log cabin

Small, one-room house built of logs notched at the ends and laid one upon another with the spaces filled with plaster, moss, mortar, mud, or dried manure. In North America they were built by early settlers, hunters, loggers, and other wilderness dwellers. They have also been built in Europe, particularly Scandinavia. Though designs vary, a common style features a sloping, single-gabled timbered roof and small windows. Modern summer cottages may be built of logs (or given log-cabin siding) to achieve a rustic effect.

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In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array) is a broadband, multielement, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA). Log periodic antennas are designed to be self-similar and thus are fractal antenna arrays.

It is normal to drive alternating elements with a 180^o (π radian) phase shift from the last element. This is normally done by wiring the elements alternatingly to the two wires in a balanced transmission line.

The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other.

The result of this structural condition is that if a plot is made of the input impedance as a function of log of frequency then the variation will be periodic i.e. the impedance will go through the cycles of variation in such a way that each cycle is exactly like its preceding one and hence the name.

Coverage example

Popular Amateur radio variations

HB9CV

The HB9CV is a very popular two-element beam which can be considered as a log-periodic dipole array with only two elements.

ZL special

This is a beam antenna which for a given boom length gives a higher gain than a Yagi antenna, it is a log-periodic where only two elements are driven with a series of parasitic elements (directors) in front of the smaller of the two driven elements.

The driven elements are folded dipoles which are linked by a length of balanced twin-lead feed line. The polar plot of the ZL special and the HB9CV is a heart shape (cardioid) while the typical Yagi antenna has a large forward lobe and a smaller back lobe.

Sources

• Federal Standard 1037C
• MIL-STD-188

External links

• LPDA Online Calculation http://www.changpuak.ch/electronics/lpda.html