Facts and figures - solid vs mesh dishes?
Hi,
I have seen it quoted here and elsewhere, that a 1.8m solid dish has almost the same performance as a 2.3m mesh dish.
However, when I've been able to find specifications, I typically see a gain figure at 4GHz of 36dBi for 1.8m solid vs 35.5dBi for 1.8m mesh. BTW, you have to be careful if the gain is quoted at somewhat different frequencies, and take this into account.
Now, by my reckoning, a 0.5dB difference would mean a 1.8m solid dish is equivalent to a 1.9m mesh dish. So what gives? Has the quality of mesh dishes improved perhaps, or am I overlooking something?
Joe.
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You also have to take into consideration manufacturers quoted figures are not always accurate.
In reality... forget about numbers... a 1.8m solid dish is similar in performance to a 2.3m mesh dish on C-Band, but will outperform a 2.3m mesh dish on Ku-Band.
If you have not yet bought the dish, other than the gain factors why are you choosing one over the other?
Esthectics? a dish is a dish, just that one you can look thru! You can always paint the solid in chamo or whatever the background is?
Wind load? The soild dish requires more mass in the ground to keep in plumb/stable as compared to the mesh. There is one exception, around 35- 40 mph (55-65kph) the air going thru the mesh causes turbulence that makes it just as solid as the solid dish!
Price? can't help you there cause i don't know which dish(s) you are buying! But as always, caveat emptor, you get what you pay for?
Gain? there is variable gain across 3.7 to 4.2ghz, and if thats an issue, LNA/LNB (which also have variable gains across the bandwidth) temp increase will help there. Check mfr's specs on the Ku gain, depends on the size of the perforation in the mesh! But as a rule MTV is correct!
Last edited by cmangle; 25-02-12 at 07:03 AM.
Thanks for the feedback. If the manufacturer's figures aren't to be trusted, what can I say? That's very disturbing. Are there any manufacturers whose specifications I can trust?
I didn't explain that I was looking into the viability of a C band setup, that was implicit in this forum's title and the 4GHz figure I mentioned. In particular, I'm interested in AsiaSat 3S and 5 (from Sydney), and have seen some very useful information here on mounting offset LNB's for this purpose. I don't see much of interest on Ku band, however, I think the mesh dishes that are dual-band must have a tighter mesh and hence be closer in performance to a solid dish on C band.
In my situation, I have limited space, so a dish larger than 1.8m would be problematic. I also have to raise the dish to get line-of-sight over a neighbouring building, so the aesthetics and weight of a mesh dish are a strong incentive. However, there's no point going to all the trouble and expense if it doesn't work at the end of the day. Hence I have to do my homework first ...
Joe.
aliashere (26-02-12)
Yes, mesh which is rated for Ku is finer than for C-Band only.
Either a solid 1.8m or 2.3m mesh dish should be fine in Sydney for C-Band on the two sats mentioned.
I've found a bit more information that's of interest, although only indirectly relevant to dish performance ...
Firstly, I've concluded that the EIRP for AsiaSat 3S (C band) in Sydney should be about 34 dBw. AsiaSat specify a maximum of 41 dBw, whereas the footprint maps at satbeams.com only indicate 39 dBw at beam maximum, so the figures at satbeams.com must be for the outer edge of each EIRP contour (if that's the appropriate word). From the distance of Sydney to its adjacent contour lines, that's what I estimate. AsiaSat 5 has a higher EIRP in Sydney, so I don't need to concentrate on this.
The other interesting information is a technical description of Dual LNB setup by AsiaSat (it's a bit hard to follow, but it complements the practical information found here) :
Joe.
