I have some doubts left:
1. If I use 2.4GHz RC gear, I understand that if I go for a lower freq for FPV (1.2/1.3G), I'm asking for trouble/going to need extra stuff that is given the name 'Low pass filter". Did I get this right?
And are there any different types of LPFs? What kind of LPF is required for a 1.2G or 1.3G VTx?

2. There's a 5.8G 400mW system avaiable on HK and hobbysea. The stock antenna gives 500m range and it says "an optional directional antenna can increase the range to 1km". Is this antenna a directional antenna?
But this clealry shows "directional", but weighs 500g! Is it fixed to the VRx or the VTx? I see some more antennae marked 'omnidirectional". So this means if I use only a directional antenna, I'll have to keep pointing my VRx in the direction of the VTx?

3. The antennae are varying in terms of Gain value measured in dBi. More dBi = more directionalism, this implies that antennae with more dBi are suitable only for intermediate/advanced guys?

4. Higher the mW => Bigger the range is NOT always true and there are many things that matter. I can't have a 1000mW FPV VTx on my quadcopter with an RC Rx if I care for my money. 'The VTx swamps the RC Rx.' What does this mean?
One parameter the range depends upon is the penetration capacity. Is this capacity independent of the frequency? If not, which freq has the highest penetration capacity at a given output RF power(mW)?

I hope someone answers my doubts patiently.
Thanks
Vineet

PS: I have posted this on another forum but didn't get many replies.

nice question, should get bump , i even want to clear some doubts

Very good questions indeed. Would be quite nice if some expert answers these as it would clear many of the doubts that, us beginners have!
Some pointers that I have picked up while doing some digging of my own:
1) Try and use 5.8GHz when using 2.4GHz radio gear. This ensures that your radio link does not get interfered by a lower freq video signal i.e. the VTx does not swamp the RC Rx.
2) Use polarized antenna for best performance(omnidirectional). Clover leaf is the way to go.
3) Directional antennae are mostly used at the ground station.
4) A 400W 5.8gig Tx should give more than 1km range when coupled wit a circularly polarized antenna easily.
5)Penetration capacity is directly dependent on frequency. Lower the freq, higher the capacity. (That is why 900MHz is used for long range fpv systems)
I may be slightly wrong in any of the points above. I have gathered these from reading many threads but have done no experiments on my own

You Should watch this video from the expert.

http://www.youtube.com/watch?v=B9DPy0zmiwk&feature=g-high-u

1. Low Band Pass filter is used for 900Mhz and 1.2/1.3 Ghz to prevent its interference due to harmonics with Two components on the plane and they are GPS receiver (1.5 Ghz) and RC Receiver (2.4 Ghz) . Both 900 Mz and Ghz produce Harmonics and its better to use a LPF filter between the video tx and antenna on both of them. LPF for 900 Mhz and 1.2/1.3 Ghz are different but does the same function. 5.8 Ghz has to have a HPF filter which is generally available as built in most Tx.

2.  For a 5.8 Ghz it is always better to go for Circular Polarized antennas both directional and omni-directional. The Link which you have posted is not a directional but a lenier polarized omni-directional antenna.

5.8Ghz performs well only on line of sight and not around objects and behind obstructions. Lower frequencies like 900 Mhz and 1.2/1.3 Ghz perform well around obstructions and has better penetration.

The second link is a directional antenna and Directional antenna is used only on the receiver and you are absolutely right about having to point the antenna always at the plane for better reception since directional antenna have a very narrow and long beam of txn and rxn. It is better to be used with a antenna tracker. Plane always has a omni directional antenna weather circular polarized or lenier polarized.. Out of the directional antennas it is better to use a Patch antenna if you are using without tracker since it has more wider beam amongst all directional antennas. However it has to be still kept pointed at the Plane always.

3.  I can agree with only this part of the statement "More dBi = more directionalism," Its not that higher gain antenna are not meant for bigginers.  Higher Gain antenna have higher reach however you cannot fly directly over head when the Omni directional antenna of receiver is vertical even at short ranges unlike a lower gain antenna.

4. Higher wattage is not always higher range it is dependent on various factors
  (a)  Like the quality of the output of the Tx. Noise to Signal ratio.
  (b)  Impedance matching of the Antenna with Tx. (RF impedance of 50 Ohms)
  (c)  Quality and type of the antenna(both directional/omni-directional).
  (d)  Gain of the antenna.
  (e)  Presence of Ambient RF transmission in the area like mobile, wifi and other transmissions.
  (f)   Rain.
  (g)  Obstructions in the area.
  (h)  Quality of the Voltage supplied to the Video Tx.

5.  Video Tx always interferes with your RC receiver no matter what frequency you use due to the higher wattages. (Try keeping the video Tx antenna closer to a servo wire and see what happens to the Servo. That servo will jitter away to death eventually. You will know what I am trying to get at) It is always a better idea to separate the Video Tx antenna from the RC receiver antenna and any other electronic component and wiring's to have a longer RC control. Higher the wattage of the Video Tx Higher separation of both the antenna is required.

6.  Lower frequency have a better penetration since it works better around obstructions.

This is a must watch for understanding polarization, gain and diversity

@vineet, @lastrites @Sundaram Sir and @girishsarwal

Thanks a lot for your help. I think I will go with 1.2/1.3GHz.
Can you link me to a fairly inexpensive polarised linear omnidirectional antenna that you use/feel is reliable?
Thanks again.

Happy to be of some help Going by your quads I am fairly certain that you can make your own. Just follow IBCrazy's Cloverleaf build logs. They use materials that are readily available here. Head over to rcexplorer.se site. There are some very good tutorials on there.

The hobby king omni-directional antenna is a linier polarized dipole antenna . Which is fairly very good for quad since a quad is not tilting beyond certain angle and will not looses polarization.

You can also fabricate yourself a inverted  'V' which is one of the most reliable linier polarized antenna, which does not loose polarization with a dipole or biquad which can be fabricated the easiest.

good to see many are planning to fly fpv

Thanks for the advice lastRites. I've seen RcExplorer, I love his builds. I'll see IBCrazy's build-logs as well.

Thanks for providing the answers Sundaram sir.

Guys, I'm posting a link to a list of good proven 1.2/1.3G FPV systems.
http://www.rcgroups.com/forums/showthread.php?t=1279902
(Lawmate combos are missing)

Thanks for your help again.