Re: what is a RF choke?
Hello,
In a supply line a choke is a component ( [lossy] inductance) to block RF frequency signals and to pass DC or lower frequency signal.
The choke function is also used to block, impede, reduce, etc common mode current on coaxial transmission lines. Common mode current does radiate, hence it is mostly not desired.
When you connect a simple half wave dipole (two quarter wave sections) directly to an unbalanced line (coaxial cable), every mA the leaves the center conductor, should return via the inner side of the screen (braid). That current can follow two paths: it can come from the other dipole half, or from the outside of the coaxial screen (undesired). To make sure most current that goes into the inner side of the screen comes from the dipole half, a choke can be used (placed directly at the feeder-antenna transition).
A simple one is a clamp-on ferrite (split core ferrite) or ferrite ring. As the common mode current (current on outerside of screen) generates magnetic field, the ferrite introduces high impedance/inductance to the common mode current. The normal transmission line currents do not generate external magnetic field, so these are not affected by the ferrite around the coaxial cable. This type of choke does not transform impedance (when load equals line impedance).
As such a choke is frequently used to couple an unbalanced line (coaxial cable) to a balanced (symmetrical) antenna, it is called a "balun". A choke can be used as a balun (as in the example), but every balun is not a choke.
Other means to impede the common mode current is via quarter wave sections (eg. Bazooka balun), tuned traps, transformers, quarter wave shorting stubs,etc.
Generally a balun is a device to enable a smooth transition between an unbalanced transmission line and a balanced (symmetrical) antenna. It may also perform impedance transformation.
Besides use as "balun", chokes are also used to avoid outer screen (radiating) RF current in situation where there is an RF voltage across two ground point (ground loop problem). Wide band versions at HF and VHF can be made with the ferrites mentioned earlier. However performance of ferrite materials as choke become less at at GHz frequencies
When you use a clip-on type ferrite (or ring) cable properties do not change, so there will be no impedance transformation, but you may measure other impedance when adding the ferrite. This is because you reduce outer screen current and that may change the antenna's input impedance.