Antenna Theory - Log-periodic Antenna

By

 The Yagi-Uda antenna is mostly used for domestic purpose. However, for commercial purpose and to tune over a range of frequencies, we need to have another antenna known as the Log-periodic antenna. A Log-periodic antenna is that whose impedance is a logarithamically periodic function of frequency.

Frequency range

The frequency range, in which the log-periodic antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands.

Construction & Working of Log-periodic Antenna

The construction and operation of a log-periodic antenna is similar to that of a Yagi-Uda antenna. The main advantage of this antenna is that it exhibits constant characteristics over a desired frequency range of operation. It has the same radiation resistance and therefore the same SWR. The gain and front-to-back ratio are also the same.

Log-periodic Antenna

The image shows a log-periodic antenna.

With the change in operation frequency, the active region shifts among the elements and hence all the elements will not be active only on a single frequency. This is its special characteristic.

There are several type of log-periodic antennas such as the planar, trapezoidal, zig-zag, V-type, slot and the dipole. The mostly used one is log-periodic dipole array, in short, LPDA.

Log periodic Array

The diagram of log-periodic array is given above.

The physical structure and electrical characteristics, when observed, are repetitive in nature. The array consists of dipoles of different lengths and spacing, which are fed from a two-wire transmission line. This line is transposed between each adjacent pair of dipoles.

The dipole lengths and seperations are related by the formula −

R1R2=R2R3=R3R4=T=l1l2=l2l3=l3l4

Where

  • т is the design ratio and т<1
  • R is the distance between the feed and the dipole
  • l is the length of the dipole.

The directive gains obtained are low to moderate. The radiational patterns may be Unidirectional or Bi-directional.

Radiation Pattern

The Radiation pattern of log-periodic antenna can be of uni-directional or bi-directional, depending upon the log periodic structures.

For uni-directional Log-periodic antenna, the radiation towards shorter element is of considerable amount, whereas in forward direction, it is small or zero.

Uni Directional Log-Periodic

The radiational pattern for uni-directional log-periodic antenna is given above.

For bi-directional Log-periodic antenna, the maximum radiation is in broad side, which is normal to the surface of the antenna.

Bi-directional Log-periodic

The figure given above shows the radiational pattern for a bi-directional log-periodic antenna.

Advantages

The following are the advantages of Log-periodic antennas −

  • The antenna design is compact.
  • Gain and radiation pattern are varied according to the requirements.

Disadvantages

The following are the disadvantages of Log-periodic antennas −

  • External mount.
  • Installation cost is high.

Applications

The following are the applications of Log-periodic antennas −

  • Used for HF communications.
  • Used for particular sort of TV receptions.
  • Used for all round monitoring in higher frequency bands.

Comments

Post a Comment

Popular posts from this blog

Cara Membuat Ikan Nila Cepat Bertelur, Agar Bisnis Pembibitan Sukses

By
Image
  Dalam bisnis ternak ikan nila, mungkin ada salah satu kendala yaitu ikan nila yang susah bertelur. Cara membuat ikan nila cepat bertelur secara tepat perlu diketahui oleh setiap peternak. Kendala dalam perawatan ikan nila perlunya untuk ditangani secara tepat. Ikan nila yang tidak bisa bertelur biasanya adanya kendala didalam sistem reproduksinya. Hal ini biasanya disebabkan adanya beberapa serangan penyakit maupun kemampuan hormonal. Jika tidak diatasi secara tepat, untuk bisnis pembibitan ikan akan menjadi hambatan besar. Beberapa juga bisa disebabkan oleh faktor lingkungan maupun jenis pakan yang dikonsumsi kurang sesuai. Hal ini akan menghambat hasil ternak ikan nila, maka ada beberapa langkah yang perlu dilakukan dan mengenali hambatan yang ada. Maka perlu sekali ditangani secara benar, yang dapat meminimalisir kualitas indukan ikan nila yang produktif. Ada beberapa hal yang tentunya diperlukan untuk menghasilkan ikan nila yang sehat, terutama untuk indukan. Lalu apa saja ya...
Read more

What is the Difference Between UHF & VHF & HF and GMRS & FRS?

By
  UHF- Ultra High Frequency (between 300 MHz and 3 GHz) UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications. The IEEE defines the UHF radar band as frequencies between 300 MHz and 1 GHz. Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz. One of the biggest pros of UHF radios is that they do a much better job of penetrating barriers in urban locations, like concrete, steel, and wood. So this kind of radio is best suited for use indoors, including buildings with multiple floors, in industries such as manufacturing, distribution, education, retail, hospitality, and healthcare...
Read more