Horizontal Weaker than Vertical - Me or the bird?

[bmcglynn]

Hi,

I am noticing that most all of the Horizontally polarized signals I recive are significantly weaker (3-6 dB) than those that are vertically polarized.

Could this be an adjustment to be made on my side, or just the way of transmission through the atmosphere? I notice this on AMC 21 (Ku), AMC 18, Galaxy 12, and others.

Has anyone seen this before? How do I tune up a single polarity?

Thanks.
Brian

 

[B.J.]

Hi,

I am noticing that most all of the Horizontally polarized signals I recive are significantly weaker (3-6 dB) than those that are vertically polarized.

Could this be an adjustment to be made on my side, or just the way of transmission through the atmosphere? I notice this on AMC 21 (Ku), AMC 18, Galaxy 12, and others.

Has anyone seen this before? How do I tune up a single polarity?

Thanks.
Brian

I don't have an explanation, but I've seen the same thing on AMC21, from the start. In my case, I've assumed that it is because that sat is one of my far west sats, and there are trees involved, so I've assumed that there must be some issue with the propagation of the horizontal signals through leaves or through extra thickness of atmosphere, although I never did try to experiment to see if perhaps rotating the LNB by 90 deg would result in the vertical being weaker, which would indicate some voltage issue, ie a difference between the LNBF working at 18V vs 13V.
Anyway, I don't have an answer, but have seen similar things with that one sat anyway.

EDIT: After writing the above, I realized this was in the C-band section, and that reminded me that I am pretty sure that I observed the same thing on AMC21 using my BUD, ie which uses a polarotor, and doesn't have the 13/18V issue, but I'm not 100% sure if this was the case. But in any event, I don't remember seeing this on C-band, just on Ku.
Right now isn't a very good time for me to experiment, since even though a month ago I could receive AMC21 via my fixed dish, motorized 3' dish or via the BUD, now I have completely lost reception on the BUD and 3' motorized dish due to the leaves coming out. With the 3' dish, I first lost all horizontal transponders, and then a week or two later lost the vertical, so it seems like it must be propagation through leaves for me. But if you're not seeing the sat through trees, I'm not sure what it would be.

 

[Lak7]

How do I tune up a single polarity?

What type of LNB / LNBF are you using?
Is it consistent across the Arc?
Give the Dish a quick String Test to see if it's warped.

 

[B.J.]

What type of LNB / LNBF are you using?
Is it consistent across the Arc?
Give the Dish a quick String Test to see if it's warped.

I'm not saying it's not a factor, but I can't think of any reason why a warped dish would favor one polarity over the other. I know that I used to read posts which suggested that the left/right of a dish received one polarity while the top/bottom received the other polarity, but this just isn't the case. I went for nearly a year using just the top half of my BUD once (not knowing that the bottom 2/3 was blocked by a pine tree), and it didn't affect the polarity reception.
I do often wonder though about side lobes, however, and whether it's possible that side lobe reception might be polarity specific. I don't think this is the case either though, because I also was once tuned in on side lobes instead of the main lobe, and didn't notice any polarity issues.
I used to read posts from people who claimed that there is a difference in polarity propogation on sats near the horizon, but AMC-21 is the first time I've really ever noticed any significant difference in signal levels of the two polarities, so I've never thought that this was a real affect. However AMC-21 is really the first KU sat that has ever been near the horizon for me, so perhaps it's a KU only affect???
Anyway, I really don't think it's a warped dish effect, because my BUD is as warped as a dish can get, and I don't see polarity differences in general. The only place I see the issue is on AMC21, and it's with ALL dishes. I think both polarities *should* reflect equally off a dish regardless of shape.

Back to the OP'er though, I'm curious whether he's using a feed with a polarotor or a C/Ku LNBF? If a polarotor, perhaps there is a mechanical blockage keeping the polarotor from turning. It might be worth looking into the throat to make sure that the probe is actually turning 90 deg when switching polaritiy. If it's a LNBF, then I think rotating the whole lnbf would be a good experiment to try.

 

[Ironsides]

Horizontal

Horizontal should in theroy be a signal that would travel further than a vertical signal. That said, I am not sure why you are seeing a weaker signal, it could be how your receiver processes the signal or how your antenna is located. It could be how well your antenna is tuned. Coax length to antenna or coils in the line could be changing the SWR before the receiver. The higher SWR can cause problems with a receiver or a transmitter. It's most common problem is with transmitters, higher SWR can cut back the transmitter watts on output. Several things could contribute to your problems...

Coils are one mistake many people make when installing coax...Coils can be made to adjust SWR or act as an inductor, but I see no reasons why a coil of coax is necessary when installing a satellite dish, other than it looks cool. The Lnbf only needs a Rain drip, all that is, allowing the coax at the dish to drop lower than the LNBf before using a tie wrap to secure it...If the coax is to long, the extra should be removed and not coiled up. Lengths of coax can be adjusted...I normally use lengths cut 25, 50, 75, 100 feet and so on. If those lengths need to be adjusted then I cut it as necessay in an even length... In my own experience the shorter the run the better the signal quality...

