Gain of a dish: how to measure it?

[telefrancisco]

Hello everyone.

I don't know if this is the correct forum to post this question, but I think many wise people is around here. Moderators: If it's not, please, move where it's appropriate.

The question is simple: in all specs sheets of dishes, it's clearly stated the Gain of that dish, calculated for a certain frequency (usually in the middle of C and/or Ku band) as well as a focal length.

So, the question is: how they do it?

I think this will be interesting to know because some people may want to know the gain of their dishes when changing focal lenghts instead of the gain of the whole system (what's measured with the signal meters that many people have in their houses).

 

[truckracer]

You have to take the dish down...then line it with plastic, connect a small water meter to your garden hose, calibrate the meter back to zero, now fill the dish like a swimming pool-the number of gallons is your gain db number!....LOL JK, im not really sure if you dont have the spec sheet.

 

[linuxman]

I don't know how it is measured from a technical standpoint.

Generally though, the flatter the dish, the higher the gain.

With that in mind, a dish with a F/D ratio of .42 or .40 has a higher gain than a dish with a .32. The downside of higher gain, is interference from other adjacent satellites. The upside with a deeper dish, is virtually no interference.

Hope that helps!

 

[harshness]

Square inches.

As it is with ice cream, the bigger the scoop, the greater the gain. At the frequencies involved, there isn't much you can do to filter out undesirable signal as you might with a conventional TV antenna.

 

[zamar23]

Some sources give this formula to calculate Dish Gain for a certain signal frequency:



were Sr - surface of dish geometric projection on a plan, perpendicular to LNB axis at the dish focal point;
lambda - selected wave length in the air;
nu - factual aperture efficiency, i.e. dish surface usage coefficient with the selected LNB (having certain fixed aperture angle) mounted at the dish focal point.

Some software packages and web calculators can show a dish gain, like

Antenna Gain and Beamwidth

When a dish manufacturer specifies Gain in a dish model spec, its a maximum theoretical gain achieved with a matching LNB by a dish of ideal geometry (if the spec data is accurate ). For more precise calculations you'd need to use RF related Engineering Handbooks.

 

[telefrancisco]

Thanks for your replies to all!

However, my question was related on how to do the measure of the gain not theoretically but practically, as this would be a good idea for DXers.

 

[zamar23]

A user can only measure comparative dish gain (i.e. compare a certain dish gain to a baseline dish with a known gain, when both have matching LNBs or the same LNB mounted - depending on the test purpose) using traditional DXers toolbox or some more accurate STBs, or a combo of Sat Card and Viewer app that gives S and Q values with known conversion formulas to signal gain in dBs and BER. Often BER (bit error rate) measurement makes more sense than signal gain along (since all kind of signals - relevant and not - receive gain after being reflected from a dish surface) to optimize a dish & LNB system for a certain sat signal reception. Some Sat Card demodulators can calculate and reflect BER on a PC screen when using appropriate software tools. The higher BER - the lower Q, and at certain BER threshold for a given Modulation & FEC scheme no sat signal can reliably be decoded. Meaning you see either dark blocks on a screen pic, or the channel picture comes and goes...only to never come back again. This whole thing means, one might be able to receive a select sat signal range more reliably at lower cost by optimizing Dish & LNB reception system rather than merely increasing dish size & specked gain.

 

[harshness]

However, my question was related on how to do the measure of the gain not theoretically but practically, as this would be a good idea for DXers.

In the context of satellite dishes, DXing isn't really a consideration.

Given their decidedly directional nature, gathering skip or other unintended reflections is probably not something one should hope for (but they might all but eliminate multipath interference).

 

[telefrancisco]

In the context of satellite dishes, DXing isn't really a consideration.

Given their decidedly directional nature, gathering skip or other unintended reflections is probably not something one should hope for (but they might all but eliminate multipath interference).

I don't know about the US, but here in Europe Satellite DXing is somewhat different to Radio DXing. While the definition you give is adequate for traditional (Ham or not) Radio DXing, DXing applied to satellite means "To receive a signal in a territory out of footprint".

For example, one of the more famous examples in Spain is trying to receive BBC Channels (with a Big Dish -with more gain-) that are, officially, out of the footprint (it covers just the British islands because of the ownership of the transmited programs).

 

[FaT Air]

Antenna Measurements

 

[harshness]

For example, one of the more famous examples in Spain is trying to receive BBC Channels (with a Big Dish -with more gain-) that are, officially, out of the footprint (it covers just the British islands because of the ownership of the transmited programs).

The same thing goes on with US programming in Mexico. It comes down to square inches and beam coverage.

There is no magic; any properly designed dish of a given size is going to net pretty much the same signal as any other satellite dish of the same facing surface area. At the extremes, there might be some problems with the LNB "seeing" the extents of the dish, but that would be a poor design.

 

[paulvr]

Dear Telefrancisco,

I don´t agree with you at all. I have seen people use that word DX for what you describe but that is to my opinion a very wrong qualification.
For me DX means to try and get a signal that only during special weather conditions can be received, be it Shortwave or FM DX-ing.

In the old days of analog radio and tv setting up a much higher antenna in order to get for instance an FM or TV station from a neighbouring country would NEVER be considered DX-ing while putting up a system with rotor in order to get for instance random reception of TV channels on Band 1 across all of Europe due to troposferic conditions would be so.
DX is just the wrong word for sat reception because whatever you pick up you can get pretty much every day.

But such silly things also depends on the country you live in. Here in Holland for instance, idiots speak of websites being "on air" in the same way people speak about radio stations being "on air". Initially I tried to correct people on that stupidity but it´s no use.

Just like the foolishness of adding "FM" to radio stations meant to sound youthful. In the USA the addition "FM" makes sense because there are stations who own both an FM and an AM frequency. But here in Europe it´s never used that way. Here in the Netherlands we even used to have a radio station on 1008 kHz Medium wave that for at least two years called itself "Radio 10 FM" while in reality broadcasting on the AM band. How foolish can you get? People seem to get dumber by the day....

 

[telefrancisco]

Dear Telefrancisco,

I don´t agree with you at all. I have seen people use that word DX for what you describe but that is to my opinion a very wrong qualification.
For me DX means to try and get a signal that only during special weather conditions can be received, be it Shortwave or FM DX-ing.

In the old days of analog radio and tv setting up a much higher antenna in order to get for instance an FM or TV station from a neighbouring country would NEVER be considered DX-ing while putting up a system with rotor in order to get for instance random reception of TV channels on Band 1 across all of Europe due to troposferic conditions would be so.
DX is just the wrong word for sat reception because whatever you pick up you can get pretty much every day.

But such silly things also depends on the country you live in. Here in Holland for instance, idiots speak of websites being "on air" in the same way people speak about radio stations being "on air". Initially I tried to correct people on that stupidity but it´s no use.

Just like the foolishness of adding "FM" to radio stations meant to sound youthful. In the USA the addition "FM" makes sense because there are stations who own both an FM and an AM frequency. But here in Europe it´s never used that way. Here in the Netherlands we even used to have a radio station on 1008 kHz Medium wave that for at least two years called itself "Radio 10 FM" while in reality broadcasting on the AM band. How foolish can you get? People seem to get dumber by the day....

Buy my definition was for Satellite only. For radio I agree with you. About the "FM" thing, absolutely agree with you, although here in Spain we use FM for FM and AM for AM (there are Stations that broadcast same on AM and FM).