Sean Mota said:
One question I have for Rod is about Rain Fade. Why? I have a Dishnetwork system and others have DirecTv 18" Dishes. A lot of us have experienced rain fade with Voom. Some had requested a larger dish and this seems to make a difference; There are three terms in the system setting menus of the VOOM box.
1-Signal Quality
2-Signal Strength
3-SNR (Signal Noise Ratio)
(a) What is the relation between the three?
(b) why the Fain Rade on VOOM (even for customers in the East)?
Sorry for the delay in answering this. I had a new release of TSReader to get finished first.
Signal Quality and Signal Strength are not really standardized. Most likely the Signal Quality is a combination of the strength and the SNR.
Signal strength is most likely related to the gain being applied to the AGC (automatic gain control) inside the tuner. The stronger the AGC is driven, the weaker the signal so you can think of the signal's waveform strength in terms of inverted AGC gain.
SNR is simple - signal to noise ratio. The trick here is if the SNR is before or after the error correction.
I'd be interested in knowing what typical values shown are for these three variables (I don't have a Voom receiver).
On to the rain fade. First, the ONLY thing you can do about rain fade is to try to get around it with gain, i.e. a bigger dish. That said, even with a big reflector if there's enough antenuation in the atmosphere, you're going to loose the signal. For example, I have a 1.8 metre dish pointed at AMC-1 in the Ku-band and it even looses signal during heavy downpours.
So, why would Voom be worse than DIRECTV or Dish Network?
Looking at their signal on a spectrum analyzer, they're a bit weaker than Dish's signals at 61.5w - perhaps 2-3dB down which is surprising. Maybe Voom is keeping the power down because their TWTAs (the amplifiers on the satellite) aren't linear enough which is a big factor when dealing with 8PSK signals. But then the 8PSK signals transmitted by Voom have a better error correction algorithm than the DIRECTV/Dish QPSK transmissions so this most likely isn't an issue.
Most likely there are three reasons:
1. Anyone west of the Mississippi will be looking through a lot more atmosphere than for the more centrally located DBS orbital slots.
2. Timing is bad. I have dishes pointed at 119, 110, 91 and 61.5 with receivers on those locations running on a single channel full-time. When it's stormy here I can see the direction of the storm by looking at which channels are out. With most storms moving west to east, I see 119, then 110, then 91 and finally 61.5 go out one by one as the storm passes by. Quite often when the storm is almost over, I'll see video back on 119/110 but still out at 61.5.
3. The skew angle on the dish is wrong. This is quite complex so bear with me.
This is easier to think of in terms of linear (vertical/horizontal) versus circular transmissions. Imagine a satellite at 100 degrees west and you're at 100 degrees west. If you point a dish at this satellite and want a vertical signal, the feedhorn will be perfectly vertical because there's no skew in the signal.
Next you pack up your bags and move to 90 degrees west. Now there's a 10 degree skew in that vertical signal so to optimize the signal you adjust the feedhorn by 10 degrees. However you need to think about what that vertical signal looks like - it's no longer bouncing off the relector straight down the middle - it's now at a 10 degree offset hence the feed needs to be lined up.
But the big issue here is dish illumination since we're dealing with a circular polarity transmission. So, you're at 100 degrees west and trying to receive 61.5 west. The signal coming from the satellite is skewed by almost 40 degrees and because of that angle the dish won't be illuminated as strongly as it should. You have to rotate the entire dish by 40 degrees to get it fully illuminated.
This is why multi-satellite feeds like the 3rd generation DIRECTV and Dish Network DishPro dishes all have a skew adjustment - to keep the dish on the same angle as the satellite arc and therefore illuminate the dish better.
I would very much imagine that Voom's 2nd generation antenna (for 61.5 and AMC-6) will have a skew adjustment.
HTH,
Rod
