Design hints

Plane size small planes are cheap and easy to transport, but they are not optimal FPV platforms. The smaller type the more compromised performance you must expect to get.

Receivers must be located as far away from transmitters as possible, at least a distance that can prove no degrade of the receiver system range when the Tx is operating or not.

It is advised to keep receivers away from all electronics, in general, speed controllers and switch mode supplies are known to be able to jam.

Electric Motor controller a range check must prove no degrade/change when motor is off, halve, full power, this also apply for gasoline and methanol engines

Transmitters for live video or data back telemetry will always have harmonics, like 2x 3x and so on of their wanted frequency the level might be low and under demanded levels, but when located near a receiver that may operate on a x2 or x 3 frequency it is doomed to fail! Plan your frequency bands to avoid using any channel at the exact harmonic of any other system you use.

GPS are often small module receivers, designed to be cheap and just cheap!! That is what we like, but cheap come with a problem price. They are not designed to be working near transmitters; some cannot handle anything not even if it is even GHZ away!! so a SAW front end is a must and a good high level capable front end is a must too, and even then some GPS modules do not like to be located near 900 or 1300MHz video transmitters first harmonic of 900MHz is 1800Mhz, a GPS works at 1575MHz its front end system is often several hundred MHz wide, even if they have a SAW, so 1300 and 1800 can jam it.
a 2.4GHz video transmitter have first harmonic at 4.8GHz so it is no big problem with the harmonics, 
it is a known fact that most 2.4Ghz video transmitters are most likely better with most GPS units.

Video stripes is often seen when a power supply is not clean or ground / signal wires are shared with power or other units on the plane, note if the stripes are constant or change with motors or servos or other items moving or operation, try to touch camera or video transmitter, see if any change in the stripes.

Grounding items with analogue signals specially video signals are super sensitive to grounding problems
you cannot supply a pulsing current on a ground wire to a video connected device, if this ground wire is the same for video signal, it is quite difficult to explain how to split signal ground with power ground. A “Star” kind of grounding has a zero current / zero voltage in the centre point.

Power supply cleanness. Try to mount external battery on different items they are super clean, and is smart to try locate a problem.

Switch mode, it is often seen SBEC or other switch mode regulators are designed to be cheap and have no really good filter, extra 100uH coils and 470-1000uF on the input and output might help, input is just as important as the output.

Shielding some devices radiate magnetic pulses, specially SBEC and cameras, if they are contained in a closed metal case the problem will be less, but weight will go up, iron cases are better for low frequency magnetic problems.

Camera some types radiate radio noise, uncased types are the worst! but cased types can also do it, case is maybe painted and therefore not fully shielded, the wires from it works like radiating antennas, ferrite torrid and aluminum-foil is a good solution, and distance to GPS and other receivers is a good idea tool.

Servos some RC servos types are only designed to be used near receivers, clear enough, but when located near a transmitter they can be jammed, moved or stopped or other weird things even periodic problems have been seen. Tricks/solutions: aluminium-foil around the servo, wire turned on ferrite torrid.

Electrical motors are also inside servos they will also radiate magnetic pulses when they move.
Some systems like video transmitters and cameras and osd systems do not like to be near this field.

Vibrations is a normal problem for receivers and transmitters, their coils and crystals and filters have microphone behaviour, pack in foam and also avoid loud sound

SWR a video transmitter with a badly matched antenna and/or badly grounded will have high frequency currents going on its signal and power cables.

When having range or interference problems, try to isolate different items, like turn it off, or change their position dramatically or bypass its function until you find a change in your problem, then you have found the noisy item.

Radio technics

Any wireless radio system contain of a transmitter side antenna and a receiver side antenna.

Both sides must have same polarization to perform most optimal, the most common polarizations are horizontal, vertical, circular left or right.

If a horizontal is “talking” to a vertical, the link loss will have an added extra loss of 26dB

If a circular left is “talking” to a circular right, the link loss will have an added extra loss of 26dB

If a circular left is “talking” to a vertical or horizontal, the link loss will have an added extra loss of 3dB
This is why it is smart to combine a horizontal or vertical often mounted in a plane, with a circular receiver on the ground, then the 26dB drop can most likely be avoided.

Pointing a whip style antenna to a plane is the worst thing you can do, imagine looking into the end of the whip, It is almost impossible to see from a distance, radio waves work this way too, make antenna most visible and right polarization.

It is normal that radio links have a 26dB extra margin in its link budget / range calculation so you don’t lose contact - when one antenna is rotated unlucky angle, a diversity system can take full advantage of its link budget, so the resulting useable range is almost 10 times as much as a non diversity system, if no other parameters are changed.

A downlink diversity system also solves one other problem, fading and nulling, the most perfect diversity system would have 3 antennas to handle signals from any angle perfect, but the gain from doing this is often minimal and cost and complexity is big, a 2 antenna diversity is the most common compromise.

All cellular systems use diversity on the receiver side and brute transmitter power on the Tx side to perfect the link, a cell side Tx is over 26dB more power full over the handset transmitter to obtain an equal quality link, also handset side have a cheaper receiver with less sensitivity, we do not have space for a diversity antenna system on a cell phone.

Wavelength and frequency - a double frequency will have halve the wavelength. Long wavelength cannot pass thru small holes, like take a 27MHz walkie-talkie and try to use it inside two cars, (cars are made of metal, end the window holes are much under the wavelength) the useable range is then really bad, now go out of the cars and see you get 10 times the range, at higher frequencies you don’t have the car window problem, but air attenuation is higher with higher frequencies, so a longer range is easier to get using lower frequencies.

Gain - Adding more antenna gain will only make the beam more narrow, point the antenna right and you get more range, point the antenna wrong and you lose signal, Any gain over 8-10dB will be hard to point to a moving target like a plane, you need a tracker system or a cheap friend that will work for free to point the antenna.

An 18dB gain yagi antenna on the receiver side and 500mW 2.4GHz video transmitter with 0dB antenna, have a proven LOS of 51km when both polarizations are right.

Range and dB - Improving a systems link budget with 10dB will increase the range by a 3 times factor, 10dB power is the same as 10 times the power.

20dB more power gives 6 times the range, 100 times more power, combining more power with more gain is often the way to get longer range, and also improving receiver side sensitivity is a good way to go.

Bandwidth vs. range - Video transmitters with audio, stereo and mono exist, the video signal is 5MHz wide, and the audio is 5KHz wide, A factor 1000 in bandwidth, so in theory the same range will be achieved on the audio as the video link with only 1/1000 of the power, that is why those systems have a much lower power in the audio, often see -20dBc to -30dBc, dBc means dB under the main Carrier -30dBc is the same as 1/1000.