2.4 Prime focus what will be its equivalence in Offset dish for Ku purposes?

[SatelliteAV]

OK, I was mainly going by the Fortec dishes. For example, I have a Fortec 36" dish. The specs show that it is 33.5" wide, and 37" tall. If you multiply the 37" height by the cos(24.6), you get 33.6 {24.6 is the offset angle}. Ie, it's clear that they've designed the thing to be a shape that looks circular in the direction of the sat, and it's clear that it would have a cross section equivalent to the 33.5" width, but they call it a 36" dish for some reason.
I don't have the h/w specs of other dishes handy, so perhaps other manufacturers are more realistic with their specs.

Most likely the dish size computation is based on the square root of the height times the width (35.20653348456789 in.) then rounded up in the favor of the manufacturer.....

The GEOSATpro 90cm is actually an 89.1487521cm, but that is a mouthful for marketing!

 

[enb141]

that would be yours truly

http://www.satelliteguys.us/1903812-post13.html

it was a test for a member in Hawaii
http://www.satelliteguys.us/free-air-fta-discussion/181927-bsc321sp-f-d-ratio.html

Thanks for your reviews Iceberg, have you tried the Invacom the one that needs the extra feedhorn, or have you tried those ones in an offset dish?

 

[Mr Tony]

no. I had the Invacom quad QPH031 for about 6 months and had issues with it but that was due to the weight of the LNB, the dish not having support arms and my northern location (LNB would hit the roof)

 

[Rafaelccs]

It's 1:1. A 2.4m offset should have a similar gain to a 2.4m prime focus.

Not 1:1

as a matter of fact:

Off set fed antennas are indeed:

A smaller subsection of the same parabolic curve used to produce prime focus antennas, but with a major axis in the north/south direction, and a smaller minor axis in the east/west direction.

The offset fed antenna design offers several distinct advantages over its prime focus counterparts. There is no feedhorn blockage, an important consideration when the antenna aperture is less than one meter in diameter. Moreover, the offset angle at which the feedhorn tilts up toward the reflector is such that if the feed looks over the antenna's rim it will see the cold sky rather than the hot earth. Due to these advantages, the offset-fed antenna can achieve higher efficiency levels than prime focus antennas can generally attain.

Check this link for more deep explanation:

Satellite Receiving Antennas

Check out spec sheet from manufacturer General Dynamics (Prodelin)

General Dynamics SATCOM Technologies - Technical Documetation

2.4 m (8 ft. / 96 inch):

Prime Focus (model 1252 & 1253):

http://www.gdsatcom.com/Antennas/Data_Sheets/24ro.pdf

C-Band (3.6 GHz to 4.2 GHz) midband gain is 37.6 dBi
Ku-Band (10.95 GHz to 12.75 GHz) midband gain is 47.0 dBi

Off set fed (model 1251):

http://www.gdsatcom.com/Antennas/Data_Sheets/5002-355.pdf

C-Band (3.6 GHz to 4.2 GHz) midband gain is 38.0 dBi
Ku-Band (10.95 GHz to 12.75 GHz) midband gain is 47.6 dBi

As you can see Diferential is +0.4 dBi for C-band
and +0.6 dBi for Ku-band

So off set has the edge on smaller wavelength.
Therefore it's vast use in DTH......

 

[pendragon]

I think you're splitting hairs. My exact words that you quoted were: "A 2.4m offset should have a similar gain to a 2.4m prime focus."

But let's take a look at feed blockage, since that's an exaggerated myth that will never die. First, we're talking about Ku. So let's consider a feed that's 7 cm in diameter. I think I'm being generous. The dish area that will be shadowed by the feed is about 0.004 m^2. A 2.4 m dish has an area of about 4.5 m^2. We're blocking less than 0.1% of the dish. This is a loss of 0.004 dB, which is entirely negligible. Of course the support arms also cause blockage, but I think you get the idea. Dish dimensions are not even specified to this tolerance.

What about noise rejection? This has more validity, but if you reread my analysis, you will notice that an offset dish on a polar axis loses this advantage as it scans toward the horizon. A prime-focus dish gains noise rejection as it scans to the horizon. You have to compare apples to apples. Actually, most Ku offset feeds cannot be adjusted for the proper gain taper. This tends to cause them to significantly under-illuminate the dish, throwing away a significant amount of gain. Prime-focus Ku feeds are often adjustable and are far easier to get the maximum CNR.

