The popularity of dual-band 2-meter/70-centimeter transceivers is high and still growing. VHF and UHF activity of all sorts on both bands is also growing. Indeed, both bands are fully populated with repeaters, and hilltop operations are becoming regular weekend events for all modes. Over the last year or so, I have received numerous inquiries into antennas that cover all of both bands. Two-band and three-band Yagis are readily available for the HF bands, so why not for the 2-m and the 70-cm bands.
When most amateurs think of a dual-band Yagi, they envision something like the version shown on the left in Fig. 1. The elements are interlaced, and the beam uses either a single driver element or two elements closely coupled. Interlacing the elements seems to promise a short boom, not much longer than required by the lower band of the pair. In the HF region, such designs are both common and highly successful.
One of the reasons that interlaced multi-band beams work so well in the HF region is that the maximum frequency ratio between the highest and the lowest band covered is about 2:1. The 2-m/70-cm combination presents a different challenge because the frequency ratio is about 3:1. Consider two dipoles, cut for 20 and 10 meters. We can easily use both dipoles with a common feedpoint. The reason is fairly simple: when one dipole shows a low impedance, the other shows a high impedance. The two dipoles do interact, but normally, we can prune each pair to show a satisfactory low impedance on each band. When rightly done, each dipole is active on its own band and relatively inert on the other band. However, if the frequency ratio is 3:1, we have a different situation. At resonance on the higher band, both dipoles show a low impedance. The activity of the longer element has many possible effects, ranging from detuning the upper-band element to dominating the far-field pattern of the combined antenna. Even where we can obtain some satisfactory performance on both bands, the operating bandwidth on the higher band tends to be very narrow.
When we try to interlace 2-meter and 70-cm elements in a Yagi design, the results are usually disastrous. As well, the high level of interaction between elements for the two bands requires that all dimensions--and everything else that has an effect on the electrical length of the elements--become so sensitive that even the smallest variation can ruin the performance relative to the desired level. One solution is to create isolation by setting the elements for each band at right angles to each other. Arrow Antennas has a hand-held satellite antenna that uses this principle. The right-angle orientation of the elements minimizes interactions. However, the antenna is not useful for using common modes on both bands. Digital and SSB operation on both bands requires horizontal polarization, while FM activity uses vertical polarization. The need expressed by my e-mail is for a beam that uses the same polarization on both bands.
The solution to the conundrum is to give up the idea of a vest-pocket beam for both bands and return to the fundamentals portrayed by the sketch on the right side of Fig. 1. In this design scheme, the lower band elements all appear at the rear. The shorter higher-band elements are either inert or nearly so when the lower band elements are active. The forward higher-band elements essentially interact only with the nearby lower-band director, which functions as a nearly inert added reflector. Note that each section of the beam has its own driver and therefore its own feedline.
The Basic Design and Its Performance
For a variety of reasons, my own design predilections run toward wide-band designs capable of covering all of a desired band. This tendency carries a limitation. First, for a given boom length and number of elements, the gain will not be as high as we can obtain from the same boom length over a narrower bandwidth. However, the seeming penalty also has an advantage. Once designed, broadband beams are more forgiving of the slight variations that occur in every replication due to small differences in the materials used or the shop skills available.
The design that we shall examine uses 3 elements on 2 meters and 4 elements on 70 centimeters. Each section of the beam covers the full band with acceptably small changes in performance level across each band. The sections each show an very good 50-Ohm SWR curve over the full band (144-148 MHz and 420-450 MHz) with less than 1.5:1 SWR. Because we are separating the two sections, the dimensions are almost (but not quite) identical to those we might employ for independent beams on each band. The final design requires a boom that is less than 50" long.
Fig. 2 shows the complete dimensions of the dual-band Yagi design. It includes element lengths and element spacing values, as well as spacing values for the combined array. Note that these dimensions apply only to the specified 1/8"-diameter elements in the sketch. Changing the element diameter will require considerable redesign of both element lengths and spacing values within each section of the beam. In a multi-band arrangement, there is no single reliable adjustment factor for changes in element diameter vs. element length and spacing. Re-design for different element diameters is best done with antenna modeling software in small and patient steps to obtain performance curves that are satisfactorily like those we shall show for this design.
With the dimensions shown, we may sample the performance on each band. Table 1 shows the free-space data and also the data for performance when the antenna is 20' above average ground with a vertical orientation.
