No. 55: What to Expect from Multi-Band Yagis

L. B. Cebik, W4RNL

In the last episode, we examined some of the physical and electrical factors that go into setting up a Yagi. For that exercise, we reviewed a large sampling of monoband beam designs for 10 meters. Our goal was to set up some reasonable expectations of Yagi antennas considering both the boom length and the element population. We discovered a number of factors that might influence our decision about which beam to purchase. The factors included the weight, the wind-survival rating, the gain and other pattern features, and the operating bandwidth of the antenna.

However, many 10-meter operators wish occasionally to use other bands. Hence, they are more interested in a multi-band Yagi, specifically, a beam covering 20, 15, and 10 meters. The effort to develop high-performance multi-band Yagis is many decades old. It has made very great strides since the advent of computer-aided antenna design. Nevertheless, the market today is filled with both modern and older designs. Fig. 1 compares in outline both types of tri-band Yagis.

At the top is a more modern design. It uses 7 elements total, but each individual element has primary utility on only 1 of the 3 bands. The element lengths are clues to the band of primary use. This particular beam uses a master-slave driver assembly, as indicated by the 2 elements very close to the long (20-meter) driver with the circle indicating the feedpoint connection. The lower tri-band beam is a hybrid. It uses some dedicated elements (without square boxes). Other elements serve more than one band, as indicated by the square boxes that mark the location of traps. Advertising hype tends to either oversell the losses of traps or to remain silent on their losses, depending on which kind of tri-band Yagi we are trying to sell. Therefore, let's pay a little closer attention to tri-band Yagi design to see if we can develop some reasonable expectations of these antennas.

1. The Earliest Tri-Band Yagis: Fig. 2 shows in outline form the general configuration of tri-band Yagis in the 1970s. These relatively early beams emerged from simple experimentation until the maker decided that the design was good enough to sell. The outline shows us two major factors to consider.

First, we find only 3 elements. From our exploration of monoband beams, we might conclude (validly) that the boom length and the element spacing are optimal on only 1 of the 3 bands--at most. Therefore, on the other bands, performance is likely to be lower than on the most optimal band. In many designs, the goal on the non-optimal bands was first to produce an acceptable feedpoint impedance for the coax feedline and second to develop at least a fair front-to-back ratio. Users who graduated from dipoles and doublets to the beam often mistook the reduction of rearward QRM for forward gain--and they still do today. Both factors are important, but they are not the same.

Second, we find traps in each element. Each trap (or equivalent device) terminates a higher-band length. On lower bands, it functions as a loading reactance. On 10 meters, the beam has no loading within the lengths for that band. However, the spacing is likely too wide for optimum 3-element 10-meter performance. On 15, the 10-meter traps load and shorten each element relative to its full trap-less length, but the spacing is likely closer to optimal. On 20, we have 2 loads per element side in each element. So the 20-meter elements are well below full size. In addition, the spacing is likely too short for full performance. Hence, in these designs, 20-meter performance tended to suffer most. (Incidentally, some tri-band designs appeared to have only one trap canister on each side of each element. However, each canister contained two traps, and the outer surface of the trap enclosure served as the intervening element section.)

Traps are not the only way to terminate an element at some specified frequency. Fig. 3 shows the schematic of a trap and its equivalent linear-loading substitute. Ordinarily, we tune a trap to a frequency at or just below the lowest frequency on the band that it terminates. So we might use 27.8 MHz as the resonant frequency of a trap for 10 meters. Now consider the linear load. It is a section of shorted transmission line that the designer has folded back toward the center of the element. Ideally, at about 27.8 MHz, the line would be electrically ¼ wavelength long, forming a very high impedance, just like an ideal trap. Like the trap, at lower frequencies, the linear load was an inductive reactance that allowed us to shorten the overall length of the element on the lower frequency. The earliest linear-loaded element designers claimed that they had no losses and hence formed ideal ways to terminate or shorten an element. Unfortunately, those claims have not proven to be correct. The fold-back construction is one reason for less than perfect performance. The 2 lines interact with the apparent main element, so the linear-loading section rarely shows perfect transmission-line currents that are equal in magnitude and opposite in phase.

