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  • Stu WØSTU

Log Periodic Antenna (G9D06)

The 2019-2023 General License question pools asks about the advantage of a log periodic antenna:

G9D06: Which of the following is an advantage of a log periodic antenna?

A. Wide bandwidth

B. Higher gain per element than a Yagi antenna

C. Harmonic suppression

D. Polarization diversity

Log periodic antenna for HF bands illustrating logarithmic length and spacing relationship of elements.
A log periodic antenna for the HF bands. Photo courtesy of Frank K7SFM.

The log periodic antenna is a specialized antenna with unique advantages. Let’s take a quick look at the general construction of a log periodic and then consider the question response options.

The log periodic looks much like a multi-element Yagi antenna in that it has parallel elements mounted along a boom, with shorter elements in the forward pointing direction and longer element to the aft. However, the log periodic antenna’s design and performance is actually very different from that of the Yagi.

While the Yagi antenna will have reflector and director parasitic elements properly spaced and cut to length for directional gain, each element of the log periodic antenna is an active (driven) dipole element. This set of variably cut dipoles is driven from a single feed point at the front end of the log periodic boom, and the dipoles are alternatively fed with opposite connection polarity. The general construction and dipole element connectivity is depicted in the diagram below.

diagram illustrating the connections among a log periodic antenna elements, with polarity switching for each fed element in the row of elements.

The log periodic antenna is an array of active dipoles of alternating polarity, fed from a single forward point. Dipole element lengths and spacing increases logarithmically from front to aft along the boom.

Further, these dipole elements are cut in length to increase logarithmically from front to aft along the boom, and the spacing between elements along the boom also increases logarithmically front to aft. Hence, the name. You may recognize this arrangement as a popular one for VHF and UHF television antennas.

This unique arrangement results in the activation of a particular dipole in the periodic array depending upon the feeding frequency. Lower frequencies will be radiated by the longer elements near the aft of the boom, while higher frequencies will activate the shorter dipoles near the front of the boom.

You may have already deduced the correct answer based upon the construction design and behavior of the log periodic. From the bottom up in response options, what is the main advantage of the log periodic antenna?

  • D. Polarity diversity: No. Polarity is determined by the orientation of the electric field component of the waveform, and the electric field orientation is determined by the radiating element orientation. The log periodic is just like any dipole in this regard, usually operated either horizontally or vertically polarized only.

  • C. Harmonic suppression: No. Log periodic antenna may promote some increase in harmonic response, but they do not suppress harmonics with any special advantage.

  • B. Higher gain per element than a Yagi antenna: While the log periodic does provide moderate directional gain in the forward direction, it is typically not as great as that offered by a Yagi of similarly numbered elements.

  • A. Wide bandwidth: Yes! (If only by deduction.) By nature of its construction with multiple dipoles cut to length across a wide range of frequencies, the log periodic will offer very wide bandwidth performance. A single log periodic antenna can operate well across several amateur bands, providing good matching impedance and low SWR. It is an excellent choice for a single-feedpoint, multi-band antenna, and the moderate directionality is a nice fringe benefit!

The answer to General Class question G9D06, “Which of the following is an advantage of a log periodic antenna?” is A. Wide bandwidth.

-- Stu WØSTU


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