2-Meter Performance of the Dual-Band Yagi Free Space Frequency Gain F-B Ratio Feedpoint Z 50-Ohm MHz dBi dB R +/- jX Ohms SWR 144 7.05 18.35 48.7 - j 9.6 1.22 146 7.08 18.38 49.4 + j 0.7 1.02 148 7.18 17.62 48.7 + j11.7 1.27 20' above Average Ground Frequency Gain TO Angle F-B Ratio Feedpoint Z 50-Ohm MHz dBi degrees dB R +/- jX Ohms SWR 144 10.71 4.3 18.34 48.7 - j 9.7 1.22 146 10.77 4.2 18.35 49.4 + j 0.7 1.02 148 10.89 4.2 17.60 48.7 + j11.3 1.27
To correlate these numbers to anticipated antenna patterns, Fig. 3 provides both free-space patterns and patterns over ground. If we assume that the primary use of the antenna will be with a vertical orientation, the free-space H-plane pattern becomes the azimuth pattern for the antenna when used over ground. However, if we rotate the antenna 90 degrees to a horizontal orientation, then the free-space E-plane pattern becomes the azimuth pattern for the beam. The figure does not show the elevation patterns for the beam in horizontal use above ground.
The fact that the patterns for three sampled frequencies do not change much is promising for consistent performance across the band. We can perform a similar evaluation by sampling the data and patterns across the much wider 70-cm band. Table 2 provides the numerical data (with the beam vertical above ground), while Fig. 4 supplies the patterns.
70-cm Performance of the Dual-Band Yagi Free Space Frequency Gain F-B Ratio Feedpoint Z 50-Ohm MHz dBi dB R +/- jX Ohms SWR 420 7.36 19.38 51.7 - j19.2 1.46 435 7.48 20.99 54.2 - j 2.3 1.10 450 7.87 22.28 49.5 + j17.1 1.41 20' above Average Ground Frequency Gain TO Angle F-B Ratio Feedpoint Z 50-Ohm MHz dBi degrees dB R +/- jX Ohms SWR 420 12.49 1.6 19.36 51.7 - j19.2 1.46 435 12.63 1.5 20.99 54.2 - j 2.3 1.10 450 13.05 1.5 22.55 49.5 + j17.1 1.41
The 4-element design used for the 70-cm section of the beam has slightly more gain than the 3-elements used on 2 meters. That slightly higher gain translates into a numerically more noticeable gain improvement at 20' above ground because on 70 cm, the beam is higher above ground than on 2 meters when we measure the height as a function of a wavelength. In these terms, the forward section (at about 9 wavelengths) is 3 times higher than the rear section (at only 3 wavelengths).
Special Note: When horizontally oriented, antennas show an initially rapid increase in forward gain as we elevated the antenna to and above 1 wavelength. The rate of gain increase slows down with further increases in the antenna height. When vertically oriented, the same antenna shows much more modest gain (largely due to the wider beamwidth), but as we raise the antenna above ground, the gain of the vertically oriented antenna increases more rapidly. By a height of about 20 wavelengths, the two gain values are nearly identical.
The patterns also return some interesting information. Although the sample patterns from across the 70-cm band are well controlled and similar, the rear lobes show more complexity than the rear lobes of the 2-meter section, especially in the E-plane free-space patterns. We can see the effects of the more complex rear-lobe structure in the H-plane patterns as well. Some of the complexity is a function of interaction with the 2-meter director, which is nearly three times longer than the 70-cm reflector. How much complexity is acceptable from a design standpoint is a compromise between lengthening the boom further, on the one hand, and developing significant impacts upon the overall pattern shape and the feedpoint impedance, on the other hand. The specified distance allows full band coverage with wholly acceptable values and lobe formation.
Since spot-sample data can sometimes be misleading, we should also perform some frequency sweeps with the dual-band array model to determine whether the performance is as smooth across both bands as the data seems to promise. Fig. 5 provides 2-meter curves for the free-space forward gain and the 180-degree front-to back ratio. Be sure to read the curves with reference to the appropriate Y-axis scale. For example, the gain curve looks like it takes a major turn upward. However, the total gain variation across the band is only 0.13 dB. Likewise, the front-to-back curve appears to be severely peaked. The actual range of front-to-back ratio is less than 1 dB. Both variations would be unnoticeable in operation.
The smoothness of performance over a given passband is a function (for Yagi arrays) of the gain, the pattern shape, the front-to-back ratio, and finally the feedpoint impedance. Fig. 6 shows the sweep curves for the feedpoint resistance, reactance, and 50-Ohm SWR across the entire 2-meter band. The resistance varies by about 1 Ohms, while the reactance shows a normal curve with a 21-Ohm range. As a consequence, the 50-Ohm SWR never exceeds 1.3:1.
Obtaining smooth performance on the 4-MHz 2-meter band is one thing, but attaining a similar goal on the wider 70-cm band is another. Allowing for the frequency differential, the 70-cm band is over 4 times wider than 2 meters. Two factors that assist in obtaining wide-band performance on 70 cm are the addition of the fourth element and the use of an element diameter that is physically equal to the 2-meter element diameter. At 70 cm, the 1/8" elements are equivalent to 3/8" elements at 2 meters. In addition, we do not always add elements to a Yagi beam to obtain gain. By careful design, an added element can improve our control of the Yagi's properties across a given passband.
