VHF Log Periodics and the Tog Scan

If you have been planning an antenna system for the VHF or UHF bands, you shouldn't overlook the log-peri odic array. Th is antenna features high gain, wide bandwi dth and reproduceabili+y.

The new look in antennas is here. Soon VHF log-periodic antennas will l>e replacing vagi arrays of practically ail types, and also replacing old standbys such as the collinear, helix, and corner reflector. This revolution has already been taking place in the television antenna business for several years and there are good reasons why.

First of all, do not be led astray on the subject of gain. Not many people interested in antennas presently know that log-periodics having about 1,5: \ bandwidths can deliver just as much gain as yagis the same size and having only a few percent bandwidth. This, as well as other new data, has been accumulated in the last few years and the properties of the planar (flat) log-periodic have been under investigation recently. Only lately has it become evident that log-periodics with small apex angles can yield as much as 12 dB gain.

Construction details of the ''Log-Scan 420" are presented later in this article. This an tenna serves as an excellent example of the advantages of the log-periodic. Here is a summary of its characteristics:

a. Gain; 16-17 dB over an isotropic.

b. Bandwidth between 1.5:1 SWR points: 50 MHz.

c. Size: 14i£" wide, 65" high, 41" deep-

d. Input impedance: 50 olims unbalanced.

e* Matching section or tuning adjustments necessary after construction: none*

As you can see, the antenna is ideal not only for general work on 420, but is just the thing for the ever-growing amateur television fraternity. Lets compare it with its nearest competitor, the yagi.

A stacked array of four yagis of the same overall size would have L5\ length booms, seven to nine elements per vagi, and each yagi would exhibit about 11 dB gain. With a stack of die above dimensions, a gain increase of only 4-5 dB would result because some aperature overlap occurs. However, even with fullwave stacking (height of 80"),

The "Log Scan 420" array built by K4GYO. Each antenna In this array exhibits about I 1.5 dB gain; the entire array yields about 17 dB.

The "Log Scan 420" array built by K4GYO. Each antenna In this array exhibits about I 1.5 dB gain; the entire array yields about 17 dB.

. r a total array gain of over 16.5 dB is unlikely. Advantage of the yagi in gain: little or nothing.

Mechanically, the yagi has the advantage of fewer booms and elements, but from every other standpoint, the log-periodic array has the upper hand. For instance: a 50 MHz bandwidth, compared to a 1 to 2 MHz bandwidth of the yagi. Even with special techniques, bandwidth of more than 10 MHz for the yagi array cannot be achieved without going to twin- or triple-driven elements at the expense of array size and/or gain. Also, log-periodics are not sensitive to tuning effects caused by element and boom diameter, Nor do small (less than 2%) variations in element lengths from those intended have much effect on gain and input impedance, In addition, no balun, delta, tee or gamma match is needed to couple the antennas to the coax phasing harness or feed line. The og-periodic design can be adjusted to provide a good match directly to coax of any impedance; this eliminates any tuning or pruning, and effectively reduces the weatherproofing problem to zero. More about these factors later.

What advantages does the planar log-periodic offer over dishes and corner reflectors? One word; size. The "Log-Scan" has the gain of a dish 7 feet in diameter, or of any other screen reflector antenna of about t !ie same area. It also has about the same gain as a 32-element collinear 62 inches square. The reason is that, like the yagi, ¡he traveling-wave structure of the log-periodic multiplies the effective capture area il exhibits, to equal a reflector antenna of much larger size (whose capture area approximately equals the reflector area). The simplicity of transmission-line matching doesn't offer too much advantage over the dish or corner reflector, but does when compared to the problems encountered with a 32-element collinear in a high humidity area,

Designing your own

Log-periodics have the extraordinary feature of being truly wide-band structures, with their electrical properties repeating at intervals occuring at a ratio equal to the factor t5 as the frequency is changed. The r factor is the factor by which the next higher set of elements on the antenna decreases in length, relative to any one set. If, for example, an array had a T factor ol 0,9, and the longest set of elements were 100 inches

Fig. I, The planar log-periodic antenna. The r factor determines the relationship between subsequent element lengths and spacing. The a angle is the apex half -angle sh own here. Both of these factors control the available gain of the array,

long, the second set would be 90 inches, the third set, 81 inches, and the fourth set, about 73 inches. The t factor is usually chosen above 0.7 so the properties of the antenna repeat close enough together percentage-wise so there is no appreciable variation in them, A log-periodic can be made to cover ¡0:1 frequency ranges or more. However, to cover, say, a 100:1 range, it would be necessary to scale down the diameters of the elements and booms in inverse proportion to the increase in frequency.

