DISH HEIGHT VS WIDTH PERFORMANCE?

[MikeinBaja]

Have been experimenting with 2 old dishes today to determine which is a beter overall performer.

Dish #1 is an old Direcway that measures 23" high X 40" wide (920 SQ").
Dish #2 is an old SKY dish that measures 33" high X 30.5" wide (1,006.5 SQ").

Used the same LNB on both on the same pole mount. Aligned each to peak on 123 with a Birdog. Weather clear (not a cloud in the sky).

Difference between the 2 on both signal strength and quality was dramatic:

#1 Max SS = 72 Max Q 69
#2 Max SS = 90 Max Q 89

Yes, Anole, I adjusted the focal length to peak for each one .

Question: Obviously dish #2 performs way better - is this due to the 9.4% additional reflector area or is the height of the dish more important that width? Dish #2 is also closer to being symetrical - effects?

Other that determining that the rescued SKY is way superior, I think all I've done is to raise alot of questions in my mind about dish size and overall design shape. These are impressive number improvements for just under 10% total area change.

What am I missing here or are these type of numbers to be expected?

PS: Am toying with the idea of somehow flipping the old DW dish 90 degrees to see what shakes!

 

[Pepper]

It's the increased surface area definitely. Also, the shape of the feedhorn will matter to some extent. If you have a feedhorn matched to the specific dish that is ideal.

 

[SatelliteAV]

Maybe an over simplification.... Reflectors with increased height will attenuate terrestrial rf (earth noise). Reflectors with increased width will attenuate signals from adjoining satellites.

If you are working low elevation satellites that are near the horizon, try using an increased height reflector then experiment for increased signal quality by rotating 180 degrees so the offset arm is hanging the LNBF from the top. This will often provide even greater rejection of the terrestrial noise that can often wash a signal that is near the horizon.

 

[MikeinBaja]

Maybe an over simplification.... Reflectors with increased height will attenuate terrestrial rf (earth noise). Reflectors with increased width will attenuate signals from adjoining satellites.

If you are working low elevation satellites that are near the horizon, try using an increased height reflector then experiment for increased signal quality by rotating 180 degrees so the offset arm is hanging the LNBF from the top. This will often provide even greater rejection of the terrestrial noise that can often wash a signal that is near the horizon.

Brian,

I'm at an EL of roughly 54 for 123 (my avitar is my test bed location , scenery kind of sucks, but someone needs to do it) and I'm shooting over the ocean.

How much terrestial noise difference shooting over salt water rather than land? More due to the salinity? Less? What effect would this have on two dishes at the same site?

Didn't flip the old DW 90 degrees this afternoon as I planned - got sidetracked with geting the golf cart running for mama . Weather has turned nice & warm!

I'm still way suprised at the performance difference between these two dishes though.

 

[Anole]

signal on 123°?

I would have expected better Q on the DirecWay dish, due to it attenuating the noise from the next bird over at 125°.

You say you used the same LNB.
I gotta think it's a round one, which much better matches the Sky dish.

Wish you'd posted pix.
But, it's still fun guessing! -

BTW, I spent some time this afternoon brainstorming with one of my buddies, how to best mount two matched LNBs 'n horns to the DirecWay.
I'll prove it's better if it kills me! Or you! -

 

[Mr Tony]

Maybe an over simplification.... Reflectors with increased height will attenuate terrestrial rf (earth noise). Reflectors with increased width will attenuate signals from adjoining satellites.

I remember a couple years ago there was a feed that was always on the same spot and had interference with an adjacent satellite so I couldn't use the 30" dish

Had to use the DirecPC dish which was 39 wide by 23 high to lock the signal

 

[Larry1]

Don't forget that when measuring SQ" that the height will be decreased when looking at the dish from the satellite's point of view or the LNB's point of view as these are offset dishes. Just comparing the SQ" of each dish is not a true comparison. What you need to calculate, is the effective height as looking at the dish from the satellite.... i.e. signals point of view. The width however is the same from the satellites point of view. (well, minus any rolled or formed edge) Also, the wider dish may be tuned for multiple focal points, and not just one focal point. This would probably meen that the curve is a compromise to achive better performance for the LNB's not at the exact center.

 

[MikeinBaja]

I would have expected better Q on the DirecWay dish, due to it attenuating the noise from the next bird over at 125°.

