Looking at the docs and designs I came up with this list:
- An impedance matching network (1:4, LC, hairpin/beta, etc)
- A balanced to unbalanced converter
- A common mode choke
- Anything else I missed?
I read online that a folded dipole impedance in clear space is about 280 to 300 ohm, but when put as a driven element in a yagi, the impedance drops to near 200 ohm. This is why there is normally a 1:4 impedance transformer present to lower the impedance to 50 ohm and match it with the feedline. I'm not so convinced about this, as I read the impedance is actually a complex number, so some measurements and adjustments with a VNA are in order. To tune an antenna, the resistive (real) part of the impedance should be as close to 50 ohm as possible while the imaginary part should be close to 0, so some capacitive/inductive elements may be required i.e. a LC network. A set of series/shunt inductors/capacitors can be added at feedpoint to finetune the impedance, and a good resource I found is this interactive online smith chart:
This is where I'm confused most. What exactly is a balun? Because different people have different representation for this keyword. In regards to a yagi, it is said that a 1:4 balun is needed, but is it really a balun or is it just a 1-to-4 (or 4-to-1) impedance transformer? Which devices do actually perform balanced-to-unbalanced tasks?
Common mode supressor
When connecting an antenna to a coax feedline, an electrical connection is made to the cable outer shield. In an ideal situation the currents in the inner coax conductor and the outer sheld are equal and opposite and the cable doesn't radiate because the effects cancel each other out. But quite often there will be some residual currents present on the outer shield and this will cause the feedline to radiate and become part of the antenna. This is undesired because it often worsens the antenna efficiency, so normally a certain type of devices (1:1 current baluns or common mode chokes) are installed to minimize this effect. I've seen some ferrite core windings being used, but those are mostly useful for lower frequencies i.e. HF; they're not so good for UHF. For my antenna I thought about using abazooka/sleeve balun, but here we are again calling this device a balun. Is it really a balun or just a common mode choke?
Thanks for any comments...