How can I design a high speed RF peak detector?
Easily enough:

Look up a diode that operates at over 1.4GHz.

Like the BAT63.

The datasheet includes the circuit used to test its RF properties:



Note that the tests were made at 2.4GHz.  It will certainly operate at 1.4GHz.

Given the Rin and the measured Vo, you can calculate the power from Vi.

Probably not what you'd call a precision power meter, but most likely good enough to tell you if you are getting close to the input limits of your AD8318.



In your place, I wouldn't worry about overloading the AD8318.

I use the similar MAX2015 for similar stuff, and find that the available signal stays well within the (slightly) narrower dynamic range.  This is looking at stuff ranging from TV satellites to the moon, as well as just looking at random stuff.

What you might do is to make you amplifiers such that you can add or remove them from the circuit easily.  

You know the gain of each amplifier.

Connect your LNB and dish, aim them at a target.  

Connect that to your level detector (the AD8318) and see what level you get.  

It should be well below the maximum of the AD8318.  The difference between the maximum input and the measured level tells you how much amplification you can add in.

Getting a clean, usable signal high enough to saturate (or damage) the AD8318 will be harder than you think.

More amplification isn't nearly as much use as you might think.  Your amplifiers add noise, so amplifying far above the minimum your detector needs just makes things noisier, not better.

As an example, the small radio telescope I built using common satellite TV receiver parts shows open night sky at about -43dBm and the moon at around -41.5dBm.  More amplification would just add noise but not make the moon standout any more.  This is at like 10GHz to 12GHz, but the signal is down converted to about 1.5GHz by the LNB.

This image was made with my little radio telescope:



The bright blob is the moon.  The stuff around it is clutter from trees and hedges.

I have a couple of 20dB amplifiers that I bought when I started that project, but I've never needed them.  The LNB and satellite dish provide enough signal to get into the input range of the MAX2015.

What I did need, though, was a low pass filter on the output of the MAX2015.  It (and the AD8318) have a very high output bandwidth - they are made to respond to rapid changes in transmitter levels as happens in WIFI and cellphone transmitters.  That doesn't help when you are looking for slow changes in relatively constant levels.  

The filter I use has a cutoff of around 200Hz because I use my "radio telescope" for other things, as well.  It is mounted on servos so I can scan it back and forth across about 20 degrees (up and down, left and right.)  That makes simple gray scale images of anything in view.  It moves at something like 40 steps per second, so the filtered  signal needs enough bandwidth to capture the changes.

For astronomical stuff, you can probably use a much lower cutoff.  Or use a higher rate and filter later in software.