What about specs? Quoting antenna gains and efficiency is always playing with fire. To measure this one generally needs a feed of some sort. The only way you can do this accurately is to match a feed design to a given dish. Do dish manufacturers do this? Of course not. Designing a feed is a combination of science, art and magic, and a properly matched feed generally requires a lot of trade-offs, compromises, simulations and cut-and-try. So the efficiency measurements are normally done with any old feed the manufacturer wants to use or has standardized on. That may or may not be a good match to the dish. There is another myth that deep dishes have lower gain. They certainly do when one uses a feed designed for a shallow dish, even if one attempts to compensate for the f/D using a scalar. But design a feed explicitly for a deep dish, and the gains can be pretty much the same as for a shallow dish. The same applies to offsets and prime-focus designs.

In the end, if I set up a 2.4m offset and 2.4m prime-focus of comparable quality with matched feeds and tweaked them for the best performance, I doubt you could measure the difference with the finest spectrum analyzer or receiver money can buy. But the cost difference could be readily grasped.

There is a place for both designs and I definitely have a fondness for my 1.2m offset on Ku. It has the best cross-polarization performance of any of my Ku dishes and has a very well-formed beam. Under normal conditions it simply is my best Ku dish. But when we get torrential downpours or ride through solar 'outages', my 1.8m prime-focus easily stomps it.

 

[pedrogarcia]

I thought I would add my 2 cents on this since I use 2.7m prime focus and larger dishes to get fringe Ku signals in Cyprus EU.
I have found that commercial prime focus dishes of the spun aluminium type can vary considerably on the quality of the reflector surface way beyond a 10% effect. On the other hand segment prime focus and large offset dishes are generally steel pressed and manufactured more accurately. My preference is always offset.
With Prime focus dishes 3m and over you have to stand on the dish to make LNB adjustments whereas offset it is not a problem.

 

[Rafaelccs]

I think you're splitting hairs. My exact words that you quoted were: "A 2.4m offset should have a similar gain to a 2.4m prime focus."

Question was about ANTENNA GAIN and even more explicit:

1:1 scale is identical match.

similar by definition is alike; having the same or similar characteristics.

I gave you a link of a well know Telecom person Mark Long, who is a recognized expert on global satellite communications technologies.

Mark Long bio

This was to give you and other readers deep explanation on the design of different antennas....

Regarding antenna gain, I advice to look into:

Definition: antenna gain

But let's take a look at feed blockage, since that's an exaggerated myth that will never die. First, we're talking about Ku. So let's consider a feed that's 7 cm in diameter. I think I'm being generous. The dish area that will be shadowed by the feed is about 0.004 m^2. A 2.4 m dish has an area of about 4.5 m^2. We're blocking less than 0.1% of the dish. This is a loss of 0.004 dB, which is entirely negligible. Of course the support arms also cause blockage, but I think you get the idea. Dish dimensions are not even specified to this tolerance.

One simple question:

If what you say is correct, then why Does DTH operators (DISHnetwork and DirecTV) advice to use off set fed antennas whith it's variations ovals, round, etc. The answer is size, what you claim (blockage) does affect when implementing small antennas (18"; 24", etc.) of course when you use 3.1 m antenna 7 cm will be a very small to nothing blockage......but one important question:

How does this feed supports it self in the antenna? the feed support also has blockage....small but it blocks.
In off set fed antennas the feed support is out of the reception area, this is why you aim 22 to 23 degrees lower than the exact satellites angle.

This is why Mark Long explains the design and use....straight and simple.
They don't advice to use Prime Focus and I quote once again from the Mark Long web page:

Welcome to MLESAT's New Look, Mission

The dish design of choice for most digital DTH systems is called an offset-fed antenna.

There must be an answer for this? if not manufacturers simply wouldn't manufacture different shapes.....they perform different!!!!