Last edited by j_ds_au; 25-02-12 at 11:20 PM.
that picture isn't exactly correct. one lnb will be dead on(prime focus)and have the most gain, the other offset lnb will ALWAYS have a little less gain BECAUSE it is offset and NOT in the prime focus position. (this assumes both sats have the same output power (eirp footprint identical)
In that picture the green lnb is prime (asiasat3) and it will have more gain that the offset (blue). The only exception to this rule is if you have a oval dish! so set your prime lnb towards the lower eirp level and let the offset lnb look at the higher power (greater eirp level) sat!
j_ds_au (26-02-12)
Offsetting additional LNBs will always sacrifice a bit of performance, which is fine, providing you have sufficient signal strength/quality left to provide reliable reception. Eg; a sufficient margin above the 'digital cliff'.
If you motorise the dish, you'll get both with optimum signal from a single LNB at the prime focal point.
Plus additional satellites.
j_ds_au (26-02-12)
Here is a great thread on mounting 2 LNB's for AS3s and AS5
Leroy
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Correct. The example happens to favour AsiaSat 5, whereas in Sydney, it is AsiaSat 3S that needs to be favoured, ie. prime focus. Both AsiaSat's have a maximum EIRP of 41 dBw (C band), so in some parts of the world (although not SE Australia), their example choice of A5 as prime focus would be appropriate. Also, in areas where two satellites have similar EIRP, the optimum configuration would be to offset both LNB's equally.Originally Posted by cmangle
Yes, as mentioned at the bottom of that AsiaSat page, the offset LNB does have slightly compromised performance. Unfortunately, they don't quantify it. However, the AsiaSat 5 signal is significantly stronger than AsiaSat 3S in Sydney, so hopefully I won't have to worry about that. In other words, I hope the mutilated scalar rings and offset position will not degrade the A5 signal below that of A3S, so I can just concentrate on achieving adequate performance for A3S. I'd consider a motorised setup as a last resort.
Agreed! After reading that thread, I decided I'd have to join up so that I could see the pictures as well!
Joe.
Last edited by j_ds_au; 27-02-12 at 08:43 PM.
It's over 37 dBw (C band), perhaps 37.5 dBw (there isn't an adjacent higher contour line at satbeams.com, so I can only guess how much it might exceed 37 dBw).
Joe.
Last edited by j_ds_au; 27-02-12 at 08:58 PM.
Well, when I started this thread, I was trying to understand why the reported performance of mesh dishes, compared to solid dishes, was poorer than suggested by the gain figures I could find in various specification sheets (once I took into account the slightly different reference frequencies that were sometimes quoted).
Having given it some thought, it occurs to me that the discrepancy is noise, since after all, that is the other half of the C/N ratio. From the typical gain figures I quoted for 1.8m dishes at 4GHz, we get (by calculation or by using the 'smwlink' software) 63% and 70% efficiencies respectively, for mesh and solid dishes.
Now, that 7% (absolute) difference in efficiency not only means about 0.5dB less signal (which equates to about 10 log (63/70)), but also that the LNB must be receiving noise instead of the missing signal. Part of that noise may be sky, in which case it should be relatively quiet (say 6K), but part of it would be earth, which is rather noisy (say 290K).
I'm not confident to calculate the additional noise in the mesh antenna case, but suspect it may be comparable in effect to the loss of signal, which would mean that for a accurately shaped mesh dish, I'd guess an effective loss in performance over a solid dish of say 1dB. Of course, if the mesh dish is not accurately shaped (historically a problem, from what I've read), then the situation would be worse.
Finally, speaking of noise and the 'smwlink' software, I've found a formula to approximate the antenna noise temperature (in the absence of actual data from the manufacturer's specification sheet) :
Tant = 15 + 30/Diam + 180/Elev (where Tant is in K, Diam is in metres and Elev is in degrees).
This formula was given in an article, attributed to the following web page (now defunct) :
Joe.
Last edited by j_ds_au; 29-02-12 at 10:08 PM.
your aa link isw dead j_ds_au
Yes, I know. That's why I said it was defunct.
