Log Periodic V Antenna
June 1963 Radio-Electronics

June 1963 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

In his June 1963 Radio-Electronics magazine article, Edward Finkel introduces the Log Periodic V (LPV) antenna, a breakthrough in VHF TV reception, overcoming the narrow-band limitations of traditional Yagi designs by employing log-periodic scaling (τ = 0.9, σ = 0.085) to achieve uniform gain (8 dB low-band, 11.5 dB high-band), constant impedance (~1.2 VSWR), and a 35 dB front-to-back ratio across 54-216 MHz. Developed by University of Illinois researchers and JFD Electronics, the LPV uses "active cells" where resonance shifts smoothly with frequency, leverages third-harmonic resonance for high-band channels (7-13), and enhances directionality via forward-V elements and a phase-reversed feeder harness to cancel rear/side signals (Figs. 5-7). Reinforced aluminum construction ensures durability, while the LPV series (LPV-4 to LPV-17) caters to ranges from 50 to 175 miles, offering flat gain curves essential for color TV and ghost-free urban reception, with future compatibility for UHF, marking a significant leap in antenna technology by eliminating the compromises inherent in Yagi designs.

Log Periodic V Antenna

By Edward Finkel*

Certain limitations have been inherent in TV antenna design for so long that they have been accepted as axiomatic. No commercial antenna has had uniform high gain over the complete vhf TV band. It has been assumed that an all-channel antenna is not possible except by a compromise design that gives up a little bandwidth to get a little gain, or vice versa. The gain curves of modern TV receiving antennas are studded with peaks and valleys that show, only too well, how they depend on frequency.

Most antennas for fringe-area reception are based on the Yagi design. The Yagi has high gain and high front-to-back ratio. But it is essentially a narrow-band antenna - it cannot cover the entire vhf TV band from 54 to 216 mc. A simple Yagi is most effective for a single channel, a spread of only 6 mc. Modified Yagis, with dipoles cut for the center of the low and high bands and an array of various-size parasitic elements for broadening bandwidth, generally have good gain at the high end of each band and degenerate at the low end. This is the fate of any antenna burdened with a large number of parasitic elements. These lower the characteristic impedance at the low end of each band, and make for signal-sapping standing waves and impedance mis-matches between the antenna and the transmission line.

For more than 8 years, a group of antenna scientists at the Antenna Research Laboratory of the University of Illinois has been experimenting with vhf and uhf antennas that have no theoretical limitations on bandwidth - are frequency-independent. Various experiments led Profs. V. H. Rumsey and J. D. Dyson to the log spiral antennas. Out of this research came the sharply directional, yet broad-band, conical spiral antenna now being used for satellite tracking,

Prof. R. H. Du Hamel next tried and succeeded in developing a linearly polarized antenna based on the conical spiral, and Prof. Paul Mayes with R. C. Carrel and D. E. Isbell further developed this design to the point where it was basically suitable for television. JFD antenna engineers worked with the University of Illinois scientists to develop the final versions of the log periodic V, or LPV, antenna for television. The LPV promises to revolutionize the TV antenna field. Although it is now designed to cover uniformly both the low and high vhf TV bands and the FM band in between, a frequency spread of 4 to 1, this antenna type can easily be extended to include uhf. The unique thing about it is that within each TV band its impedance, gain, reception pattern and front-to-back ratio are virtually constant. The gain for each channel is as high as that furnished by a comparable sized, single-channel Yagi.

Log Periodic Concept

Essentially, the LPV antenna incorporates two separate design concepts: the log periodic factor, which determines the size and spacing of the elements; the forward V shape of the elements, which permits multi-mode operation and determines its directionality. Let us first consider the periodic function.

The basic planar log periodic antenna is an array of dipoles in which the length of each element bears a fixed ratio to the length of the preceding element. This ratio is called the scale factor and is designated by the Greek symbol T (tau). The spacing between adjacent dipoles may also be fixed by a ratio, a (sigma). These relationships are shown in Fig. 1, where h denotes element half length and d represents the spacing between dipoles.

The actual values of tau and sigma were derived from many experimental models and tests and finally selected from tables which combine these test results. The directivity of the antenna increases with increasing tau, and sigma must be small to obtain higher mode (harmonic) operation, important for high-band reception. (The mode desired multiplied by sigma should equal 0.2 to 0.4.) Since, for TV, the third mode is desired (as will be explained later), a good value for sigma is 0.085.

