in that range on 123W..

When aimed at 123W I get a signal level of 64 (The noise floor seems to be between 58-60 w/ my current LNB - Course I have no idea of units).. The Q is pegged at 0 and displays a very choppy picture with about 3/4 of the frames missing..

When aimed at 125W I get a signal level of around 70 and Q's between 85 and 90.. Doesn't seem like it's that far off.. 125W is the furthest West Sat I ever look at so it at the very very end of the arc..

Maybe my skills aren't that great, but I find it very hard to not get something out of alignment tightening down the motorized dish setups.. One extra turn of the bolt my luck always tweaks it slightly out of whack so the weaker transponders don't want to come in that well..

 

[brentb636]

You are correct in guessing that this is a "2 degree problem". Smaller dishes frequently pickup adjacent strong signals, because the focus is not good enough on the small dish to exclude those signals. I used to have the same problem with my Fortec 90cm dish on those same birds. I managed to find a free 1.0M dish to replace the 90cm and that kind of problem disappeared.

 

[SatelliteAV]

Some 90cm dish models are not 4 degree compliant and adjacent satellites may cause interference or bleedover (especially strong transponders). The dish specs should indicate the attenuation at a specific degree.

 

[B.J.]

Although the 2 deg compliant thing is an issue, the fact that the dish involved is a 90CM Fortec suggests to me that it is more to do with the dish itself, and more specifically the likelihood that the LNBF isn't at the focal point. My 90 CM had the lnbf positioned more than 2" away from the real focal point, and although you can still get fairly good reception there, you also start getting reception from other sats. The LNBF arms on the Fortec dishes are easily bent.
Just my guess.

 

[Cletus2k]

How do I measure the focal length distance? I know moving the LNB in or out didn't seem to make any differences when aiming, but now I am curious..

Fortec Star Satellites

Fortec Star's website says "focus length" is 510mm ~= 20.1 inches.. I just very roughly measured from the end of the LNB to the chord that you would put between the side edges of the dish and it looked like it was about 21 inches.. If you went to the center of the dish it was a few inches more.. Keep in mind this was just very rough measurement, which may have been done totally wrong.. I just wonder if the weight of the Invacom LNB and lack of support arms on that dish coupled with moving it numerous times have bent it over time somewhat..

I also have picked up a Dish Network Transponder or two (whatever is at 119W) when blind scanning 116.8W which is 2.2 degrees separated and of course the dish birds are much more powerful than SatMex 5/6 (whichever it is can't remember off the top of my head)..

 

[B.J.]

How do I measure the focal length distance? I know moving the LNB in or out didn't seem to make any differences when aiming, but now I am curious..
...

I wasn't referring to the focal length, that doesn't change much when the lnbf arm is bent since the bend in the arm is pretty close to the center of the parabaloid. However, when the arm bends, it puts the lnbf off center. Look at the 2nd half of this page I put up several years ago:
FC90-sg2100.html
Basically, I put a bunch of small mirrors on the dish, and aimed it at the sun to see where the focal point was. What I saw, was that you couldn't get all the mirrors to focus at the point where the LNBF was, but after much experimentation, I found a point about 2 1/2 " off center where all the mirrors did focus. Basically, with the lnbf arm in the wrong place, different parts of the dish are bringing different areas of the sky (ie different sats) to the lnbf.
After doing the above experiment, I also took a measuring tape, and measured the distance from the lnbf to mid way up each side of the dish (I think there was a little hole in the side of the dish that I measured to. I found that the distance wasn't the same, ie that the arm was bent a bit to one side. I eventually tried to bring the lnbf up to the proper place, square to the dish, using some steel wire tied to a nut/bolt through that little hole, however I didn't get it quite in the right place. I do think I improved it a bit though.
Basically though, I don't think the Fortec dish is very good. My old 3' Primestar, which was dumped out of a truck into a local transfer station (I told the manager to watch out for sat dishes, and he pulled it out for me), outperforms the fortec dish. The Fortec dish does lock most signals I see reported though.