You say you used the same LNB.
I gotta think it's a round one, which much better matches the Sky dish.

Wish you'd posted pix.
But, it's still fun guessing! -

BTW, I spent some time this afternoon brainstorming with one of my buddies, how to best mount two matched LNBs 'n horns to the DirecWay.
I'll prove it's better if it kills me! Or you! -

Could the fact that the DW dish is "wider" actually amplify the interference from 125 (closest sat)? The "narrower" old SKY dish might actually help to cut the interference.

I still have the SKY dish up on the test bed (but it's dark) will try for pics in the morning.

Anole: have a DW dish for you next time I'm up that way. Missing the arm right now, but I've got a solid line on a freebee arm - if you are still interested in messing with these.

 

[MikeinBaja]

Don't forget that when measuring SQ" that the height will be decreased when looking at the dish from the satellite's point of view or the LNB's point of view as these are offset dishes. Just comparing the SQ" of each dish is not a true comparison. What you need to calculate, is the effective height as looking at the dish from the satellite.... i.e. signals point of view. The width however is the same from the satellites point of view. (well, minus any rolled or formed edge) Also, the wider dish may be tuned for multiple focal points, and not just one focal point. This would probably meen that the curve is a compromise to achive better performance for the LNB's not at the exact center.

I hear you loud & clear - but the S&Q number differences are staggaring to me - and probably the difference between watching something and going below minimum Q in weather.

 

[SatelliteAV]

You are very fortunate to have such a beautiful open sky! Terrestrial noise is probably not great since the elevation is at 54 degrees, but it will still be a factor.

BTW.... Terrestrial noise is not just the RF generated from our personal electronics, power lines, broadcasters, radar, etc. It also originates from solar flares, signals trapped and bouncing between the earth and ionospheres, etc. The earth also radiates a great amount of RF energy.

The purpose of an LNBF and reflector combination is to reflect the greatest amount of desired signal into a properly illuminated LNBF and reject unwanted signals (noise) I.E. Side lobes or noise generated by the earth. The orientation of the reflector is unimportant as long as it optimizes these objectives. The simplified measurement available to a hobbyist is an increase in Signal Quality or SQ (ratio of good signal-vs-noise will provide less errors in decoding the signals).

I respectfully question the assumption that the orientation of the dish needs to match the polarity of the satellite. From the perspective of the satellite, the ratio of the horizontal to vertical axis is irrelevant. The reflector gain potential is a combination of surface area and ability to focus the signal to a point with high efficiency.

The LNBF (F = feedhorn) must efficiently capture the reflected signal without under illuminating (only seeing a portion of the reflector surface) or over illuminating (picking up signals from beyond the edges of the reflector). Think of a flashlight with an adjustable beam aimed at the dish. When the flashlight beam is zoomed in, only a portion of the dish is illuminated. Widen the beam focus and light spills over the edges of the dish. Feedhorn under/over illumination occurs when the feedhorn is not designed to match the reflector's shape. This results in the LNBF gathering unwanted signals or not picking up all the reflected signals that the reflector is providing. End result? Lower SQ.

One should not assume that a reflector with a higher horizontal ratio is designed for multiple satellite reception. It may have been designed for single satellite reception and rejection of nearby satellites. Usually a hobbyist can determine the design of the dish by observing the LNBF configurations provided by the manufacturer for the specific reflector.

The test results are probably exaggerated by a feedhorn mismatch and possible damage to the reflector. In my exhaustive LNBF testing, LNBFs that perform well on oval dishes often perform very poorly on elliptical dishes. Feedhorn design is extremely import on reflectors with extreme height to width ratios.

 

[B.J.]

Very interesting discussion.
As mentioned above, the differences are really significant.
I must admit that I would have predicted different results, at least quality wise, because despite the fact that all the equations I have seen say that resolution or beam width is only dependent on gain, it just intuitively seems like a wider dish should be better at eliminating adjacent satellite interferrence, which I thought would have made the oval dish have better quality.
The suggestion above, that perhaps the dish was not a true parabaloid (or ellipsoid) section, but a compromise for receiving multiple satellites is possible, but I doubt that they would do that for a Directway dish that I thought was only intended to receive 1 satellite. And a perfectly shaped dish will only have 1 focal point regardless of whether it is oval or round.

