Bell TV in NYC - LNB spacing and aiming

[CanuckInNY]

Hi guys, I'm new to the forum (but not FTA), and wanted to start by thanking all the contributors whose posts I read to get me going in the right direction!

I now know that a 20" Dish Network setup will not get 82W reliably (at all?) down here (in NYC), but a 30" dish should work, so I ordered the Winegard DS2077 plus a DishPro Plus LNBF from Solid Signal. I probably just turned some heads with the idea of mounting the DPP on a 30" dish I guess I should have done more reading about LNB separation vs. focal length, but I think it's still worth a shot. if you're interested in discussing the math with me, I'm all ears.

This post is long, sorry. If you have any feedback on any portion, I'd love to hear from you. I have separated two aspects that I'm interested in: LNB spacing, and aiming.

LNB SPACING:

I started with reading the post by MJFlash: "Offset LNBs: Theory & Practice", and thought, "oh no, the LNB spacing is going to be wrong". After scratching my head some more, and reading up on spherical coordinates and how to calculate the angular separation of two points with known lat,long or az,elev coordinates, I think there is reason for hope. Here's what I've got:

DS2077 dish: d=760mm, F=448mm (F/d = 0.59 from spec sheet)

DPP LNBF: center-center spacing of two throats = 68mm

angular separation = atan (68/448) = 8.6 degrees

Now the less obvious question: what angle separates 91W and 82W from my location? My first knee-jerk answer was "9 degrees" but then I realized that's only from the earth's center (or another location 42164 km from the satellites). I am somewhat closer (many "look angle" calculators report distance to sat), and the angle ends up being 10.1 degrees. This number doesn't agree with the formulas in MJFlash's post, where he uses the difference between elevations to each sat for the vertical offset angle (I agree), and the difference in azimuths for the horizontal angle (I'm suggesting this is a problem). The problem I see with this is that azimuth lines get closer together as you increase elevation, so a 10 degree change in azimuth represents an angle smaller than 10 degrees if you're looking anywhere above the horizon. If anyone follows this argument, please let's chat!

So, assuming that 10.1 degrees is correct (again, I'd love to discuss this with any interested members), I've got a small error of 1.5 degrees between 8.6 and 10.1. What is the effect of this error? There are no satellites that close to either 91 or 82 with circular polarization, so I won't be getting interference, but I will have off-peak gain. According to a formula on Wikipedia, the half-power beamwidth of a "typical" parabolic dish is about 70*24mm/760mm = 2.2 degrees, so I will lose about 1dB of signal if I split the difference (0.75 degrees off-center for each sat). I know this isn't the best I can get, but given that I have a DPP LNBF in my hands, it seems to be the logical first step. I will end up buying a pair of single LNBs if this setup doesn't work.

AIMING:

The next issue is how to aim a DS-2077 with a dual LNB (no way to adjust offset) when the dish mount doesn't have a skew adjustment. I thought about big pliers and twisting the support arm, but then I remembered the polar mounted dish I used to have at my last place. If the dish is polar mounted, the skew takes care of itself as the dish is swept through the arc of satellites (i.e. satellites are always on a line that is parallel to the horizontal axis of the dish). So instead of mounting the dish on a vertical pole, I mount it on a pole that's tilted north at (90-my lat) degrees from vertical.

But the dish needs to be set for "-5 degrees" elevation for this to work, and the range is only 20-70 degrees. More poking around got me to think about mounting the dish upside down (with the LNB arm above the dish) to get the correct elevation. It looks pretty funny in the living room, but the math says it should work. I can post a pic if anyone wants to see it.

It's pretty cold right now, so I'm not exactly rushing to the roof to try this out this week, but I'm interested in hearing from others if they've tried a similar setup, and if it worked or not.

I'm a displaced Canadian who's pining for some curling on TV and the local news from home, and I'm a bit of an antenna geek, so this seems like a perfect project!

Thanks all,
Dave

 

[Slartibartfarst]

In NYC, I would expect a std 20" Dish to work just fine for BOTH birds.

 

[Satellite Expert]

Be aware that N6 will have the same coverage as N4

 

[CanuckInNY]

In NYC, I would expect a std 20" Dish to work just fine for BOTH birds.