What about noise rejection? This has more validity, but if you reread my analysis, you will notice that an offset dish on a polar axis loses this advantage as it scans toward the horizon. A prime-focus dish gains noise rejection as it scans to the horizon. You have to compare apples to apples. Actually, most Ku offset feeds cannot be adjusted for the proper gain taper. This tends to cause them to significantly under-illuminate the dish, throwing away a significant amount of gain. Prime-focus Ku feeds are often adjustable and are far easier to get the maximum CNR.

I don't understand well the use you give to noise rejection.

Noise is as I understand and I quote:

An undesired disturbance within the frequency band of interest; the summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources. (188) 2. A disturbance that affects a signal and that may distort the information carried by the signal

From:

http://www.its.bldrdoc.gov/fs-1037/

I don't understand well your point here, but of what I can see. f/d of off set fed antennas are 0.7;
LNBF's designed for off set fed antennas are designed to match this aperture of 0.7
scalar adjustments for prime focus antennas must match the antennas f/d....

What about specs? Quoting antenna gains and efficiency is always playing with fire. To measure this one generally needs a feed of some sort. The only way you can do this accurately is to match a feed design to a given dish. Do dish manufacturers do this? Of course not. Designing a feed is a combination of science, art and magic, and a properly matched feed generally requires a lot of trade-offs, compromises, simulations and cut-and-try. So the efficiency measurements are normally done with any old feed the manufacturer wants to use or has standardized on. That may or may not be a good match to the dish. There is another myth that deep dishes have lower gain. They certainly do when one uses a feed designed for a shallow dish, even if one attempts to compensate for the f/D using a scalar. But design a feed explicitly for a deep dish, and the gains can be pretty much the same as for a shallow dish. The same applies to offsets and prime-focus designs.

Mayor manufacturers who I have used their products like: General Dynamics, Andrew (Commscope), Patriot Antenna Systems.

Prodelin:

General Dynamics SATCOM Technologies - Prodelin

Long Time ago known as Channel Master latter as Andrew (Commscope) and now ASC Signal:

ASC Signal Earth Station Antennas

Cobham plc some time ago know as Patriot Antenna Systems:

Cobham plc :: Avionics and Surveillance, SATCOM, Albion, Products

They as manufacturers MUST certify their operativity in the range of frequency the user will implement their product. I have been invited to one of their facilities and have seen in their lab how they actually run the antenna gain test. Plain and simple, they have a microwave transmitter that emulates the conditions they want to test on and design based upon resonse, of course this is done under ideal conditions (in the lab) other enviromental factors can't be 100% matched; but at least they have methods to messure in dB to certify their product, of course the products are expensive, therefore they must be certified.

In the end, if I set up a 2.4m offset and 2.4m prime-focus of comparable quality with matched feeds and tweaked them for the best performance, I doubt you could measure the difference with the finest spectrum analyzer or receiver money can buy. But the cost difference could be readily grasped.

Have you actually done this? I have messured db levels on a 3.8 m. prime focus compared with a 3.8 m off set fed antena and it actually gives more gain on my spectrum analyzer.

I have installed in the caribbean area:

Only Professional grade (solid)

Off set fed:

4 ft.; 6 ft.; 8 ft.; 12 ft.

Prime Focus:

2.4 m; 3.1 m; 3.8 m; 4.0 m; 4.2 m; 5.0 m; 6.1 m; 6.3 m; 7.5 m; 9.0 m.

In all my installations I must use spectrum analyzer....

I was a regular member of DBSForums, my speciality is signals in Fringe areas since 1988.

Bottom line:

off set fed antennas do outperform prime focus antennas at smaller wavelength frequencies, but after a certain size prime focus technology is cheapper therefore I have implemented it and have had to design my own feed. Manufacturers sell large off set antennas much more expensive than it's counter part prime focus antenna...what they claim is the funds spent in the developement of this 3.8 m off set fed antenna was so that we have to sell it at this price......

For example a Prodelin 3.8 m off set fed could be sold over $11,000 and even up to $17,000 depending on your requirements.
a 3.8 m prime focus could be sold arround $3,500 to $6,500 depending also on your requirements......

If we go to the size in question:

2.4 m off set fed antenna depending on manufacturer could be found arround $1,500 and $2,500 depending on configuration and manufacturer.

2.4 m prime focus antenna could be found arround $400 and $1,500......

Best Regards.