 

[bmcglynn]

I am using a DMX 741 for the LNBF, and have a clear line of sight for AMC 21. For AMC 12 - there are some trees nearby, although for 105W - there is a clear shot to the sky and I still see the issue.

The only thing I could think about would be a potential voltage issue for the 18V that Horizontal polarization needs. I have tried my First Strike FS1 meter, as well as an AzBox and see the same results.

The dish as a 10' Channel Master that visually looks in good shape. I will give the dish a string test to see if there is any warpage.

 

[Lak7]

I will give the dish a string test to see if there is any warpage.

If your dish has a hole in the center, you could disconnect the Arm and carefully flop the Dish over and String both the Front and Back. Then eyeball both sets of "cross hairs" to see if you are centered too.

 

[Hermitman]

Here's a couple of thoughts for you.

Concerning the lnbf 13/18 voltage - These are just the switching voltages. Every lnbf I've torn apart always has a separate 8 volt voltage regulator IC powering the lnbf. The 13/18 voltages merely switch the flow of the power voltage inside the lnbf. So if the lnbf is switching polarities, power to the lnbf is not the problem.

Its possible for a warped dish to cause more signal to be receive in one polarity than the other. But only if the dish is warped to an ellipse shape with the long side aligned with the vertical signal. That would allow for more vertical signal to be reflected to the lnbf than the horizontal signal, resulting in a stronger vertical signal. But it seems to me it would have to be really obviously warped.

If your lnbf is not pointing directly at the center of the dish, cocked off in the horizontal polarity, then that might cause more vertical signal than horizontal to be sent down the lnbf's throat.

Besides adjusting the illumination of the lnbf/dish, the scalar ring also straightens out and attenuates some of the unwanted waves reflected from the dish. If the scalar ring isn't parallel with the dish and/or set at the proper distance, this might cause stronger signals in one polarity over the other.

Hope this hasn't confused you too much. Good luck.

 

[Long Hair]

I had problems like that on W5 (4DTV) it ended up being
a bad coax. My Quality was about 15 to 20 differences.

 

[B.J.]

Horizontal should in theroy be a signal that would travel further than a vertical signal.

Why? I can see this sort of thing for some kinds of non line of sight earth to earth propagation beyond the horizon, but I'm not sure why this would be the case for sat to earth transmission, where the only difference is really going through a bit more atmosphere.

That said, I am not sure why you are seeing a weaker signal, it could be how your receiver processes the signal or how your antenna is located.

Relative to receiver processing, there is absolutely no difference between horizontal and vertical. The receiver doesn't know, and doesn't care whether it's horizontal or vertical, other than sending a different voltage to LNBFs. Re "how antenna is located", I'm not sure what you mean by this.

It could be how well your antenna is tuned.

Not sure about this one. I can't see any logical reason for this being different for H vs V, unless somehow the sidebands are different for the polarities, but I just don't see why that should be the case. If the dish surface had a regular pattern parallel to one of the two polarities, I can see how the dish could behave differently for the two polarities, but dishes are supposed to function like solid surfaces. Anyway, I just don't see a reason that this should be polarity specific.

Coax length to antenna or coils in the line could be changing the SWR before the receiver. The higher SWR can cause problems with a receiver or a transmitter. It's most common problem is with transmitters, higher SWR can cut back the transmitter watts on output. Several things could contribute to your problems...

But again, why would it be polarity specific, since by the time it's in the coax, there is no difference between the two polarities.

Basically, I just have a hard time coming up with any logical reason for one polarity to be favored over another, other than perhaps coming in at a shallow angle relative to one of the atmospheric layers, which I assume is related to your first reason above, but you'd think that this would only be a significant issue with sats VERY close to the horion, but maybe not. I was also under the impression that this sort of thing was more likely with lower frequencies. I never heard of this being an issue up in the Ku band. But it's the only thing that seems at all possible to me. Otherwise, I'm at a loss for an explanation.

EDIT: After typing the above, I thought of one other factor that I hadn't thought of before, and that is terrestrial interferrence. Ie, I wonder if horizontal terrestrial signals could be interferring with the horizontal sat signals?

 

[Ironsides]

ARRL

My comments are from personal experiences and come from the ARRL handbook. While I am not up on Satellite to Ground theroy and should have made that clear and I apologize for my mistake.

In my experiences Polarity does cause certain receivers problems if not correctly tuned. Example a Dreambox can tell the differences between vertical and horizontal. Or at least that is my experiences. Not all receivers are the same and some do process the information given to them differently and not all receivers are dumb little boxes.

