When it comes to the vtx foxeer has a 25mw 200mw 400mw and a 400mw 600mw 1000mw and is like a $3 differnce. 
Question is is there a point of no benifit and to high of a mw?

Well, it can get complicated. I covered it before in my VTX guide: https://oscarliang.com/choose-video-tran...ower-range

The more power you have, the "louder" the VTX is. So longer the range.

However, most VTX antennas transmit in a sphere (or doughnut shape). You have to distribute this energy over the surface of the sphere. So say you have a certain amount of mW/area. The further away you move from the center of the antenna, the larger the area of the sphere, the less mW/area, so the signal is weaker. You're range is going to be like the radius of this sphere.

Because of the properties of the sphere, doubling the radius will not result in a doubling of the area. In fact, a tiny increase in the radus, can result in a massive amount more area of the sphere.

Likewise, doubling the power, and then going to a distance where you have the same mW/area, will not mean you have doubled youre range.

So you cannot go from a 400mW, to a 800mW, and expect to double the range. More like 1.2-1.5 times the range.

Negatives of high mw VTX:

- draw more current and generate more heat. So overheating may be an issue
- high mw VTX generally are more expensive
- high mw VTX may not be legal, most places in the world only allow 25mW
- It makes it harder for others to fly with you

Generally, the maximum you will want to go is 200mW when flying with others. but it's nice if you can get everyone on 25mW. 
If flying alone, 600mW is fine if it's legal where you are flying.

As for range, I found antenna plays a much much bigger role
Here is my antenna guide in case you're interested: https://oscarliang.com/best-fpv-antenna/

Excellent, thanks so much I will read through these articles

i fly 2000mw, its illegal and I dont care if it interferes with others because there aint much around where i fly....howere you are right i cant get that much improvement from 600mv to 2000mw, its not linear at all....i got about 1.5 times range going from 600 to 2000, however, stronger signal does a heck of a better job getting thru non-dense obstructions, and this is where 2000mv makes a diff over 600mw....but you are also correct antennas is where the gain is at....

Haha i noticed more than once that even my 200mW vtx provides more range than my rc transmitter... so i really don't need more.

it doesn't matter?

(28-Jul-2017, 12:41 AM)Oscar Wrote: - draw more current and generate more heat. So overheating may be an issue

I can attest to this issue! I've burnt out a VTX by overpowering and poor wire management. If you go with higher power you want to be sure that your VTX has lots of airflow and that you don't leave it powered on while stationary for too long because over time that thing will overheat.

Mine was in a spot that didn't have much ventallation and the wires were kind of close and at 600 it melted the wires, caused a short, and burt out. I am lucky that the whole thing didn't catch fire after I saw what happened.

(07-Sep-2017, 10:24 AM)Dennis Pfeiffer Wrote: it doesn't matter?

I think the overall message here is that it does matter, but not as much as the numbers suggest and you should be careful with heat and flying around others and the legalities of it. In general I only fly 200 these days just because it's easier than switching back and forth.

(07-Sep-2017, 10:41 AM)Carl.Vegas Wrote: I think the overall message here is that it does matter, but not as much as the numbers suggest and you should be careful with heat and flying around others and the legalities of it. In general I only fly 200 these days just because it's easier than switching back and forth.

Wise words.

I don't use more than 200mW as anything capable of actually producing 600mW and more is going to need a large heatsink on it just to get rid of the heat. My quads don't have room for anything like that or the regulated power budget to provide power to such a VTX.

High power also requires an antenna that's perfectly tuned - the less well tuned the antenna, the larger the proportion of the transmitted power that is reflected back into the output circuitry. The reflected power generates lots of heat and will eventually destroy the output circuit.

Also, higher power means that unwanted signals, like harmonics, are generated at higher power levels and these unwanted signals can start to cause interference on many other frequencies. Given the low quality of many of the super high powered Chinese transmitters, you can bet they are throwing out plenty of harmonics.

The increase in range from doubling your output power is (in theory) 1.41 times, but it's more complicated than that as the 5.8GHz frequency we use for FPV is more affected by free space path loss than a lower frequency. For example, the free space loss over 1Km at 5.8GHz is 107.72dB. For 1.2GHz, it's only 94.03dB.

If I calculate the effective range that my Skyzone goggles have using Pagoda antennas and a 200mW (23dBm) 5.8GHz transmitter, I get 1.7Km (best case). If I push the VTX power up to 1W (30dBm), I get 3.8Km. Five times the power gets just over twice the range!

For really long range FPV, lower frequencies are needed. The problem with lowering the frequency is that the available bandwidth for a given channel spread also decreases so you end up using wider and wider channels just to have enough bandwidth for the video signal.

Additional range is not linear in terms of power, but additional power can make a huge difference in perceived penetration through dense objects like underbrush, buildings over the crest of a hill, etc. I usually fly at my home so I don't need to worry about interference with others and in one quad I am flying 200 and another is at 500.  With the one at 500 I can fly over the other side of my house, and down my driveway which passes through a lot of dense vegetation and get all the way to the end (about 450ft) before the signal becomes un flyable. With the exception of going over the roof, I am usually flying at 3 - 4 feet off the ground. With the 200  mW VTX, as soon as I lose altitude immediately on the other side of the house, the signal becomes un flyable.  So the additional power does have it's advantages.  I realize that range is limited by solid objects (particularly metal) due to the short wavelength, but increasing the power level will help to increase your range significantly when the cause of the limited range is lack of LOS.  Works for me anyway.