dbi vs db

[steffo78]

I am wondering about dBi and dB.
When you look at specifications for dishes some manufacturers use dBi (triax,Andrews) and others dB (Gibertini,Fortecstar).
I thought that dBi is the gain you get compared to a isotropic radiator (non-directive antenna), and dB is used to compare with a dipole.
The dipole would be something like 1.5dB better than iso.
This is a big difference for a dish!
Does someone know if its always the dBi figure that should be used???

 

[PaulR]

dBi is as you say, gain compared with an isotropic radiator.

dB is just a way of comparing one figure with another. It's like saying "three times as much" or "half of". In other words you have to know what it's being compared with for it to make any sense. It might be compared to a dipole or it might not. It might even be relative to an anglepoise lamp but unless they tell you you don't know!

 

[Likvid]

Same thing goes by dish manufacturers that are low quality, they often only say like 40dBi and doesn't specify the gain at what frequency which most often means they want to hide the real figure or the dish isn't performing that good so they can say the real gain.

 

[steffo78]

So then I have to assume that it is always dBi since only cheaper dishes is specified with dB.
This would be the most logic answer.

I thought that dB when you measure gain on a dish was something predefined, you compare it to something, guess its not.

About the frequency, that is really annoying.
It is not easy to compare when you measure at different frequencys.
Usually it is around 11700, but gibertini is 10950 and all cheap stuff you can assume to be at the highest possible 12700.
Also efficiency is not for real, like Jonsa claims 75% on some of their prime focus dishes... Have even seen over 80% on a prime, dont remember name right now.
Would be good with a standardized method of measuring, then you could see what you are paying for and performance!

 

[Likvid]

So then I have to assume that it is always dBi since only cheaper dishes is specified with dB.
This would be the most logic answer.

I thought that dB when you measure gain on a dish was something predefined, you compare it to something, guess its not.

About the frequency, that is really annoying.
It is not easy to compare when you measure at different frequencys.
Usually it is around 11700, but gibertini is 10950 and all cheap stuff you can assume to be at the highest possible 12700.
Also efficiency is not for real, like Jonsa claims 75% on some of their prime focus dishes... Have even seen over 80% on a prime, dont remember name right now.
Would be good with a standardized method of measuring, then you could see what you are paying for and performance!

You guessed it right..... Commercial quality dishes specify gain at either 11.3 or 11.7GHz, that is the norm. Low quality dishes specify it very high, like at 12GHz to gain 1dBi or something to make it seem better. Some doesn't want to reveal the real gain at 11.3 or 11.7, like Gibertini so they specify it at 10.95GHz. And forget all dishes made in China, they are all crap, Fortec, Jonsa, Wavefrontier.......

 

[steffo78]

Thanks guys!
I am now enlightened, and perhaps other people too!