 

[Cletus2k]

Interesting reading about your Fortec 90cm dish.. Your mounting bracket assembly doesn't look like mine, and I wonder if your instructions were for my style of mounting bracket (see attached photo). When did you get yours? I really have no idea when I got mine now but I think it was sometime around 2002.. I am fairly certain that my dishes elevation markings are not correct from aiming it numerous times. Usually when aiming the Fortec I just eyeball the elevation and tweak from there because I never trusted the markings on the dish. The elevation angles on the motor (although I have a DMS not Moteck SG2100) are correct.

Also have you had any problems with your motor u-bolts and bolts rusting? I ended up replacing all of mine when I remounted the dish the other day.

 

[guapoharry]

That's a great writeup, BJ.

Thank you for posting the link.

 

[B.J.]

Interesting reading about your Fortec 90cm dish.. Your mounting bracket assembly doesn't look like mine, and I wonder if your instructions were for my style of mounting bracket (see attached photo). When did you get yours? I really have no idea when I got mine now but I think it was sometime around 2002.. I am fairly certain that my dishes elevation markings are not correct from aiming it numerous times. Usually when aiming the Fortec I just eyeball the elevation and tweak from there because I never trusted the markings on the dish. The elevation angles on the motor (although I have a DMS not Moteck SG2100) are correct.

Also have you had any problems with your motor u-bolts and bolts rusting? I ended up replacing all of mine when I remounted the dish the other day.

I wasn't sure when I got mine either, but I looked it up, and it was back in 2006, which was about when my BUD started to give me problems on Ku due to dents from hitting it with a broom in the winter to knock snow off. THe mount you have is the so called U-bolt mount that Sadoun sold. I got what they called the pole mount. Everyone I've talked to who got the U-bolt mount has said that the elevations were off by about 5 degrees, so I really have assumed that it was a generic mount, not specifically made for the 90CM. I haven't noticed any rusting, but mine isn't very old yet.

But back to the lnbf arm, I also found that if you're not careful about how you tighten the bolts that connect the brackets to the dish that the lnbf arm can aim left or right of center, ie not be square to the dish. And re the arm bending, there was one guy on the Sadoun forum whose 90 CM had it's lnbf about 6 or 8" out of position, almost to the extent that it looked like a prime focus. I really trust dishes that have 2 side braces in addition to the main arm, because there is no way that the lnbf can get much out of it's proper position with 3 arms. The good thing about the Fortec dish is that it's light, and isn't much of a load on the motor.

 

[Tron]

Cletus2k, the motor U-bolts made for the SG-2100 at that time were very problematic. They were subject to breakage and stripping... It was very easy to strip them without even tightening them much. They have since been replaced by far better hardware.

 

[Anole]

reference material, Fortec dishes

As long as the subject of elevation accuracy came up in this thread, let's set the record straight.
And, knowing the facts, it's easy enough to work with .

Sadoun discusses Fortec Star dish elevation confusion:
http://www.satelliteguys.us/free-air-fta-discussion/173704-general-fta-question.html#post1809188

 

[B.J.]

As long as the subject of elevation accuracy came up in this thread, let's set the record straight.
And, knowing the facts, it's easy enough to work with .

Sadoun discusses Fortec Star dish elevation confusion:
http://www.satelliteguys.us/free-air-fta-discussion/173704-general-fta-question.html#post1809188

Thanks for pulling back the old thread.
Strange. I sort of remember that thread, but somehow I missed Sadoun's response to one of my posts (in response 17). Ie I had always assumed that the pole mount was specific to the 90CM and the U=bolt either generic or specific to some other dish, mainly because the pole mount was fairly accurate with my 90CM, and so many people have posted (over on the Sadoun site) about the U-bolt not being accurate with the 90CM, however it was apparently the other way around. So it's interesting to read with respect to which mount is actually for which dish. Only explanation in my case would be that my lnbf arm was bent enough to make the pole mount seem more accurate. No big deal either way I guess, since that adjustment is generally just a starting point unless you're going to use it as a fixed dish.
Anyway, strange that the mount intended for the 80CM generally works better on the 90CM and the mount intended for the 90CM works better on the 80CM.