Feedlines can cause a receiver problems and that is something I know for fact.

Those cute little coils of coax are not necessary from my experiences. Those coils can change the VSWR by acting as an inductor. I learned that from building antennas and reading the ARRL handbook. Mark that down to personal experiences...It doesn't make me right about anything it's just something I have seen for myself. A rain drip is about all that is needed on a dish install and that is just something I learned for myself...

VSWR if not tuned correctly can and will cause problems. I have to assume the satellite dish design has taken SWR into account and should have made that clear. However, the feedline can and does change the SWR that is a fact and not a guess.

Earth Based objects do interfer with transmissions of certain types or modes and polarities. Personal Experiences, ARRL handbook...I learned that from 30 years messing with Radios and Electronics...Years ago I had a satellite dish that worked great all winter long but in the summer months all I could get was the vertical channels....It turned out to be a tree limb that caused that problem....explain that one.. LOL...It would seem to me if it blocked out one signal it would've blocked out both but it didn't. Why? I have no idea! It was just marked down as a fluke of nature...


Horizontal signals according to personal experiences and the ARRL handbook in theroy do travel a greater distance here on earth. That said, I should have pointed out the fact I wasn't particularly meaning from a satellite to an earth based antenna. I should have also pointed out that I am not up on Satellite to Earth Based antenna theroy and apologize for any confusions. I should have clearly pointed that out...my mistake!

The Atmosphere and the Sun can change how signals work and again this can either cause problems for a polarity or not. ARRL handbook for ref...

It was not my intent to debate or argue or to outshine anyone or dissect another guys post...I did not realize it was a competition to get all the answers correct, LOL! We're just here to help each other...

I was just offering something that had not been thrown out there. If I confused anyone, I apologize for my attempt to help. From personal experiences and from reading and studing the ARRL handbook for many years I have learned that things do not always work as they should. Just because someone carved it into stone does not always mean it will work as mentioned and many factors can weigh in. Just because we've not seen every signal problem out there does not mean the solution can't be as simple as a bad coax cable. Sometimes we over look the most simple things in life that can often solve our problems...

I can only go on knowledge I have gained from my own personal experiences and research, what works for me, may not work for someone else. Being old worn out and not seeing the best sometimes I make mistakes and don't see things clearly perhaps this again is the case.

In my case I am more often wrong than right so I'll leave the help to the experts from now on....

I'll add that some information is better than none, even wrong information can be helpful. Anyone taking the time to try and help another is better than no one ever showing up. Which is often the case...

 

[truckracer]

i know amc 21 has a difference in the reception of the H and V. so does satmex 5 here. my dish is well tuned so i know thats not it. G1 HBO max has different signal levels between V and H. i wouldn't worry too much if the other sats are good.

 

[B.J.]

It was not my intent to debate or argue or to outshine anyone or dissect another guys post...I did not realize it was a competition to get all the answers correct, LOL! We're just here to help each other...

I was just offering something that had not been thrown out there. If I confused anyone, I apologize for my attempt to help. From personal experiences and from reading and studing the ARRL handbook for many years I have learned that things do not always work as they should. Just because someone carved it into stone does not always mean it will work as mentioned and many factors can weigh in. Just because we've not seen every signal problem out there does not mean the solution can't be as simple as a bad coax cable. Sometimes we over look the most simple things in life that can often solve our problems...

I can only go on knowledge I have gained from my own personal experiences and research, what works for me not work for someone else. Being old worn out and not seeing the best sometimes I make mistakes and don't see things clearly perhaps this again is the case.

In my case I am more often wrong than right so I'll leave the help to the experts from now on....

Sorry if I made it seem like I was not appreciative of the post, and seemed argumentative. I'm really trying to figure out an explanation, and happy to see any suggestions. It's just that you mentioned a bunch of items which affect the quality of reception, and many of them while important issues just don't address differences in polarity, since once the signal gets to the LNB, the signal no longer has any polarity, so things involving the coax or receiver, while important, just don't seem to possibly be related to this specific issue. I am really having a hard time coming up with any explanation though, so I'm mainly just rambling, trying to come up with an idea while typing.

I have lots of ARRL books too, and years ago I remember reading some things about polarity affecting propagation related issues, but I can't remember the specifics, as it's been years since I read them. When I asked "Why?", it's not that I was arguing, but was hoping for an explanation to refresh my memory. I am familiar a bit with ducting and stuff in VHF, and sunspot affects on 10M propagation, all the long range propagation involved down in the lower HF bands. I sort of remember issues related to vertical vs horizontal on 10M propagation, as I had both a vertical (actually a converted CB antenna) and a horizontal 10M dipole antenna. I remember differences in performance relative to long distance communication, but I can't remember the specifics. But I don't remember any propagation effects like this up in UHF and microwave bands, but that doesn't mean they don't exist, just that I never heard of them.

