Why do we use a folded dipole in Yagi Uda antenna?

I was reading about this Yagi Uda Antenna and I came across this--

"If the distance between driven and parasitic element is decreased the it will load the driven element, irrespective of its length. Thus input impedance at the input terminals of driven element reduces. This is why a folded dipole is invariably used as driven element so that reduction in input impedance is compensated i.e. raised."

Could you please explain me this paragraph completely(from the first sentence).

Thanks in advance.

Folded dipoles with equal diameter wires in both parallel lines increase the impedance by 4x. This will increase the lowered impedance (caused by the parasitic elements) to be more nearly equal to 50 ohms and make the impedance matching to the coax easier.

flatulent said:

Folded dipoles with equal diameter wires in both parallel lines increase the impedance by 4x. This will increase the lowered impedance (caused by the parasitic elements) to be more nearly equal to 50 ohms and make the impedance matching to the coax easier.

Click to expand...

There are two questions that I have. May be it's because I am new to antenna theory.

How do you say that parasitic elements decrease the impedence?

How does the folded dipole increase the impedance by 4 times?

Thanks in advance

A folded dipole on its own is 300 Ohms impedance compared to a "straight" dipole which is 75 Ohms.
But when either of those dipoles are used as driven elements within a Yagi, the effects of the parasitic elements, primarily, the reflector and the first couple of directors will have a huge effect on the feedpoint impedance. It will lower that feedpoint impedance substantially. The 300 Ohms of a folded dipole can drop as low as 200 Ohms and a "straight" dipole of 75 Ohms can drop to as much as 20 Ohms.
In the case of the now 200 Ohms feed impedance of the folded dipole, this is quite an advantage, as a simple 4:1 BALUN transformer will match standard 50 Ohm coax to the feedpoint.

In reality tho, contrary to the comment in your first statement, straight dipoles are virtually as common as folded dipoles in Yagi antenna systems. Delta and Gamma matches are a common way to effectively impedance match 50 Ohm coax to a straight dipole within the Yagi.

cheers
Dave