how to improve RF meter?
These people are scammers, and do not deserve your money


  How can I improve the circuit, in order to detect also cordless phone radiation, like the dect 6.0 cordless phone, which, actually, operates on 1.9GHz?


With this circuit, you can only get exactly one band. I'd suspect this circuit was meant for GSM in the 900 MHz band (they don't mention – could be 800 MHz or 1800 MHz, too, one reason more to stay away from that product).

The circuit itself is kind of funny: it's two things in one: 


a loop antenna (hence the circular layout)
a voltage multiplying diode cascade, which I can only guess works by exploiting parasitic capacitance.


This device will not work. Even if we assume the LED gets this bright with but 0.5 mA (it will not), a single 1SS86 would have a forward voltage of about 0.18 V; nine of them in series would have a forward voltage of 1.62 V. Then add the forward voltage of the LED (something around 2V). Without a very large antenna, you cannot get peak-to-peak voltages that high from an EM field as weak as legal for GSM.

I tried to watch the video, but it was so full of bullsh*t that I couldn't stand to finish it. Don't buy anything from these people. If you put your cell phone in something that tries to block electromagnetic radiation, you will be subject to more energy, not less.

One thing that struck me was: in the assembly instructions, there was no external antenna whatsoever, whereas in the video where they demonstrate this thing lights up, the have about 15cm of wire attached to the device. That's a scam. Not only doesn't the device work on its own, it's almost certain they can now use whatever secondary transmitter (and by the length of that cable, maybe a 433MHz analog FM radio transmitter that can send 100s of watts – much much much more than cell phones) to light up the LED.