extend my tv aireal by joining 2 cables?

Yes you can buy joining Gizmos Female to Female - there will be some deterioration of signal but very probably not enough to notice.

http://www.tme.eu/en/katalog/connectors-coaxial_100364/#id_category%3D100364%26

An alternative is to buy or make up or have made up an aerial cable that makes the connection in one piece.

Peccavi
November 2009