About Transmission Lines

   Most CB radios or other 2 way communication radios uses 50 ohm characteristic impedance coaxial transmission line, but those dual CB antennas uses a pair of 75 ohm characteristic impedance coaxial transmission line at a certain length that are harnessed to 1 coax connector, or each coaxial line hooked up to a T coaxial connector adapter.

What is the definition of characteristic impedance?

  Characteristic impedance is the transmission line's characteristic that is measured in ohms that is capable of delivering the RF power to the antenna load impedance in ohms that is match equally to the ohms of the transmission line's characteristic. Characteristic impedance can not be measured by a ohmmeter, but only by a SWR meter, or a reflected power meter along with a  along impedance load terminated on the end of the transmission line that has an impedance equal to the characteristic impedance coaxial transmission line, or by an antenna analyzer.

What is the definition of antenna load impedance?

  That is the total opposition of AC current of an antenna that is measured in ohms.

For example:

  If you would use a 75 ohm coax cable connected to a 50 ohm antenna, the SWR would be 1.5 to 1, or the reflected power would be 4%. If you would connect up 75 ohm antenna to a 50 ohm coax cable, the SWR would be 1.5 to 1 or the reflected power would be 4%. If you would use a 100 ohm CAT 5 twisted wire pair data line connected to a 50 ohm antenna, the SWR would be 2 to 1 or the reflected power would be 11%. If you would use a 300 ohm twin lead line connected to a 50 ohm antenna, the SWR would be 6 to 1 or the reflected power would be 51% in which that would be a redline antenna impedance mismatch, and so do not use that twin lead transmission line.

Here is that formula:

Table Of Definition
ZLINE = Characteristic impedance of transmission line in ohms
ZANT = Impedance load of the antenna in ohms

VSWR =

Standing Wave Ratio or VSWR or SWR in the number of ratio to 1

 

If ZLINE > ZANT:

VSWR = ZLINE / ZANT

 

If ZLINE < ZANT:

VSWR = ZANT / ZLINE

 

   So always use the right characteristic impedance of coaxial cable that is specified for your CB radio equipment like those coaxial cable type RG-8, RG-58, RG-213, RG-214 or any other 50 ohm coaxial cable.

Related Links:

Conversion Formulas For
SWR and Reflected Power
Click Here

SWR And Reflected Power
Conversion Chart

Click Here

Transmission Line Behavior
Click Here

Dual CB Antenna System
Click Here

 

 

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