The best I can do is refer you to the most recent copy at the Internet Archive :
This has a lot more in-depth information about noise and the like. For our purposes, which is to have data to feed the 'smwlink' software, specifically the antenna noise temperature needed to calculate the C/N margin, the formula I extracted is of most interest, because all too often this data is not specified.
Joe.
Jee I wonder if it has anything to do with the wavelength at ku band
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the earth noise only comes into play at very low look angles, correct?
Good point, I'll look into that.
Yes and no. The lower the angle, the higher the noise.
Joe.
Well, I still haven't found the origins of that noise temperature approximation formula. One would think that any term involving diameter would probably also involve wavelength, but it could also involve the amount of blockage by the LNB or something.
While Tony Townsend's article (linked above from the Internet Archive) does mention (and hence imply) Ku band, I've found a thesis paper from 2002 which also uses this formula for S band (2.4GHz) :
The earliest copy of Tony's article in the Internet Archive is from 2004, which is not conclusive, but suggests the above thesis paper precedes Tony's article. If that is the case, and if there is a dependency on wavelength, then it becomes more likely that the approximation formula is based on S band rather than Ku band. OTOH, it could also mean (if both articles are correct) that the formula is independent of wavelength.
I've also found a program for calculating antenna noise temperature "properly", but haven't figured out how to use it yet :
Joe.
The holes in the mesh dish have to be 1/10 or less in diameter of the wavelength for there to be no loss.
As for noise AND G/T a number of factors can affect it
The quality of the dish.
The material used to construct the dish.
Is the LNBF prime focus or offset.
And fineally what is used to coat the dish.
Noticed I have not even mentioned sky or ground noise yet
As for the software its pretty basic.
First calculate the G/T then calculate the temperature degrees Kelvin value then use these to calculate Noise temperature
Who really gives a shit about the noise figure any way all it does is raise the noise FLOOR of a system.
Last edited by Vic; 09-03-12 at 08:45 AM.
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Thank you Vic. You have drawn attention to a very important point here, and one which is often overlooked by newcomers to (satellite) communications systems.
The dish is only one of many components in a system, (dish, lnb(f), cable, connectors, receiver etc.), and the system is only as good as its weakest link (each individual component plus quality of installation workmanship).
I personally would not recommend using a mesh dish for the reception of Ku-band signals, as in my experience, the surface integrity is not of a high enough standard to give good efficiency - unless you could obtain one of the better American mesh dishes, such as those manufactured by companies such as KTI, Paraclypse or Orbitron, who did certify their mesh dishes for Ku-band use. Even so, they were not perfect.
Having stated that, I have fitted Ku-band LNB's to already-installed mesh dishes, but only where (more than) adequate signal strength allowed me to do so, thus guaranteeing on-going trouble-free Ku-band reception, under "normal" variation in weather conditions.
One has also to take into account, that as orbiting satellite systems age, their power output drops, resulting in less available signal for reception systems.
Manufacturer's specifications need to be taken with some skepticism, particularly in today's world, where turnover and shareholders' dividends are taken to be the benchmark of company success, rather than product quality and pride in workmanship.
One should also understand that manufacturing tolerances also affect the final performance of a product. Performance specifications can only be taken as a rough guide at best, unless we refer to products manufactured for the professional/business market, where reliability and performance are paramount, and purchase price is of far less consideration.
I would certainly accept the claims of world-class dish manufacturers such as Patriot, Prodelin and others, particularly in regard to the products manufactured for broadcast use, which (naturally) have to adhere to much higher standards than those for private consumer use. I have not yet seen a Patriot dish installed in somebody's back yard!
Commonly and readily available mass-produced dishes manufactured in Asia are "made to a price" - a price, which the average purchaser is happy to pay, because the product gives results, which are judged to be acceptable. But please don't rely on claimed performance specifications, particularly at Ku-band, where such dishes perform poorly, usually having many side lobes due to poor surface integrity.
See also my comments regarding LNB noise figures in post #5 at .
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