Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many (if not most) applications require using an antenna over a range of frequencies. An antenna's bandwidth specifies the range of frequencies over which its performance does not suffer due to a poor impedance match. Also in the case of a Yagi-Uda array, the use of the antenna very far away from its design frequency reduces the antenna's directivity, thus reducing the usable bandwidth regardless of impedance matching.
What type of antenna has impedance at a specific frequency?
resonant

The first law of thermodynamics asserts that energy (but not necessarily thermodynamic free energy) is always conserved and that heat flow is a form of energy transfer. For homogeneous systems, with a well-defined temperature and pressure, a commonly used corollary of the first law is that, for a system subject only to pressure forces and heat transfer (e.g., a cylinder-full of gas) without chemical changes, the differential change in the internal energy of the system (with a gain in energy signified by a positive quantity) is given as
What asserts that energy is always conserved and that heat flow is a form of energy transfer.
first law of thermodynamics

Balansiyya had a rebirth of sorts with the beginning of the Taifa of Valencia kingdom in the 11th century. The town grew, and during the reign of Abd al-Aziz a new city wall was built, remains of which are preserved throughout the Old City (Ciutat Vella) today. The Castilian nobleman Rodrigo Diaz de Vivar, known as El Cid, who was intent on possessing his own principality on the Mediterranean, entered the province in command of a combined Christian and Moorish army and besieged the city beginning in 1092. By the time the siege ended in May 1094, he had carved out his own fiefdomwhich he ruled from 15 June 1094 to July 1099. This victory was immortalised in the Lay of the Cid. During his rule, he converted nine mosques into churches and installed the French monk Jerome as bishop of the See of Valencia. El Cid was killed in July 1099 while defending the city from an Almoravid siege, whereupon his wife Ximena Diaz ruled in his place for two years.
When did El Cid die?
July 1099