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dbi gain vs range

PostPosted: Fri Jun 08, 2012 11:04 pm
by francoishaha

I just wonder if there is a way to evaluate the possible range of high gain antenna versus omnidirectional antenna.
I actually use the stock antenna for the plane and the receiver. I think it's 3 dbi stock antenna, not shure about that.
but what range should i expect from a 8 dbi directional antenna? is there a "formula"

Re: dbi gain vs range

PostPosted: Sat Jun 09, 2012 12:20 am
by RCModelReviews
Perhaps I ought to do a video or article on this but the inverse-square law applies.

This means that to double the range you have to quadruple the amount of power the transmitter produces (or appears to produce). Doubling the (effective) power is an increase of 3dB. Quadrupling it is an increase of 6dB.

Now since we're not easily able to fit a hi-gain antenna to an FPV transmitter (because it will be too directional) we have to use such hi-gain antennas on the receiver.

So, if you want to double your present range and you're currently using a zero-gain (0dBi) antenna, you'll need to use a 6dBi one instead. (what does the 'i' mean? It means "greater than isotropic" - isotropic being no gain at all)

However, if you're already using a 3dBi antenna, you'll need a 9dBi antenna (an extra 6dB) to get that range-doubling effect.

So if you use an 8dBi antenna you'll have 5dB more than your present 3dBi whip.

5dB of extra gain over your existing antenna means you will get less than twice the range. I could do the math but it would serve little purpose because there are many other factors involved. Suffice to say that in the real-world you can expect between 60% and 70% more range than you're getting now -- SO LONG AS you have the 8dBi antenna pointed in the right direction.

To give you an idea of what I've found -- using CL and SPW antennas (which have a very low gain - under 2dBi) you can easily get about 1.2Kms with a 200mW 5.8GHz system. Fit a 9dBi helical to the receiver and, so long as it's pointed right, you could expect to get as much as 4Kms. However, with 5.8GHz, the actual range is dependent on many other factors such as humidity, terrain, etc.

Re: dbi gain vs range

PostPosted: Sat Jun 09, 2012 3:47 am
by francoishaha
Thanks Bruce to bring some light in this dark matter :)
so it's a square root pattern for the range.
Now i wonder if my equipment is cheap, because the best range i get out of my 600 mW transmitter is 1.1km with the stock antenna, and you tell me that i can easily get 1.2 km with a 200mW... And with my CL, SPW antenna the results are not better. I know there is manny other factors that can affect the range, like antenna emplacement, the RF noise, the area, temperature, etc. I think I got to make more test to get better results with my 5.8ghz system.

Re: dbi gain vs range

PostPosted: Sat Jun 09, 2012 3:54 am
by RCModelReviews
If you're using the Immersion RC (FatShark) 5.8GHz system then you won't get nearly as good performance as with the FoxTech/modules we use. The Immersion RC receivers are quite a bit less sensitive than the others.