Transmission lines in parallel...

[Externet]

Connecting radiofrequency transmission lines; say from a transmitter to its antenna, by ladder lines, or coaxial, or twisted pair; in parallel (doubled)...
Will its characteristic impedance be doubled or halved ?

Can two RG58 (50 ohm) coaxial lines be paralleled to obtain a 25 ohm transmission line to use a 25 ohm output impedance transmitter to a 25 ohm impedance antenna? Or would it result in a 100 ohm transmission line ?

 

[BigBoss]

Characteristic impedance of a transmission line is well defined by its' mechanical and electrical specifications.Therefore it wouldn't change.
Symmetrical 25 Ohm Transmission Line should be defined for a proper RF propagation/transmission.
Or using Baluns at the source and the end can be a good choice to drive an antenna..

 

[Externet]

Thanks.
..."It would't change"... Then will the resulting characteristic impedance of two paralleled 50 ohm transmission lines still be 50 ohm ?

 

[biff44]

if you had two identical lengths of 50 ohm line, connected at the ends in parallel, it would act like a 25 ohm characteristic impedance line. that trick is actually used in stub tuner circuits, where you desire a lower impedance than the microstrip can provide...u use two stubs and the effective impedance is halved.

BUT, u need to make sure the voltage along the length of the two transmission lines stays in phase! Otherwise some really weird notches may result. so for a long line length, u probably have to tie the lines together every wavelength of two.