There are two ways to build up the stock Radian, light for casual/short range flying, and heavy for distance/speed.
This is the easiest and fastest way to get your Radian FPV ready.
Pan and Tilt
The Radian is THE perfect candidate for a P&T setup. Its such a slow and stable platform that you can look around without fighting to keep the plane on course. For pan and tilt, it's best to mount the P&T straight to the canopy. Because on this build, we won't be doing any structural mods to the wing saddle joint, its best to not try to mount the P&T onto the fuselage above the canopy. In this photo, I'm using the P&T mount from Low Price RC
ESC, RC Rx, and servo placement
If you purchased the ARF or RTF Radian, then the stock Eflite 30A ESC is plenty adequate for this setup. Use it and you can mount the ESC in its stock location. Same with the servos and RC Rx, continue to mount them in their stock locations. If you purchased the foam only and are building it from "scratch", then any Turnigy 25-40A ESC will be adequate and should fit nicely in the stock location. HXT900s or any other mini servo is plenty for this build and should be mounted in their stock locations.
For this lightweight build, I recommend using the Zippy Flightmax 2200mah 20C batteries. They're cheap and small! Without any modification, they should slide nicely into the stock battery cutout but will require some tail weight to balance (more on that in a min). If you use any other battery and it doesn't fit, spend a few minutes with a breakway utility knife and carve out the foam needed to keep your battery snug. Also, remove the plastic mount and velcro strap for the battery that's glued inside the fuselage. You won't be needing that.
Motor and Prop
The stock motor is THE most efficient motor I have found so far for the Radian. Believe it or not, Eflite actually got something right. On this build, use the stock prop and the stock prop only. For a lightweight build its super efficient and can cruise at 18mph pulling 3-4A. This plane really needs some down thrust built in, FPV or not, its very pitchy on the throttle. Just toss a washer or two to get some down thrust in and it will fly so much better.
If you haven't read it before, avoid APC folding props (although on this setup its such a low wattage that you would probably be ok) they have been know to break. Also, don't get silly an experiment with other random props at the hobby shop because they were out of stock of a carbon prop in a larger diameter, or this will happen
That's an Eflite 11x7 folding prop I found at the hobby shop and during a full throttle motor run in the house, blade broke at the hub and slung it through the ceiling. Before I could get the throttle closed, the now unbalanced motor did this to the nose
You have been warned...
OSD, wiring, etc
With this lightweight build, the plane isn't heavy enough to punch through winds over 10 mph very efficiently. Accounting for wind and battery capacity, its safe to say this is probably a 2 mile out and back setup MAX. On a calm day or with a tail wind, you can easily go further, *BUT* you don't have the weight to punch through any kind of headwind. The plane will slow down to 5mph and gently fall straight down. *IF* you were lucky enough to get back it would have taken you 20 min to get back from 2 miles in a 10mph headwind if you were just attempting to glide back, assuming you had the altitude to pull it off. All of that brings us to this point, a "full house" OSD is useless on this build and only increases the cost of replacement if something was to go wrong. The HK Simple OSD is all you need.
If you are using a 12V camera and VTx, just run everything straight off the balance plug. The motor/ESC combo does not produce any noise in my video at any throttle setting. At the time that the following picture was taken, I was using the "buck inverter" to keep a constant 12V to the camera and VTx regardless of how low the flight pack got. This really isn't necessary because *most* 12V gear will operate much lower than your flight pack EVER needs to go and much lower than ESC low voltage cutoff will allow.
If you have 5V equipment, use this step down regulator
I would pretty much say the HK Simple OSD is a must for this airplane. Due to the insane flight times you can get, and due to how easily you will be distracted looking around with P&T, you don't want to find yourself 1 mile out and at low voltage cutoff.
So just run all your wiring to the canopy and tape/velcro it in place
VTx and VTx antenna Placement
Because of the extra weight we added in the nose with the P&T, OSD, and battery, we need some weight in the tail to balance. So, once you have everything in the nose built and assembled, move you VTx around on the back of the fuselage until you get the plane to balance. And while we're on the topic of CG, I think the Radian flys the best and has the flatest glide with the CG about 1/4-1/2" behind the CG finger holes on the wings.
For the lightweight build, glue a piece of velcro to the back of the fuselage and just velcro the VTx in place. Don't go removing foam and trying to mount the VTx "clean" into the fuselage to keep it more streamlined. Remember, not only are we trying to make this a fast build, the speeds this lightweight version will see, having the VTx hanging out in the wind wont affect a thing and all you would be doing is weakening the tail. Disregard the VTx antenna placement in the following picture. You want to mount the VTx antenna as far away from the RC Rx antenna. The only downside to mounting the VTx on the back of the fuselage is when you turn back home and fly straight at yourself, the video would drop out occasionaly. I never noticed it when I was flying this specific build, but on the heavier builds, the big battery and everything in the nose was blocking the video signal from getting out clean. I noticed it more of a problem on 2.4 than when I was on 910. So, just for good practice, get your Vtx antenna mounted high in the air, because I would notice whenever I turned home and lost video, if I pointed the nose down the video would come back. So would flying at an angle back home, but we can eliminate that problem or make it easier to deal with by getting the antenna 4-5 inches up.
EDIT: I can confirm after a couple test flights, the antenna 4" higher fixes my problems with video drop when turning home and flying directly back at me. Will do some distance flights this week and see if it still works at range.
Since using the stock whip antenna, regardless of your video frequency, doesn't get it high enough up. Not to mention, they usually aren't tuned well to your exact frequency. Build yourself an IBCrazy Inverted-Vee antenna. SUPER SUPER SUPER easy to build and will kick your stock antenna's butt.
If on 910mhz, you would need to make a support frame for the antenna because of the long 1/4 wavelength wire lengths. Popsicle sticks, balsa sticks, etc, it all works. Use the vertical stablizer as you mount and use the balsa sticks, etc, as needed to support the wire that will hang off thefront of the vertical stab.
All ofther frequencies, the vertical stabilizer should have enough surface area to tape the Vee antenna to without neededing any suport structure to keep the wire lengths straight. Again, mount it as high up on the vertical stab as possible
Build/Buy you an IBCrazy cloverleaf and build/buy it with a 4-5" coax extension from the antenna to the SMA connector. This should get it high enough to elimitate that problem or at least make it better. You will need something to keep the antenna to stick straight up. If you build it yourself, before you solder up the SMA connector, slide on a 4-5" piece of heatshrink over the coax. Once you solder up the SMA connector, put a 1/8" diameter wooden dowel, pushrod tube, anything thats somewhat rigid and shrink the heatshrink. Now you have a rigid coax length to keep the antenna straight. If you buy one and don't have this option, glue a wooden dowel, or fiberglass rod in the back of the fuselage to run the VTx antenna coax up and use tape or something to attach it to the rod.
Again, like I said, I never really experienced this problem on my simple/lightweight build, but just preparing you for what is a possibility. The best thing to do is go fly with what you got and if you experience this problem, go back and try to get the antenna higher.