coaxial cable impedence

If i have a coaxial cable how can i know what is its impedence i mean any simple method to measure its impedence

I guess normally coaxial cables are available with 75 ohm or 50 ohms impedence.
is this 75/50 ohm impedence is only the characteristic of material or this impedence changes with length.

if the impedence is dependent on length then is it possible that by cutting to proper length i can get 50 ohm impedence out of a 75 ohm coaxial cable(I think if the impedence of a coaxial cable 50 ohm or 75 ohm is per meter then i can do this)

Normally RF modules need 50 ohm impenece for antenna.if i can get a coaxial cable of 50 ohm can i make a omni directional antenna with this cable only.I mean the antenna as shown in figure


|
| central wire of coaxial cable
| with length equal to the λ/4
|
|
---------------------- Metal piece connected to GND of cable
| |
| |
| |
| |
| |
| |
____________


RF module with antenna feed connected to central wire and GND connected to GND of coaxial cable

Note: The frequency of RF module is 418 Mhz

I have another antenna design in my mind.The connector of a TV antenna has impedences 75 ohm on one side and 300 ohm on other side.if i use this connector foe impedence matching between my antenna and RF module will it work(impedence of RF modules input port is 50 ohm so how much power loss i will have in case i uses TV connector)

Plz see the attachment for antenna


I am a student and haavent yet studied microwave engineering courrse or anything like this just experimenting different things for a project so if i am doing a very stupid thing plz inform me

The impedance is determined by the physical dimension ratio of the diameters of the two surfaces across the dielectric as well as the dielectric constant. Impedance is independent of length. The cable should have a model number printed on it. If not and the center conductor is solid the impedance is probably 75 ohms.

Your idea of the quarter wave perpendicular to a conducting plane is a standard antenna type and you can have the plane radius an integer multiple of a quarter wave.

Your loop antenna idea will not work well because the diameter that makes the real part of the impedance 300 ohms is very critical. I would suggest a standard half wave dipole would be easier to make and function properly.