 

[pendragon]

1:1 scale is identical match. similar by definition is alike; having the same or similar characteristics.

Frankly, in this world 1.001:1 and 1:1.001 qualify as similar gains as I originally stated. My first professional satellite installation was in 1979 and I have worked off and on in this and the business of RF long before that time. I'm a physicist by training and have designed and modified antennas/feeds for decades. I think pedro hit the nail on the head. The efficiency and gain of a dish is dependent on the materials and quality of manufacture. After all the physics here is pretty simple. Beyond that one needs a feed matched to the dish for maximum efficiency. Unfortunately in the world of FTA this rarely happens.

There are differences between prime-focus and offset. You've conveniently tried to redirect my feed blockage calculations from a 2.4 m Ku dish to a 0.5m dish. But OP didn't want a little DBS dish. Using your DBS examples to extrapolate to a FTA dish 23X larger is specious. Nevertheless, feed blockage on a 0.5 m still less than 0.1 dB for a 7 cm Ku feed. The real reason the DBS companies use offset dishes is economy. At these smaller sizes, offset dishes are cheaper to manufacture and ship, and for the installer far simpler and faster to set up. As pedro also points out, at lower latitudes prime-focus dishes are hard to work with. Even at 40N I can't easily reach the feed on my 3.2 m at true south.

But economy is a double-edged sword. As the size of dish increases, offsets become less economical to manufacture as you adroitly point out. When I drive by commercial dish farms, all the big dishes are prime-focus. But this is a FTA forum and my comments here are primarily directed there. At 2.4 m, new FTA prime-focus dishes are less than half the price of a 2.4 m offset. In fact they're often free. If you have to ship one, a 2.4 m FTA is normally sectioned and fits in a box often smaller than a 1.2m offset. When you're paying dimensional weight for shipping, that makes a big difference.

Also in keeping with this being a FTA forum, people here don't have the luxury of locking one 3.8 m dish onto each bird. We also are limited for the most part to using one-dimensional drives on a polar axis. Offsets are a real problem here, because motors for much more than a 1.2 m pretty much don't exist in the land of FTA. Even in the commercial world offsets are often fixed. It's the prime-focus dishes with the AZEL drives. Why am I arguing this? Because the offset advantage for noise rejection applies best if the dish is perfectly vertical. That's practical for a fixed dish, but doesn't apply to an offset on a polar mount. My 1.2m offset noise rejection becomes abysmal as I rotate it down to the eastern and Atlantic birds in Colorado. Thank goodness for prime-focus dishes that improve at the lower elevations.

The bottom line is OP asked if a 2.4 m would beat a 1.5 m offset. The answer is yes, no matter how you try to spin the arguments. And yes, a 2.4 m offset would be in the same ballpark as a 2.4 m prime-focus, all things being equal.

 

[pendragon]

I don't understand well the use you give to noise rejection. <snip> I don't understand well your point here, but of what I can see. f/d of off set fed antennas are 0.7; LNBF's designed for off set fed antennas are designed to match this aperture of 0.7scalar adjustments for prime focus antennas must match the antennas f/d....

Since you keep editing your post, I feel as though I'm responding to a moving target. So I'll now respond to something you apparently added after I started writing my previous post.

The purpose of a satellite dish is manifold. First it must collect and focus energy from the bird. Second it must shield the feed from environmental noise sources. Generally feeds are designed to taper their beam so the outer portion of the dish is not actually used to collect energy from the target. In fact it is mostly used for rejecting thermal and other noise. That's what I meant by noise rejection. The application of this is as much art as science.

Optimizing a feed design isn't as simple as moving a scalar to a different point on the feed for a given f/D as you imply. A scalar can be part of matching, but it is not the only factor and it is not the only way to do this. A good feed design will consider the relative power of the E-plane, H-plane and cross-polarization as a function of angle across the frequency band(s) of interest. There's a lot of compromise involved and maximizing CNR is often an intuitive call because it will depend on how a given dish is pointed relative to unknown noise sources. It may be as or more important to balance the planes or minimize cross-polarization.