Each of the dipoles in the antenna is equal to an adjusted half-wavelength at a different frequency, making the dipole resonant to that frequency. The scaling factors τ and σ are so chosen that the desired frequency range is covered with elements whose resonances overlap. Thus, as the frequency changes, resonance moves smoothly from one dipole to the next.

Typical values of tau and sigma are 0.9 and 0.085, respectively. These in fact are the actual values used in one of the many experimental models developed in the JFD laboratories. This is a seven-element antenna, 92 inches over-all, with h₁, the half length of the longest element, 56 inches, approximately one-quarter wavelength at channel 2. Lengths of all other elements are determined by the equation in Fig. 1. A diagram of this antenna is shown in Fig. 2.

In designing the larger LPV models it was necessary to depart slightly from the log periodic formula, to make the antennas commercially and mechanically practicable.

Fundamental Operation

Just as the largest dipole of the LPV antenna corresponds to a half-wavelength on channel 2 many of the other dipoles more or less correspond to the half-wavelengths of the other channels in the low TV band. Although one particular dipole - the one closest to the resonant length - absorbs the greatest amount of signal at any particular received frequency, the adjacent elements also absorb signal energy. How much is shown in Fig. 3, a curve representing the distribution of current at the terminals of each dipole of a nine-element LPV antenna on channel 5. Note that while maximum energy is absorbed by one dipole, No.5, two other elements, Nos. 4 and 6, absorb 60% as much, and even elements 3 and 7 absorb substantial amounts of signal (30%).

The resonant or near-resonant dipole together with those adjacent elements that contribute substantial signal energy at the received frequency, plus the crossed phasing harness, constitute the "active cell" for that channel. As the frequency of reception increases, the active region moves toward the front of the antenna; for each channel a different active cell is formed.

The tau and sigma used in the design of an LPV are the key in providing a wide active reception region for every channel. When these two factors are selected properly, the dipoles of the active cell present a low impedance at their terminals, resulting in high energy absorption. This low impedance results from a combination of element length and the spacing determined by the log periodic equations, as well as the thickness of the elements.

High-Band Operation

For channels 7 through 13, the large elements at the rear of the antenna constitute 3/2-wavelength dipoles. Therefore, they resonate to the received frequency at the third harmonic mode. The large elements at the rear of the antenna are 3/2 wavelength at channel 7. As the frequency increases toward channel 13, the 3/2 wavelength elements, and therefore the active region, shifts toward the apex of the antenna. The actual gain realized by third-harmonic operation is shown in Fig. 4, the vhf gain curves for the JFD LPV-11, an 11-element antenna. From these curves it is apparent that there is an average increase of 3 1/2 db in gain on the high band vs the low band. This is in accordance with good TV antenna design, which requires greater gain on the high band because of the greater transmission signal losses at these frequencies.

In all other respects, operation is the same as on the low band. Active cells embracing several elements for each channel and low impedance at the received frequency are basic to the antenna.

A close inspection of Fig. 4 shows that the gain of the LPV-11 is uniform across all channels for each band. This guarantees good color TV reception. For color fidelity, the gain on the bright-ness and color carriers within each channel must be nearly the same. Obviously this can only hold true if the antenna has a flat gain response curve for the entire channel.

If the input impedance of an antenna varies appreciably from that of the transmission line at any point in the bandwidth of the antenna, a mismatch will exist between the antenna and downlead. Such as mismatch decreases signal power to the TV set and introduces standing waves along the line. This leads to further signal reduction and ghosts.

The LPV is unique in that it maintains essentially constant impedance across the full bandwidth of the antenna. An important reason for this is that the input impedance of the LPV depends primarily upon the impedance of the feeder network, which can be easily controlled. In the JFD LPV series, the feeder consists of a crossed network of solid bars whose diameter, length and spacing are determined to give an exact match to 300-ohm transmission line. That this is the case is proved by measurements of the vswr which are consistently in the area of 1.2 to 1.

Directivity, Front-to-Back Ratio

As important as high gain and constant impedance are in fringe-area reception, the antenna would be worthless without good directional sensitivity. Even in the heart of cities, directivity is needed to reject the ghost-causing interference signals that bounce from building to building. In fringe areas, interfering signals from adjacent channels picked up by the antenna from the rear and sides cause venetian-blind and herringbone effects, fading and other picture distortions.