I'm going more from intuition than knowledge on this subject, and it seems like layers in the atmosphere could have an affect like this, but only if the signal is traveling somewhat parallel to the layers, sort of like if the layers are like window blinds, and the horizontal waves fit through the slats, but vertical waves don't fit type of thing, however if the signal is coming from space more perpindicular to the atmosphere, then I wouldn't expect atmospheric layers to be polarity sensitive, but that's just my intuitive guess, not that I'm suggesting it's fact.


I just got real interested when the OP mentioned something that's been puzzling me for a couple years now. I welcome all suggestions, and sorry if I sounded argumentative.

 

[Hermitman]

Here's another thought for you to ponder. I just noticed you're using a DMX741. That c/ku combination lnbf is similar to the BSC621. In the BSC621, the alignment of the probes in the feedhorn is critical to pulling in good ku signals. Maybe your DMX741 has a probe alignment problem causing the signal differences. Or maybe one of the probes has a slight defect causing signal reception problems. The way to check if the lnbf is the problem is to rotate the lnbf 90 degrees and see if the problem follows or stays the same. Good luck.

 

[Ironsides]

Hi hi

A Good Tech will consider all, even things that may not seem to fit the problem. A simple solution will often times be over looked because we don't consider the over all effect or affect on the system. Pointing out Checking a Coax, checking for a bad connector or bad switch. Looking for a Wasps nest or Ants or even the location itself or the tune of the Dish "lnbf skew" may not sound as though it address a given issue. But we have to consider all things when dealing with an unknown problem. Or better said, a problem we don't know the answer too. While we may think simple things may not have bearing on the workings of our systems, those simple things are part of our systems.

 

[B.J.]

..... Looking for a Wasps nest or Ants or even....

This was something that occurred to me last night, that might be another thing that could have a polarizing effect on the incoming signal. Those nests are so geometrically regular, I could easily see them having more of an effect on one polarity than another.

I'm not familiar with this DMX 741 lnbf. If it is one that electrically switches between two fixed probes, perhaps it's as simple as one of the probes being shorted by some foreign material (or insect as above) around one of the probes, or perhaps one of the electronic components which accept the signal from one probe has degraded. If either is the issue though, it would affect all the sats, but maybe it's only noticed on weak signal sats? In MY case, I only see the effect on one sat, but it sounded like the OP'ers problem was on multiple sats. But turning the LNBF 90 deg should determine if the problem is in the LNBF.

 

[Ironsides]

DMX 741

I have a DMX 741 and really I am not sure about this but I could see a Wasp nest or an Ants nest getting to the probe and causing troubles. Again, I am not really sure about this; but I was thinking the DMX 741 only had one probe inside?? I am not sure about that.. But What I was thinking is, if it had 2 probes then the inner probe could be having an interference issue with the front probe??? Maybe because the front probe is being blocked by a nest? If it has an inner probe but how it is made it would certainly be difficult for wasps to get to the 2nd probe...

I know it has 2 lnbs and I thought the lnbs used a switch for the polarity changes.

The reason I thought this is because I had once tried using 2 receivers on mine and one receiver would over take the other and change the polarity. If it had 2 probes it would seem to me the polarity would remain the same on one band. Unless you changed both receivers???

I also know proof postive that the DMX 741 is picky about skew and being tuned. I have tried it with every receiver I own and it works ok with my Neusat 6000 SP. It also works ok with my Pansats...

I tried it with my Dreambox 7020 and 2 other Dreambox 500's and those boxes wouldn't even scan a transponder because the polarity was inverted. When I installed it I accidently tuned it wrong and I know that for a fact. The Neusat don't care about the Polarity at all, so it will scan with some editing of the transponders. Making the verticals, horizontal or the other way around...The Dreamboxes would not scan unless you edited ALL the satellite information and completely reversed it. Heck of a lot easier to just re-tune the lnb.

Now I tried this same experiment with an a couple of other lnbs because I had thought it was a problem with the receivers. But I saw the same issues pop up on the other lnbs. Which leads me to think a Dreambox is smart enough to know what signal it's supposed to see. I don't know, that is just what I saw happening when I tried it. Or maybe the software or firmware of the Dreambox has to match the data it gets from the satellite??? I have no idea, I never really understood that issue. Again I just figured the Dreams was intelligent enough to know what signal it should be seeing for a given satellite. I just assumed that, but the experiment with other lnb's being inverted kind of does prove the point to me...

Anyway, I know I am off topic a little here but I do know and can say the DMX 741 is a decent lnb and does an ok job. That said, it is picky as heck on tuning and really hard to tweak. Or it was for me...I have noticed that it is weaker on some satellites than others depending on polarity. I never really gave that much thought other than just explaining it as dish pointing and some satellites being stronger than others...