Unfortunately there's no standard when it comes to dish and feed design, and that is as much true for offset dishes as for prime-focus. With offset dishes, one is largely stuck with what the feed designer chose. If you happen to put a feed designed for a f/D of 0.7 on a 0.5 dish, you will lose gain. The converse would be even worse, so my belief is offset feeds are designed on the high side. Offset dishes are available for f/Ds of say 0.5 - 0.8. That's a sizable range when you have no means to match/adjust the feed.

Prime-focus feeds are only a little better with a scalar adjustment. This is not feed matching in any real sense of the word. In FTA we have to live with these limitations because we like to mix and match. Other than setting up a lab there's not much one can do to really match feeds and dishes. I try and I have a fair amount of test equipment, but I'm well aware I'm partly hunting in the dark. The commercial world can be better, but economy often drives designs to less than great optimization.

 

[Rafaelccs]

Frankly, in this world 1.001:1 and 1:1.001 qualify as similar gains as I originally stated.

so 47.0 dBi is the same than 47.6 dBi ?

To make such a statement you must have proof that in deed they're equal, what proof do you have?

By the other hand my proof is widely known.....physically talking. I have posted links that illustrate the main difference.

They're only equal if you compare from where they're conceived....based upon that one is the most efficient area (off set fed) from the surface of the other (Prime Focus).

What's different? the design, shape, efficiency.

antenna efficiency: The ratio of the total radiated power to the total input power. Note: The total radiated power is the total input power less antenna dissipative losses.

antenna gain: The ratio of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance.

Both concepts extracted from:

Federal Standard 1037C: Glossary of Telecommunications Terms


To compare Gains and that they would be equal you need to compare a smaller off set fed antenna to a larger prime focus antenna.

more deep in concepts:

The aperture efficiency is a function of where the feed antenna is placed (in terms of F and D) and the feed antenna's radiation pattern.

extracted from:

Antenna-Theory.com - Parabolic Dish Reflector Antenna(Page 3)

Off set fed are the most efficient due to the fact that f/d is 0.7

Regular Prime Focus antennas are designed for a f/d between 0.20 and 0.45
(Check chaparral feeds to see their specs and verify my post).

Therefore although both are Prime Focus as I have stated one is more efficient than the other. Efficiency is paramount to obtain the most.....

The efficiency and gain of a dish is dependent on the materials and quality of manufacture. After all the physics here is pretty simple. Beyond that one needs a feed matched to the dish for maximum efficiency. Unfortunately in the world of FTA this rarely happens.

Prodelin Antennas which I posted as an example use indeed a micromesh inside their plastic injected molds to manufacture their antennas.

Of course no doubt the best surface is completly solid as Patriot antenna manufacture their prime focus and off set fed antennas....

At the level I have posted all manufacturers have high standards of manufacturing.

The bottom line is OP asked if a 2.4 m would beat a 1.5 m offset. The answer is yes, no matter how you try to spin the arguments. And yes, a 2.4 m offset would be in the same ballpark as a 2.4 m prime-focus, all things being equal.

He asked and I quote:

Hi guys I was looking to buy a bigger sat but unfortunatelly prime focus bigger than 1.5 M are so expensive so instead of that I was thinking to get a 2.4 M prime focus for the price of 1.5M offset so I would like to know what would be the equivalent size of this 2.4 Prime focus into offset.

you answered and I quote:

It's 1:1. A 2.4m offset should have a similar gain to a 2.4m prime focus. However there's also real life and you need to compare apples to apples. You need feeds and LNBs matched to the dish in question. Using an offset LNBF on a prime-focus dish is guaranteed to get less gain than the dish is capable of providing. There are Ku feeds for prime focus dishes and they work very well. You also did not specify the construction of the 2.4m you are considering. For Ku-band, you probably want a solid 2.4m instead of a mesh or a perf because surface imperfections can eat up gain for Ku on big dishes.

Do you still have the same answer?

If my english isn't so bad (not my primary language) he's asking the equivalent size of off set fed antenna to a 2.4 m prime focus antenna.....based upon antenna gain.

The real answer should be 2.23 m

In other words if he buys a 2.4 m prime focus antenna the equivalent gain on a off set fed based antenna design is 2.23 m no manufacturer has that size....but he might get a rough idea!!!!