Anyway, after reading all the above, I started thinking that perhaps the width of the oval dish is not really being illuminated by the lnbf. Ie if the lnbf of the oval dish is illuminating a 23" circular pattern, which is likely, then the extremes of the width of the oval wouldn't even be seen by the lnbf. This is basically what I've observed with the Ku feeds on my BUD, ie only about the center 4' of the dish are actually illuminated.

Anyway, I'm thinking that the oval dish is really effectively a 23" dish, due to poor illumination of the width.

I'm curious relative to what the FLs are for the 2 dishes . Since most LNBFs are designed for similar F/D ratios, I'm guessing that if you calculate the F/D ratio of the oval dish based on it's height, which is the logical way to do it, that the oval dish would be poorly matched to the LNBF used.

Just a guess on my part.

 

[MikeinBaja]

Here are the pics of the SKY dish on the test stand.

Edit: No idea why the first pic is rotated?

 

[harshness]

Might there be a difference stemming from how many slots each dish is designed to capture?

 

[primestar31]

Here are the pics of the SKY dish on the test stand.

Edit: No idea why the first pic is rotated?

I'm envious! You live in that house year around, and you are watching sat tv!

I'd be out on the beach like this:

 

[Anole]

You say you used the same LNB.
I gotta think it's a round one, which much better matches the Sky dish.

There ya go!
If you used the same LNB shown on the SKY picture, then the DirecWay won't perform worth diddly.
That LNB doesn't see the entire dish properly.

My DirecWay dish has a matching feedhorn, but not the LNB.
And, I have nothing that'll bolt to the feedhorn.
So, I'll be testing with the hardware shown below.
And hoping it'll match (see / illuminate) the dish pretty well.

Didn't you have a spare DirecWay LNB around there?
Seems I recall you saying you did.
So, slap it on the DirecWay and then do the comparison again.
That's the only way to make for a fair test.

BTW, your view kills me!
I don't know how you can put up with all that eye-abuse!

Anole: have a DW dish for you next time I'm up that way.
Missing the arm right now, but I've got a solid line on a freebee arm - if you are still interested in messing with these.

I thought we were looking for the big molded DirecTV dishes?
But another DirecWay? Cool!
Properly installed and tuned, they can be pretty nice.
You might want to use one for an orphanage.
So, I wouldn't want to beat you out of it.

Of course, maybe the SKY dishes are easier to find.
I have some LNBs that look like the one you show.
Perhaps that combination is the best way to go for the kids.

Picture of bandstacked LNB with oval feedhorn, is from Global-CM web site. I have some.
The little Universal LNB came from a close-out deal at IvyStonePlace couple of years back.

 

[MikeinBaja]

There ya go!
If you used the same LNB shown on the SKY picture, then the DirecWay won't perform worth diddly.
That LNB doesn't see the entire dish properly.

My DirecWay dish has a matching feedhorn, but not the LNB.
And, I have nothing that'll bolt to the feedhorn.


Didn't you have a spare DirecWay LNB around there?
Seems I recall you saying you did.
So, slap it on the DirecWay and then do the comparison again.
That's the only way to make for a fair test.

BTW, your view kills me!
I don't know how you can put up with all that eye-abuse!



I thought we were looking for the big molded DirecTV dishes?
But another DirecWay? Cool!
Properly installed and tuned, they can be pretty nice.
You might want to use one for an orphanage.
So, I wouldn't want to beat you out of it.

Of course, maybe the SKY dishes are easier to find.
I have some LNBs that look like the one you show.
Perhaps that combination is the best way to go for the kids.

Picture of bandstacked LNB with oval feedhorn, is from Global-CM web site. I have some.
The little Universal LNB came from a close-out deal at IvyStonePlace couple of years back.

I need to dig into the shed, but I'm positive I've got another LNB for the DW. Picked up that other DW arm this afternoon, but no LNB on it . Rest of the hardware is on it though. Will let you know what I come up with.

Yea, the view sucks, but someone needs to do it. No grass to cut either .

You know, I had that DW dish mounted on the test bench and never got really great Q numbers on it - but I'l mount it again with the OEM LNB on it and report back on numbers.

I don't use the DW dishes for the orphanages. Way too heavy, and the tripod mounts are a bear to come by. The stamped international dishes work well for that and just use standard J poles.

SKY dishes like this one are all over the place down here - but many of them are still in use (even though folks hate the service). LNB that is on them is crap though - but they are linear round ones so replacing is a breeze.