Hmm, that's not what I gathered from reading posts about the tightened up N4 footprint. Can you elaborate?

 

[catamount]

It's not a big deal to mount a twin LNB to a 30 inch fixed dish. Years ago, I put a legacy twin on a 30 inch round starchoice dish.
I cut off about 2 1/2 to 2 3/4 inches from the arm. Then mounted the y-adapter from a bell/dish 20" dish.
Mounted mine vertically on deck fascia. Instead of straight up and down, I positioned the baseplate at about 10 o'clock and 4 o'clock. I also put a 6 inch length of 1/2" copper pipe under the right side (facing dish) to give me the approximate skew.
Signals were low to high nineties, plenty of signal

Catamount

 

[CanuckInNY]

It's not a big deal to mount a twin LNB to a 30 inch fixed dish. Years ago, I put a legacy twin on a 30 inch round starchoice dish.
I cut off about 2 1/2 to 2 3/4 inches from the arm. Then mounted the y-adapter from a bell/dish 20" dish.
Mounted mine vertically on deck fascia. Instead of straight up and down, I positioned the baseplate at about 10 o'clock and 4 o'clock. I also put a 6 inch length of 1/2" copper pipe under the right side (facing dish) to give me the approximate skew.
Signals were low to high nineties, plenty of signal

Catamount

Thanks Catamount! I'm glad to hear that this setup worked for you. I bought a Y-adaptor as well, but noticed that the part that connects to the arm is meant for a rectangular arm, and I have the D-shape. I'm going to try one sat at a time first with a centered LNB (the DPP has two "spigots" that will mount directly to the arm) to see what the best signal is for each sat on-focus, and then see what degradation there is off-axis.

I might try cutting off a piece of the arm to compensate for the length of the Y-adapter, and then modify the Y-adapter to fit on the D-arm. My only fear is once I do this, and find that for some reason I need to go with two single LNBs, I'll have to add that length back somehow. Where there's a will, there's a way

I'll let you know how it goes...

 

[catamount]

Yes, it would be a shame to cut off part of the arm on a new dish. The channelmaster 75E elliptical dishes have arms that are bolted on. You can easily remove these and drill a new hole to compensate for the y adapter.

Also, you might try taking a piece of rectangular arm from a scrap DTV or DN dish and mount it on top of your existing arm. The lnb's may sit a bit higher, but since their circular signals, I've found their isn't a lot of difference in signal quality.

Good luck, Catamount

 

[CanuckInNY]

Update (5 months later!)

So I finally got around to modifying the Y-adapter to increase the LNB spacing to 105 mm (calculated from a number of sources, based on the angular separation of 91W and 82W -- 10.1 degrees, and some ray-tracing). The wider spacing is required as a result of the longer focal length (448 mm) of the Winegard 30" dish compared to the 20" dish (estimated at 315 mm) that the Y-adapter is designed for. I can post some pics if people are interested in seeing how I modded it.

Once I hacked off a length of the original arm to account for the length of the Y-adapter, I attached the Y-adapter and two single LNBs, and aimed based on online look angle calculators. Got 91 back, had to do a check switch for the receiver to recognize 82, and then peaked the signal strength. Confirmed that both 91 and 82 are peaked at the same position.

Old TPs at 91 are between 86 and 100, new Nimiq 6 TPs are between 0 and 55. This is no worse than when I had a single LNB pointed at 91.
Signal on 82 ranges from 56 to 71. Not spectacular, but decent.

Early observations suggest that the Nimiq 6 footprints are tighter than Nimiq 4 at 82 (I can only lock onto half of the eight N6 TPs, but get all TPs on 82). Time will tell if the rest of the N6 TPs are as tight.

Bottom line: two LNBs on a 30" dish is possible for 91 and 82, but the spacing is wider than on a 20" dish. DPP dual LNB is not a good fit for this (fixed spacing is too narrow). Hope this info is helpful to others who might be thinking about the same kind of setup.

 

[CanuckInNY]

Here are a couple of pics to show what it looks like.

 

[Aust54]

I am in Northern New York during the summer but reside on Long Island during the winter
Can you tell me what you have lost with the new bird? I am concerned about what I am coming back
too
Austin