This why I posted and I quote:

Bottom line:

off set fed antennas do outperform prime focus antennas at smaller wavelength frequencies, but after a certain size prime focus technology is cheapper therefore I have implemented it and have had to design my own feed. Manufacturers sell large off set antennas much more expensive than it's counter part prime focus antenna...what they claim is the funds spent in the developement of this 3.8 m off set fed antenna was so that we have to sell it at this price......

For example a Prodelin 3.8 m off set fed could be sold over $11,000 and even up to $17,000 depending on your requirements.
a 3.8 m prime focus could be sold arround $3,500 to $6,500 depending also on your requirements......

If we go to the size in question:

2.4 m off set fed antenna depending on manufacturer could be found arround $1,500 and $2,500 depending on configuration and manufacturer.

2.4 m prime focus antenna could be found arround $400 and $1,500......

Best Regards.

P.S. sorry I have to correct a lot my posts....I think in spanish and then I must translate to english....some gets lost in translation so then I must correct so readers can get my main idea.

 

[pedrogarcia]

Rafaelccs

I have been in RF engineering commercial and military for what feels like ions and I learnt very quickly in the commercial world, nth degree theory matters little.
There comes a time when it is the product quality that is looked at in terms of cost of manufacture (inc development) v number of sales. Basic economics and that's when they get involved with accountants (with no RF knowledge) and corners are cut. Then its the marketing guys (with no RF knowledge) who dream up a bunch of figures for the adverts to its potential customers.

A little cynical but true, happens in nearly all industries.

Military is a different ball game it's called lets fleece the tax payer.

Guys in this forum want the best they can acheive at a reasonable price and this forum is a really good pool of reviews, experience and assistance to the FTA guys.
So back to the original post my view is as previously posted, offset or segmented dishes provide the better signal quality

 

[Sadoun]

So back to the original post my view is as previously posted, offset or segmented dishes provide the better signal quality

Usually, you can achieve the same signal gain on a smaller (surface area) offset dish than on a bigger prime "segmented" focus dish.

 

[pendragon]

so 47.0 dBi is the same than 47.6 dBi ?

Is 47.6 dBi the same as 47.596 dBi? I'm sorry, but I'm not going to engage in a proof by example debate. That has no merit in the realm of logic or science.

The apparent question on the table is whether an offset dish of a given diameter necessarily has a non-negligibly higher gain than a prime-focus dish of the same diameter. To do this properly we would first have to define what constitutes identical diameters for the two design types. It's not obvious to me how to accomplish this in a manner that does not bias the result. We have not taken that first step and it probably would be quite hard to come to an agreement on even that. Perhaps that has something to do with our debate.

While you have cited a number of pedestrian definitions, none has addressed the crux issue of why offset dishes theoretically should have a higher gain than an equivalent prime-focus dish. However it's rather interesting you provide the following non sequitur above the graph you present:

Off set fed are the most efficient due to the fact that f/d is 0.7

If your claim had merit, one would expect the aperture efficiency of a reflector with a f/D of 0.7 to be larger than reflectors with lower f/Ds. However in contradiction, your graph shows the same maximum aperture efficiency of ~0.8 can be obtained for f/Ds from about 0.37 to 0.73. Thank you. This is basically my claim.

I could go a step further and point out in the reception world we are invariably interested in maximizing CNR rather than gain. The converse is true when transmitting. Gain can be a misleading specification that manufacturers like to use as it can easily be taken out of context.

As a citation, refer to "Microwave horns and feeds" (Olver, 1994). On p218 we have "In low-noise applications it is desirable to use reflectors with small f/D values. The required feed must therefore have a wide-angle pattern. Such feeds are often difficult to design and seldom give high gain factors with low crosspolarisations. With proper computer optimisation, however, they can be designed to give satisfactory characteristics." Unfortunately offset dishes are better at delivering gain than CNR. This is generally applied in the realm of radio astronomy where low f/D reflectors (0.25-0.4) are far more common those around 0.7. If you're looking for a deep space signal, you need every fractional dB of SNR you can get. Are we doing something different in FTA?

