Note....... I have tested my Futaba 6ex 2.4ghz out to 1.75 miles with no problems, but it probably can not do 4 miles. The idea is to fly the UAV out of transmitter range autonomously. This is for sure illegal and this video system is hypothetical and has not been tested. So um... I don't endorse it.
I am planning to use this setup for a school project UAV.
I hope this can save other people from searching the internet for hours on end for the best deal.
Yes it is fully approved but as with all video transmitters over 10mw you have to have a amateur ham radio license to use it legally. But if you fly in the country I don't think anyone would notice if you didn't have a license.
It is a good idea to keep the uav gps unit away from any transmitter on the plane so if you have it a yard away there should be no interference.
Also it would be good to keep it away from the rc reciever.
As for people on the ground noticing, I don't think anyone would because 1280mhz is a very clear, unused channel. And it would not mess with the crowded 2.4ghz channel which is used for wireless home phones, bluetooth, most wifi modems and rc radios.
There is one component you have given little attention to in your notes: the antenna.
Don't under-estimate the antenna's role in the performance of your rf link - in fact, when it comes to rf links in RC/UAV/FPV projects (keeping in mind frequency and power output restrictions), antenna's are the one component you can develope in-house and realise significant gains with in doing so.
Antennas are the one un-restricted component to rf-links, but always seem to lag behind in development terms when it comes to putting together video/data/telemetry links for RC/UAV/FPV projects.
Quite why this is the case with RC/FPV and UAV projects, I can't figure out, but there you go.
Granted, the size of most hobby/amatuer UAV airborne platforms severly restricts what can implemented when it comes to antenna's - and its made even worse when the dynamic nature of the enviroment in which they fly, and how they fly, is taken into consideration - but, there is no such restriction when it comes to implementing an antenna (or suite of antennas) for the ground-station part of your project.
So long as you are not too concerned about the mass/weight of your ground station antenna/s, there is a considerable amount, performance wise, that can be achieved - lots of oppurtunity to realise very worthwhile real-world gains.
Another plus point about antennas: they are the one component in the overall rf hardware set-up, that can be treated independently i.e. changing, adjusting, altering an antenna very seldom impacts on the other components e.g. changing a video camera often means ensuring the replacement spec's are compatible with the rest of the hardware, or changing an amp will more than likely require the input/output levels of hardware to be plugged into the amp are not to low/not to high .... etc etc......
Changing an antenna usualy requires only an understanding of the before/after gain values, and possibly also radiation pattern changes. This is a somewhat simplified statement on the subject, but the point is: it's generally a lot easier to change an antenna to realise real beneficial performance gains, than to change other hardware components which are power output/voltage/current, or otherwise restricted in some way or another - as most hardware components in the UAV/RC/FPV rf-link will be .......
I'm going on a bit here - antennas have been my profession all my working life - so I'll end off with this suggestion: budget & time permitting, throw your chosen antenna to the dogs (before actually buying it) and go read up some on axial mode helical antenna's and the advantage they offer over just about every other antenna type that exsists, and why they are so idealy suited to UAV/RC/FPV projects.
Just read the wikipedia on axial mode helical antennas. It sounds like a good way to increase receiver sensitivity and you wouldn't have to point it at the plane like a patch antenna. But... how to make or buy? That is the question.
Nope - you will have to point it towards the aircraft, but how accurately it has to be pointed will depend on the beamwidth of the helical antenna versus the beamwidth of the patch. You wil have to compare the 2 - the wider the beamwidth the less accurately it will need to be pointed. The figure you need to look for is the 3dB beamwidth, which is often given as part of an antennas' published typical Vertical and/or Horizontal Radiation pattern. The wider the better.
Understand why I suggest the axial mode helical versus the patch(?)
It's all about radio signal polarisation - which in the case of axial mode helicals is circular (well, strictly speaking if you measure it with lab equipment it will be found to be slightly elipitical, but for all intents & purposes it can be considered circular).
Antennas that are designed to receive or transmit rf signals as circular polarised [signals] are significantly less sensitive to changes that occur in signal polarisation i.e. using a verticaly polarised antenna to receive a horizontally polarised transmission will result in enormous signal loss, and using a horizontally polarised antenna to receive a verticaly polarised transmission will also result in enormous signal loss. Using a circular polarised antenna to receive will result in loss - yes, but it will in theory always only be 3db, no matter what the polarisation of the transmitted signal that it is receiving. That transmitted signal could be verticaly polarised, or horizontaly polarised, or anything in between, or it could be made up of components of both vertical and horizontal polarisation - but the loss will remain constant on the recieve side (in terms of polarisation). In other words: factor in that 3dB loss and you will have factored for your worst possible signal loss scenario due to polarisation.
Whys this an issue for flying models?
Because the antenna on a flying model changes its orientation in relation to the ground station as it flys (e.g. when turning to change direction) - and when that change in oreintation occurs, so to does the polarisation of the signal of the sigmal it is transmitting. In short, so long as you factor in for the 3dB loss, you will eliminate signal drop that occurs as a result of change in polarisation as the position/angle of the flying model changes - which, based on studies conducted with small/medium tactical type military UAV's with ISM type video downlinks, accounts for between 43% - 47% of dropped video signal!
I don't know what you feel, but to me 40% plus is a significant amount of time - certainly worth eliminating.
The remaining time in which video downlink signal loss occurs is down to primarily (but not only):
a) overall signal strength (insufficient Tx/Rx gain - rf power in other words).
b)and, radiation pattern not actualy reaching the ground station antenna - as when a UAV banks towards/away from the ground station [antenna] so much so that the ground station antenna "falls" outside of the UAV Tx antenna beamwidth.
Anyone reading these comments, who has an understanding of antennas and how they work, will quickly appreciate that I have not covered all the issues related to Keegans original question. Correct - I have concentrated on one aspect only: that aspect I believe Keegan can most easily address, and which overall is likely to give in the most consitant improvement to video downlink/reception for the given video hardware setup. The great thing about ground station antennas is that there is no legislated max size or sensitivity or configuration that users are confined to - one is at liberty to do as they wish, more or less. The freedom RC/UAV/FPV flyers have to implement ground station antenna Tx/Rx solutions can go quite a long way to overcoming both the legal (freq & power output) and practical (airborne platfrom size) constraints.