Mosley and some others give up gain and front/back to get around 50 Ohms.
More typical for an antenna designed for decent gain and front/back ratio is
about 20 - 25 Ohms. Generally a beta match, hairpin, or gamma match is used.
You could also use a 2:1 (Impedance, not turns ratio ) transformer if you
can find it. A 1/4 wave section of 35 Ohm coax, made up of two pieces of 75
Ohm coax in parallel, might also work.

Note that the transformer and 1/4 wave sections are not adjustable, the
others are.

Tam/WB2TT
"Avihai dahan" wrote in message
om...
Hello there,

I would like to ask the following please,
1.What is the typically impedance of most popular Yagi antenna?
2.Does transformer is necessary or recommended to achieve best match
any way? (does it depend on the cable length etc)

Regards,

Avihai.