Relative to OP's original question and my response, the issue was whether a 2.4 m prime-focus was a good alternative to a 1.5 m offset. The basic answer is yes. While I could have said just that, I believe the additional detail I provided was important to maximize OP's use of a 2.4 m dish for Ku. You have offered no proof that a 2.4 m prime focus is equivalent a 2.23 m offset through some physical law. As both are sections of a paraboloid, there is no theoretical difference. To rays emanating from a satellite, the dish collection area defines the maximum achievable gain. Absorption, surface imperfections and departures from a perfect paraboloid will reduce this. A practical feed will also not collect the entire energy reflected and should taper its gain before reaching the edge of the reflector. All of these apply equally well to prime-focus and offset designs. Any departure would violate the conservation of energy. You're welcome to reply and have the last word, but I think I have invested enough in this discussion. Cheers.

 

[pedrogarcia]

Usually, you can achieve the same signal gain on a smaller (surface area) offset dish than on a bigger prime "segmented" focus dish.

from personal experience I would certainly go along with this.

 

[zamar23]

It sounds like the most valuable thread reviewing satellite dish performance factors in public FTA forums. If a bit more can be added to compare small size DBS type dishes form factor & performance comparison versus targeted sat range, this thread would be perfect and more rounded up from FTA community learning standpoint. May be 2.4 number should be removed from the thread title, as it becomes more generalized in content.

 

[Rafaelccs]

This is what your state (all I'm stating is they won't work 1:1):

It's 1:1. A 2.4m offset should have a similar gain to a 2.4m prime focus.

Is this your post? or am I altering your original post?

IF you Claim that a 2.4 off set fed antenna operates 1:1
compared to a 2.4m prime focus.

post definitive proof that they indeed operate 1:1

so I can send to all antenna manufacturers (I know) and tell them to stop their production line on off set fed antennas.....and start manufacturing only prime focus.
That there is no need for that design (off set fed) and it was a mistake in the first place.

Books (theory) and Manufacturers can't be wrong!!!

Mr. Paul Wade has published tests...actual tests and I quote from his tests:

Summary

An small off set parabolic dish can deliver more performance than a conventional dish of the same size.

find this extract here (page 16):

http://www.qsl.net/n1bwt/chap5.pdf

or check his web page:

W1GHZ (ex-N1BWT) 10 GHz Amateur Radio Page

W1GHZ Microwave Antenna Book ONLINE

Mr. Paul Wade (w1ghz@arrl.net) : tell him to take off from his web site this chapter and that prime focus perform 1: 1 compared to off set fed antennas as you state here on Satelliteguys.com

Please send your deep explanation that Off set fed antenna performs as good as prime focus, so he throws away all the research he has dedicated to off set fed antennas and how they perform, what should be the correct feed (conical, scalar, etc.) all was a waste of time.

post a copy here so I can send also to other publishers that have entire chapters dedicated to off set fed design.

That link I found on Wikipedia

[ame="http://en.wikipedia.org/wiki/Offset_dish_antenna"]Offset dish antenna - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/Filearabolic_antenna_types.png" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/2/24/Parabolic_antenna_types.png"@@AMEPARAM@@commons/2/24/Parabolic_antenna_types.png[/ame]

who also has a page to give concepts and links.....

after all of them agree I will agree....

If you're wrong will you agree????

I have stated that prime focus is cheapper (no doubt about that) but I think it's bad for any kind of forum to post inaccurate information, this is the reason why I keep posting on this, some readers might get the impression they wasted their money on off set antennas and that they should have bought a prime focus (much cheapper) the reality is that off set fed antennas are more efficient than regular prime focus........

 

[Rafaelccs]

from personal experience I would certainly go along with this.

I appreciate some people do agree.....I tried to give a rough idea that a 2.4 m prime focus antenna should perform approximately like a 2.23 off set fed antenna....that was math manufacturers don't concieve that size as commercial (I guess based uppon db / manufcturing price, etc.), they go for 4 ft. 6 ft. 8 ft. 12 ft. etc.

Best Regards.

 

[zamar23]

Given the multitude of factors affecting efficiency of a single (often first time) Satellite Dish setup in real world conditions by an unexperienced limited in funds and readily available equipment choice FTA fan, I tend to agree with Pendragon's logic.

Another interesting amateur read on the subject:
Online Microwave Antenna Book (Dish & Feedhorn performance explained)

Any heroes to compare 18 - 24 inch small dishes, and logic behind their selection to receive available clear channels on circular sats?