Yagi antennas obtain good directional sensitivity and high front-to-back ratios with parasitic elements (directors and reflectors). The LPV obtains its sharp forward pattern from the V-ing of the elements and the phase-reversed feeder.

Consider Fig. 5, a simplified diagram of a four-cell LPV antenna. front-fed, using a twisted phasing harness. Note that because the elements of the adjacent dipoles are not fed in parallel, they are in phase opposition. This effectively cancels reception from the sides. Furthermore, the length of the harness plus the space between adjacent elements adds up to produce a 360° phase shift between the signals reaching the first and those being picked up by the second element (or between any two adjacent elements) in the forward direction (toward the feedline, at the small end of the antenna). This 360° phase shift actually puts both waves in phase for additive signal strength.

Toward the rear, on the other hand, there is only a single 180° phase shift, due to the crossed harness. This effectively cancels reception from adjacent elements towards the rear.

The Signal Finds Itself in somewhat the position of a motorist going down an avenue that has phased traffic lights. Arriving at the front (small end) of the antenna, it finds each element in turn phased in its favor, and gives up a maximum of its energy to the antenna. If it arrives from the rear, it finds each alternate element phased against it, and is effectively cancelled out.

Directional sensitivity is increased and reception from the rear further reduced by V-ing the elements forward. A straight half-wave dipole receiving a signal three times its resonant frequency has a radiation pattern like that shown in Fig. 6-a. The signal sensitivity is dissipated in three forward lobes. If the elements of this same dipole are directed forward into a V, the pattern becomes Fig. 6-b. The two side lobes are brought together and merged with the center lobe as the elements are brought toward each other. The rearward lobes are "phased out" in the feedline.

Reception patterns for the complete LPV TV antenna are shown in Fig. 7-a for the low band, sharpening up to 7-b on the high band. This type of pattern is maintained through the FM band too. In actual tests the LPV-11 with 9 active cells and 2 directors maintained a front-to-back ratio of 35 db, with a gain of 8 db across the low band and 11 1/2 db across the highs. In comparison, a somewhat longer Yagi antenna, adjusted to a front-to-back ratio of 25 db at the middle of its band, fell to 15 db at the edges, and more important, had a bandwidth of only 7%, at a gain equal to that of the LPV.

Although reflector elements are unnecessary for the LPV, directors are desirable to "peak up" the high end of the upper vhf band, particularly for fringe-area reception. The director spacing is determined experimentally since it must not affect the input impedance of the antenna itself. Laboratory tests recommended a spacing of approximately half the distance between the two shortest active elements of the antenna. Director length is shorter than the shortest active elements - theoretically, it should be 0.46 multiplied by the half-wavelength of the frequency to be "peaked".

City and Far Fringe

Since the frequency independence of the LPV depends on the scaling of the cells, any number of intermediate cells may be narrowed without affecting the essential characteristics of the antenna. To narrow an antenna, a smaller value of tau is chosen, so that the shortest element is approached faster, omitting some elements in between. Narrowing the cells will reduce the gain but will not affect the front-to-back ratio, directivity and constant-impedance characteristics, which do not depend on the number of elements used, only on the adherence to the proper scaling factors and equations.

When a shortened LPV is used in a strong-signal area, the increased signal strength will compensate for the fewer total signal-absorbing elements. At the same time, it is no less important that suburban and city viewers use an antenna with high front-to-back ratio and low vswr to eliminate ghosts caused by signal reflection from tall buildings.

There are presently six models in the LPV series made by JPD. The shortest, the LPV-4, contains 4 active cells and is recommended for use up to 50 miles from the TV transmitting antenna; in other words, in city and most suburban areas. The largest is the LPV-17 with 8 active cells and 10 passive elements. This one is designed for use up to 175 miles from the transmitter under virtually ideal conditions. Between these two are four other models for any reception area.

Since element spacing and V-ing are critical, special mechanical innovations were needed to assure antenna rigidity. The crossarm is made of extra-heavy-gage aluminum, 1 inch square. Every element has sleeve reinforcements to prevent bending. The phasing harness is made of 1/8-inch solid aluminum rod, cold-welded into position. Other mechanical features are "flip-quick" construction for ease in erection, gold alodizing and the inclusion of a double U-bolt assembly.

A fortunate dividend in the LPV design is its "compatibility" with uhf. When and if combination vhf-uhf antennas find an increasing market, it is almost certain that the LPV will be one of the leading all-band designs.

* Executive vice president, sales and engineering, JFD Electronics Corp.