As shown in the 70-cm sweep graph for free-space gain and the front-to-back ratio in Fig. 7, the Yagi exhibits a normal set of curves. The total gain range is 0.51 dB, a very small variation for a 7% bandwidth. As well, the front-to-back ratio varies by 2.9 dB, also a relatively small change over a 30-MHz spread. Operationally, we could not detect the performance difference by switching from one end of the band to the other.
The feedpoint impedance values are equally smooth across the band. As shown in Fig. 8, the source resistance varies by only 5 Ohms, while the change in reactance across 30 MHz is about 36 Ohms. As a consequence, the 50-Ohm SWR remains below 1.5:1 throughout the band.
Remember that a broadband beam is not just for individuals who wish to use it everywhere within a given band. For the relatively new antenna builder, broadband beams have another advantage. Small variations (I shall not call them errors) from the design very often allow virtually full performance with a broadband design, but the same variations are often enough to detune a narrow-band design to the point of ruining the anticipated performance.
Some Notes on Building a Beam Like This One
These design notes are not a construction article. You should use the materials and techniques that you have mastered. However, a few notes drawn from the design of many dozens of Yagis and the construction of many operating and prototype antennas may be useful to newer builders.
The design shown and analyzed in these notes emerged from NEC-4 models that fall well within the software guidelines for accurate models, that is, models that perform to expectations when constructed so as to coincide with the parameters of the model. All NEC models presume that all elements are insulated and isolated from any conductive boom material or use a non-conductive boom, such as a PVC or a fiberglass tube. (If you use PVC, then be sure that the type you use is well protected from UV. The UV protection of white PVC tends to vary by region of the country.) In fact, NEC has no way to model directly the effects of a boom on elements that contact it or that pass through it via insulating sleeves.
The design specifies the use of 0.125" (1/8") diameter elements throughout. Even changing the diameter to a 3/16" diameter rod amounts to a 50% change in the diameter, enough to require considerable redesign to restore the performance curves in all categories. Larger diameter changes will require patient redesign of the element lengths and spacing values to re-center all of the performance curves. I occasionally receive e-mail asking if I can redesign an array for someone's local material. Unfortunately, my time does not permit custom design work. At a certain point, a prospective builder has to master the skills needed to redesign a beam or simply pass up a design as interesting but not feasible. About the only elements susceptible to diameter changes are the drivers. Fatter drivers normally only require a change in length to restore their SWR curves and do not usually affect performance in other categories.
The construction of this beam likely should avoid virtually all nut-bolt combinations. In the HF region, even #10 hardware (stainless steel, of course) creates no measurable detuning effects. However, at frequencies 10 or more times the upper end of the HF region, the lumps created by #6 hardware may make a noticeable difference in performance. The closer the hardware lump is to the center of the element, the more it will likely detune the element relative to its modeled performance.
Similar cautions apply to cutting and spacing elements. The builder of UHF antennas needs to master the art of measuring to 32nds of an inch, and preferable to 64ths of an inch. (Better yet is the use of a rule that calibrates the space between inch marks in tenths and hundredths.) A 64th of an inch is about 0.4 mm. Such precision, especially from 70 cm upward, requires that we set aside our woodworking concepts of cutting elements in favor of a "sneakier" process. If we measure our element material, then we should cut it with the entire mark showing. Then we can trim the ends down while smoothing them. For aluminum elements, use a bench sander. For harder metals, you may use a grinding wheel (which will gall up if used on aluminum).
For the present beam--as much as I like aluminum rod--I would likely recommend brass rod. The reason for this choice is the fact that we shall add very small solder "blobs" to the parasitic elements (reflectors and directors). Fig. 9 shows the general idea. In a non-conductive boom tube (round or square), drill a 1/8" hole to pass the element. More precisely, drill a hole just large enough to pass the element with difficulty until it is centered with the centerline of the boom tube. On each side of the tube, add a very small amount of solder to the element to prevent the element from moving in the hole, but not enough solder to make a significant "lump." As an alternative that will work equally well with either brass or aluminum, you may obtain from a local home center one of the epoxies rated for adhesion to metal. Most of these epoxies will be non-conductive, but use only enough to prevent the element from moving under the most severe stresses that nature might eventually throw at the beam.
The driver elements require somewhat different treatment. We must split these elements to allow direct connection of a 50-Ohm coaxial cable (with a few ferrite beads near the feedpoint to attenuate common-mode currents). Fig. 10 shows one system that has proven effective. Construct a small plate to support the element. Plate size will vary with the size of the element, but it should be long enough (from side to side) to keep the driver in line and parallel to the other elements. At the same time, it should not be so large (especially on 70 cm) that it forms a significant substrate for the element. A substrate with a dielectric constant that differs from the value of air will detune the driver element. A pair of sheet metal screws through the plate and boom should hold it in place.
