Large Directional HDTV antenna

The LPDA(log periodic dipole array) or Large Directional antenna has several dipoles arranged in echelon and criss-cross fed from the front. The name comes from the geometric growth, which is logarithmic.The LPDA burst upon the antenna scene in the 1960s as a practical antenna with directional gain and an exceptionally wide (theoretically unlimited) frequency range. Like the Yagi-Uda array, it used linear elements. At the upper HF range and above, the elements might be aluminum tubing or rods
By experimentation with various designs of LDPA antenna it has been established, that the antennas having greater extent boom, can give very much greater gain. So, for example, 13’s elements for frequency of 145 MHz at distribution of its elements on length of 7 m boom has power gain a little bit greater 16 дБ. Such big gain can be received also at use of multidipole antennas or aerials « the wave channel », arranged in some levels, but thus their design and a power supply circuit is much more complex. Therefore for reception of the big gain(almost in 40 times on capacity) recently radio fans even more often use LPDA antenna with greater number of elements.
The big power gain of the LPDA antenna with wide range of elements is reached due to significant reduction of a passband of the antenna; it at the same time means, that the sizes of the antenna are very critical
In LPDA antennas with greater number of elements, elements usually are produced from the conductors having minimal (from the point of view of durability of a design) diameter. The more diameter of elements of the antenna, the less received power gain. The thin conductor owing to a small surface possesses small capacity, but simultaneously has greater inductance and, hence, has big attitude L/C . Therefore such element we shall compare to a narrow-band resonant oscillatory contour.
The element having greater diameter of a conductor, accordingly has smaller attitude L/C and, hence, possesses less strongly pronounced resonant properties, that in this case is equivalent to reduction of gain of the antenna. For reduction of losses due to superficial effect the surface of thin conductors should possess good conductivity. If diameter of an element is equal 3 mm (the most suitable diameter for a range 2) the element can be executed from copper or aluminium tubes if smaller diameter of conductors their surface should be silvered is used (for example if bicycle spokes as elements of the aerial are used). For reduction of losses, it is desirable, that thickness of a silver covering was whenever possible greater. At a covering of elements silver disappears necessity for additional protection of a surface of elements from influence of weather while the surface of aluminium or copper elements should cover with a protective varnish. For this purpose the colorless varnish with good isolation properties and a small dielectric constant is used.

Examples of large directional(LPDA) antenna may be TV38 from Terk and HDTV Digital Antennas CM 3671, CM3679 from Channel Master.