The Q of a standard trap ranges up to about 250--a good value but not a perfect value. Each pair of traps in an element--when they function as loading devices on lower frequencies--tends to reduce the element's gain by about 0.5 dB. We cannot eliminate the loss with an ideal trap--such as a perfect linear load--because part of the gain loss comes from the shortening of the element. Hence, even ideal traps, of which there are none, would create some gain loss. (Unfortunately, many trap-haters attribute all of the gain loss to power dissipation, which is not true.) In our aboriginal tri-band design, 15 meters would show losses associated with a pair of traps in each element. On 20 meters, the losses would amount to the sum of 2 sets of traps in each element and the double shortening of the overall element length.

There is no easy way to estimate accurately the gain of the early 3-element designs. You might use the monoband performance table in the last episode and compare the boom length and associated gain. But remember that on 10 meters, the boom length might be too long for 3 elements. Then you can come up with an estimated gain on that band. Next, for each trap that is active as an element-shortening load on one side of each element, subtract 0.5 dB. On 15 meters, we shall subtract about 1.5 dB. On 20, we might subtract as much as 3 dB from the potential gain. Since 20 meters is already short in boom length, we would wind up with very little forward gain (perhaps 2 to 2.5 dB) over a dipole at the same height. However, the front-to-back ratio might be useful to us.

You can perform the same exercise on the hybrid beam shown in Fig. 1. However, only count the most active elements on each band. On 10 meters, we have 4 elements. To estimate the baseline potential gain, use the actual distance between the 10-meter reflector and the forward-most element. Since no trap loads the element, gain should be close to an optimal value for the boomlength. On 15 meters, the boomlength is between the rear-most element and the next-to-forward-most element. We find 1 trap on each side of 2 elements, the driver and the reflector. So we might reduce the potential gain by about 1 dB relative to what the boom length and 3 elements suggests for a monoband beam. (Remember to adjust the boomlength for the frequency change.) On 20 meters, we find 4 traps on each side of center for the full array. So we would subtract about 2 dB from the potential gain of a monoband 3-element beam with the same boomlength.

These estimates are very rough and ready, but they prove out in all too many cases. Advertisers tend to make claims that cite the peak gain of the array on its best band for gain and let the buyer assume that they apply to all bands. So if you count traps and estimate the boom length on each band, your revised likely gain figure will in most cases be close to correct.

2. Modern Tr-Band Yagis: Modern designs, like the upper sample in Fig. 1, do not use traps. Hence, we do not need to make adjustments for them. Each element serves a single frequency band. These designs have more elements and more aluminum tubing to bend or break in bad weather. However, their performance tends to be closer to monoband beams, if we know how to estimate it. Fig. 4 enlarges the small graphic of Fig. 1 and identifies each element. Note that the boomlength is different for each band of operation. We have a 2-element driver-reflector Yagi for 20 meters and another separate one for 15 meters. The 10-meter beam consists of a driver plus 2 directors.

Since the element spacing on 20 and on 15 is close to optimal for these bands, we can expect fairly standard 2-element Yagi performance on these bands. Although we do not have a 10-meter reflector, we can expect 3-element short-boom performance on that band, or close to it.

Element interactions will moderate these numbers to some degree. On larger modern tri-band Yagis, some 10-meter elements may be very close to 20-meter elements. They prevent the 20-meter elements from shifting the passband lower and hence add very little to the 10-meter gain. Element interaction also tends to reduce the front-to-back ratio relative to what we expect from 3-element or larger Yagis. Anticipate a front-to-back ratio of 12 to 17 dB from 3 or more elements on a band, rather than the standard monoband minimum value of 20 dB.

All multi-band Yagis are compromises. We pay for the convenience of having 1 beam for 3 bands by obtaining lesser performance on many bands compared to monoband Yagis with relevantly similar boom lengths. If we know how to adjust our performance expectations, we shall end up neither overly disappointed nor overly enthused.

In our exploration of tri-band Yagis, we have largely bypassed all of the physical considerations that go into a Yagi installation. Be sure to fully inform yourself about all of the important data so that your installation will be effective, secure, and safe.

Updated 04-01-2007. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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