Hope you all don't mind if I chime in here. I was a wireless systems integrator and engineer for many many years so I'd like to share a few simple guidelines you can use to get the best range from any RC system.
Polarization
One of the first rules is to keep the polarization the same between transmitting and receiving antenna(s). Typically that's vertical, horizontal, cross or circular. The flight control receiver antennas in the P2 are attached to the landing gear vertically so keeping your controller antenna vertical gives the best general signal. Having the TX antenna sideways is worse and pointing it at the aircraft is way worse (explained in next paragraph). Clover leaf antennas are popular for the P2 video transmitter and they're circularly polarized. About any antenna will receive this polarization closer in but to get good range you need the same polarization at the receiving end.
Also, the strongest signal is radiated 90 degrees off an antenna element so if your controller TX antenna is oriented vertically and your aircraft is located straight out from it 90 degrees, you're at the best TX/RX angle.
Line of Sight
One picture in an earlier post shows a nice panel antenna used to receive video that's mounted directly to the back of the control transmitter. That's great but it was also directly in front of the controller TX antenna, effectively blocking the signal forward to where the aircraft would be. 2.4Ghz and 5.8Ghz signals can be blocked, reflected and absorbed easily. Line of site is critical. That's why when at reception limits the video many times gets noisy when the drone is turned around for the return trip. The gimbal and camera block line of sight from the antenna to receiver. So any antenna must be positioned line of site to the target antenna to get maximum range.
Another earlier post showed an attempt at hanging the TX antenna way below the aircraft. Right idea but too aggressive. Higher frequencies don't travel through coax well and have a tendency to dissipate into the cable. Connectors also cause a small amount of VSWR (Voltage Standing Wave Ratio - or in other words a small of amount of TX energy is reflected back that effectively reduces the output to the antenna - too much VSWR will burn out the transmitter). The more connectors, the worse it gets. That's why most microwave and satellite receivers use a signal amplifier if there is any appreciable cable length. For serious microwave systems, wave-guide is used. It's like water pipe for radio frequencies.
Locating the video TX antenna just far enough below the aircraft to clear the gimbal would be ideal, although it isn't usually practical for every day use (unless you have extended landing gear) and most ready-made video TX antennas are too short to accomplish this. A 4 to 6 inch or so cable extension might be an acceptable option to a custom-made antenna since connector loss is actually pretty minimal. I might just try that.
Antenna Gain
Here's the big one. Antenna gain is the ability of an antenna to effectively increase its signal strength by implementing an array of single physical or virtual elements that together create the electrical equivalent of a much larger antenna. Gain figures typically correspond to TX characteristics but are used as a general rule for receiving signals as well.
First, increasing the video TX power on a drone is the
least effective way of getting appreciably better range. Yes, that's a proven fact. 200mw to 600mw transmitters are completely adequate if you use the right receiving antennas. Flights of distances outside the nominal usable range of a P2 would require a different type of drone and RF transmission system anyway.
Because of the way RF works, increasing the gain of a receiving antenna by 3db is effectively the same as doubling the TX power in the direction of the gain. 6db would be the effective equivalent of 4 times the power and so on. However, you don't get something for nothing. As gain increases in one direction it drops in another. For example, a rubber duck or stick antenna that has 3db gain will implement that gain in the horizontal plain while sacrificing the same amount in the vertical plane, generally. There are ground plane and other specialized antennas that can behave differently but we're talking basics here.
As you increase gain with a directional antenna you also decrease the corridor of reception, or beamwidth. For example, the panel antenna below has a gain of 14dbi. DBI stands for Decibels over Isotropic. Isotropic means radiation in all directions equally, 0 gain or omni-directional. This antenna specs a vertical beamwidth of 30 degrees and horizontal of 45 degrees. This means that when the panel is rotated between 15 to 22.5 degrees from direct center of the TX signal source, the receive signal will drop by 3db. This antenna is from Helipal for $30.
This antenna below specs at 23dbi, which is huge. That's fixed point-to-point, long-range system specs. It also has a vertical and horizontal beam-width of 11 degrees. That means you lose 3db of signal only 5.5 degrees off center. The signal would go away very quickly after that. Keeping this one aimed correctly would be tough unless you knew exactly where your drone was at all times, or had someone trying to keep it aligned for you. I'd love to build a tracking system with motorized tripod that interprets the GPS telemetry from the drone to keep the panel aimed directly at it. But I digress. Plus they're already out there if you have the bucks. This antenna is also from Helipal for $100. By the way, I pulled the images at random so no promotion going on here.
Using a diversity receiver like the Black Pearl with a lower gain antenna on the second receiver or second port is also a good idea. I'd use something a little beefier than the ducky in the picture, however. A signal amplifier in conjunction with the 14dbi antenna would probably be a safer bet for long-range video in my opinion. But whatever works, right? Just keep in mind that great video doesn't do you much good if you fly out of controller or battery range. Morbid curiosity aside, of course.
I was interested in this thread because just for fun I plan to fly a slightly modified P2 to the top of a mountain near my home just to see if it can be done. The temps are too cold here now but next summer after the snow melts the game is on. One critical element of that project is to have an OSD that displays GPS lat/long in case it goes down somewhere. I use a little DVR with the Black Pearl to record video. That way the telemetry can be played back with the last known location. But let's hope that won't be necessary.