How do I determine the impedance of a coax cable?

[alyas]

I bought those thin coax cables rg178/rg179 a long long time ago ..... just can't remember where I was using them either for cable tv extension or for a wifi antenna.  There is actually no markings or labels on the cable or whatsoever.

How do I determine if its 50 Ohms or 75 Ohms? 

[zer0w1ng]

Kung may SWR meter ka sir, just put a 50ohm resistor load at the end and measure the SWR.
Kung 1.0 siguradong 50ohms Zl impedance siya.
Kung hindi (the reading depends on the length per 1/4wavelength), it is a 75ohm one.

If the coax in question is multiple of 1/4wavelength, the SWR reading would be:
Zin = Zo^2/Zl = 50*50/75 = 33.3 ohms
SWR = 50/33.3 = 1.5

[zer0w1ng]

Edit (may mali):
Zin = Zo^2/Zl = 75*75/50 = 112.5 Ohms
SWR = Zin/50 = 112.5/50 = 2.25

The SWR reading will 2.25 if multiple of *odd* 1/4 wavelengths coaxial cable is used.

[Ka irnie]

Sir good day tanong lang regarding sa impedance,
          Kung may antenna ako halfwave dipole may rg6 ako na cable 75 Ohms tama b na 150ohms ang matching x'former or balun ang ilagay ko para maging balanced?

[zer0w1ng]

Sir good day tanong lang regarding sa impedance,
          Kung may antenna ako halfwave dipole may rg6 ako na cable 75 Ohms tama b na 150ohms ang matching x'former or balun ang ilagay ko para maging balanced?

AFAIR, a halfwave dipole is about 73 ohms radiation resistance, so if the Zi and Zo is 75 ohms, you don't need a balun/transformer.

If it is a folded dipole (approx 300ohms radiation resistance), you need 1:4 balun transformer (75*4 = 300).

[rdpzycho]

ang kailangan lang sa 75 ohm to dipole ay current Balun na 1:1 para mabawasan 'yung common mode voltages. kung isang frequency lang gagamitan pwede na ang coaxial cable as balun.

[ahlburuto]

Edit (may mali):
Zin = Zo^2/Zl = 75*75/50 = 112.5 Ohms
SWR = Zin/50 = 112.5/50 = 2.25

The SWR reading will 2.25 if multiple of *odd* 1/4 wavelengths coaxial cable is used.

If wala akong SWR eh anung ibang option para malaman ang impedance ng coaxial?

TIA

[zer0w1ng]

If wala akong SWR eh anung ibang option para malaman ang impedance ng coaxial?

TIA

If you know the material of the coax's dielectric and its coefficient you could compute the impedance by measuring the diameter of the center conductor and the inner diameter of the ground (outer conductor):

Zo = (138/sqrt(e)) x log (D/d)

where:
e = dialectric constant
D = ground diameter
d = center conductor diameter

[webscatter]

If you know the material of the coax's dielectric and its coefficient you could compute the impedance by measuring the diameter of the center conductor and the inner diameter of the ground (outer conductor):

Zo = (138/sqrt(e)) x log (D/d)

where:
e = dialectric constant
D = ground diameter
d = center conductor diameter

tamasi zerow1ng

[bravokilo]

google those coax na alam mo naman kung ano sila.

BK