how to construct a jammer 433mhz ?it's possible? 
Sure. Also illegal more than likely.
ok.. it's illegal in your country... but learn how to make it's not illegal..
The first step in learning how to make one is learning how to use an Internet search engine like Google.
Some home security systems with 433MHz have a 'jammer' detection.
Tell us first what you want it for, something bad ?
Well a quarter wave Cavity Resonator tuned to 433 MHz and driven by an SRD pulsed at a 1 KHz rate might do it... for perhaps 3 to 5 meters and If the rise time of the 1 KHz pulse was < 10 ns... you might generate a harmonic every 1 KHz for several MHz around the center frequency of the resonator... I made one for a "T hunt" about 20 years ago... Worked a charm...
Doc
I don't like the sound of "cavity resonator". Is that just me? Especially if we are jamming things...
Could be a bit sticky but if pectin is modulated into the mixer circuit it should produce a stable base
I think this could be done using one of those new Pi micro-processors with a butterworth filter
thank you bigred1212..
Caltoa I want to keep my gate closed.
@wvwv When a 433MHz signal is used to open/close something without a good secure protocol, you are in trouble anyway.
For example sending radio noise to such a device will someday open it. So you end up opening it yourself.
Someone else could keep a 433MHz transmitter closer to it and still open it.
There is only one good solution to this: make the signal to open it a secure and encrypted signal. Using an encryption with rolling code.
Someone made something like that for a garage door opener, but I can't find it right now. I think he used AES for encryption.