The low cutoff frequency occurs when the longest set of elements is about 0,47 wavelengths long. The high cutoff frequency occurs when the shortest set is about 0,38 wavelengths (for that frequency). If it is desired to maintain gain and pattern closely over all of a given band, the cutoff frequencies should be set 10% below and above the band limits for r factors of 0,9 and above; 20% for smaller factors.

The a angle is the apex half-angle, as shown in Fig 1* The r factor and a angle together control the available gain; it being higher for a smaller a angle (which means a longer boom) and a higher r factor (which means more sets of elements).

It can be seen why an antenna can be duplicated with different element and boom diameters than the original, without affecting the performance. All that will happen is that the high and low cutoff frequencies wall be shifted slightly, This factor makes building log-periodics much easier than building and adjusting vagis.

Looking at Fig, 2, it can be seen that the planar log-periodic is actually a balanced transmission line with elements fed from along its length. Notice that each set of

200 Ohm Ohm Feed Line Match Chart
of the booms and connected to both,

elements is reversed in feed polarity from the previous set, Tiie antenna will not work unless this is done. The antenna structure is fed at the high-frequency end, and its feed impedance appears somewhat less than the characteristic impedance of the boom structure. It is possible to match impedances from 50 ohms to 200 ohms by adjusting the boom spacing. The only restriction is that low impedances should be used only with high r factors, although the reverse isn t true.

The L-P balanced structure can be fed by coax, without using a separate balun, by feeding the coax through one of the booms from the back of the antenna, The shield of the coax is connected to the carrier boom only at the very front, and the center conductor is connected to the end of the other boom by the shortest possible path. Currents on the other surfaces of the booms drop almost to zero toward the rear of the antenna, and the boom completely shields the coax from antenna fields a]ong its length. The coax can be taken from the rear of the boom to the mast at about a 45 degree angle, without producing noticeable effect on antenna pattern, or line SWR, Notice that bath booms must be insulated from the support mast and should be spaced from it by at least twice the gap between the booms*

It should be kept in mind that the smallest possible booms should be used for building VHF arrays, because this will lessen the amount by which the halves nf an element set are out of line with each other* The fact that the halves are not directly in line causes some shift in polarization away from horizontal. This can be minimized by using high t factors and by using square booms with the elements inboard toward each as far as possible. This was done in the "Log-Scan7' (see Fig, 9)

The gain is related to the « angle and r factor as shown in Fig. 3 on the left scale. Antennas will work with other combinations of ct and but these combinations are optimum for maximum gain. Fig, 3 also allows estimates of the size of an antenna for a given gain and bandwidth.

Let us design a L-P array as an example. Suppose that you wanted to build a fairly high gain L-P to cover 144-225 MHz, including 2 meters, channels 7-13? and Vi meters, ihe antenna is to have as much gain as possible without exceeding a boom length of 10 feet (h). First, calculate how many wavelengths at 144 MHz are equal to 10 feet; n =h x fiower/985

Then calculate the bandwidth ratio, BW:

Then, going to the graph, draw a straight line from 1.56 on the right scale, through L46 oil the center scale, and find its intersection on the left scale. Roughly, a = 4.5 degrees and r = .95. The gain available is 11.5 dB. This gain is equivalent to a 2-meter yagi of the same length, with a typical bandwidth between 1.5:1 SWR points of 2 MHz

—not even enough for the whole amateur band!

The next step is to calculate the longest element length, This length , 1*i, is equal to 0.47 at the lower cutoff frequency: &co = 0.47 x 985/fiow«

The second set of elements has a length of;

= 3,22 x 0.95 = 3.06 feet The rest of the element lengths are calculated in turn by multiplying each length by r to obtain the next length. To know how many sets are needed, calculate the ideal shortest element, hm; equal to 0-38 t at the high cutoff frequency; Isfiu = 0.38 x 985/ftiPP*r

Then continue the originsj table of elements until a length of less than 1,66 feet is reached, ¡ his is the shortest element needed (don't necessarily use 1.66 feet).

To determine the location of each element, start by determining the distance di from the longest and rearmost) element,

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