Very close to the center, drill two small (1/8") holes through the plate. Each driver half will make a very small bend at the hole and penetrate into but not through the plate. The function of the bends is only to lock the driver element position at the boom. The spacing between the holes should be the same as the spacing between the center conductor and the braid of the coaxial cable used. (Remember that the driver lengths are from tip-to-tip and include the gap created for feedline connection.) Solder the cable center conductor and braid to the element halves using zero-length leads. Coating the cable end and connections with a plastic dip-type coating (such as Plasti-Dip) will weather proof the cable end and solder joints. The length of the cable will depend on a subsequent step. However, clamp the coax to the plate or the boom to remove all stress from the connections.
Without further pinioning, the driver elements will swing. One step in preventing the swing is to cut a shallow groove in the plate from one end to the other so that the element can rest in the groove. At the ends of the plate, you can cut small block with an inset just large enough to pass the element. With the correct epoxy or plastic cement, weld the plate and the end blocks together.
Making all of the mechanical connections and drilling holes in the boom requires careful alignment. I recommend that you make up from scrap wood a jig to firmly hold and align the boom while adding the elements. For holes, a drill press--even the rudimentary type that clamps a hand drill--is almost a necessity. However, it will prove invaluable for many future antenna and household projects.
Any other system that achieves the same goals with minimum metal connected to or in very close proximity to the driver elements will do as well.
The beam needs support. If you use the beam horizontally oriented, you can place a standard fitting at the center of the boom to mate it with the mast or the stub that attaches to the mast. The center point of the boom lies between the 2-meter driver and director. The two coax leads from the drivers can lie along the boom or within the boom to emerge at the hub.
The greater challenge lies in using the beam vertically oriented. In most cases, the safest routing for the coaxial cables is to the rear of the 2-meter reflector so that the lines do not detune the elements of the beam. Fig. 11 shows one scheme for accomplishing this goal. Since the boom will be about 50" long on one side of the mast, it will require some form of brace (or counterweight) on the other side of the mast for balance. This configuration also places a limitation on the choice of boom material. It must be rigid enough not to sag significantly with the added weight of the elements. The elements will usually be an insignificant addition to the weight of such boom materials as PVC, so a length of boom that remains straight when supported at one end will likely be sufficient.
The sketch shows the brace installed on the side of the mast away from the antenna. This system is more apt to PVC and similar materials in which the cements virtually weld the junctions of pipes and fittings together to form a continuous length. However, a triangular support that meets the boom on the antenna side of the mast will also work if it does not interfere with the beam elements.
The sketch in Fig. 11 also shows a device called a duplexer. If you use the antenna with separate 2-m and 70-cm rigs or if the transceiver has separate 2-m and 70-cm input/output connectors, you may run the individual cables all the way to the rig. However, if the transceiver covers both bands with a single input-output connector, you will need a duplexer to combine the single paths on the transceiver side but to isolate the two antennas from each other on their side of the device. Feeding a 70-cm signal to both antennas at once will divide power between the two sections of the beam with very serious degradations to performance on that band.
A cross-band duplexer essentially consists of a pair of filters: a low pass filter for 2 meters and a high-pass filter for 70 cm, as shown in general form in Fig. 12. The cut-off frequency for each filter is a frequency that can be almost anywhere between the two bands. The key is that the cut-off frequency must allow the filter to exhibit virtually maximum attenuation at one frequency while showing virtually no attenuation at the other. The number of filter sections and their exact design will vary from one unit to the next. However, for dual-band rigs with a single input/output connector, the device is essential.
You may place the duplexer anywhere behind the 2-meter reflector. The most convenient mounting position will vary with the construction of the support system. You now also know the lengths of the feedline cables to the individual antennas: just long enough to reach from the driver element to the duplexer. Most commercial units will use cable connectors on both the antenna and rig sides of the unit. However, if you build your own duplexer, you may solder the antenna-side cables directly to the board holding the filters and simply seal the protective box openings--being careful to clamp the cables to remove stress from these connections.
Conclusion
We have examined a design for a dual-band directional antenna for 2 meters and 70 cm. The antenna falls into the category of utility beams, since it has modest forward gain, good front-to-back ratio, and a very wide operating bandwidth in all performance categories. Using separate 2-meter and 70-cm section arranged for minimal interaction results in a beam that one might reproduce with relatively good reliability and assurance of performance to specifications.
Just as important to the success of the beam as the electrical specifications is the care that we put into the construction of such a beam. Attention to all of the construction and support suggestions--or to variations that yield the same results--will allow the beam to achieve the performance of which it is capable.
Updated 08-08-2007. © 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.
Go to Main Index