No. 53: A Short-Boom Wide-Band 3-Element Yagi

L. B. Cebik, W4RNL

In the course of these episodes, we have examined a fairly large number of Yagi designs. They include short beams with only 2 elements up to long beams with 8 elements. Some have used matching networks to raise their low feedpoint impedances to 50 Ohms, while others used a direct 50-Ohm feedpoint connection. A few designs just barely managed to show SWR values of under 2:1 across the first MHz of 10 meters, while other have been wide-band designs. So why should I add another Yagi to the collection?

The simple answer is that the design in this episode is a bit different. A normal wide-band 2-element reflector-driver Yagi is about 5.5' long. A wide-band 3-element Yagi with a direct 50-O feedpoint is close to 11.5' long. There is considerable difference in the performance. The 3-element Yagi provides about 7.1 dBi free-space gain (add about 5.5 dBi when 1 wavelength above ground) and manages about 20-21-dB front-to-back ratio across the band. In contrast, the 2-element beam manages about 6.0-dBi gain across the band with only 10-12 dB front-to-back ratio. Now suppose that we could find a way to achieve about 6.7 dBi gain with about 16-17 dB front-to-back ratio and still keep the beam only 5.5' long. That beam would be worth at least a second look.

Fig. 1 shows 3 overlaid patterns for the beam to cover the low, middle, and top parts of the usual 1 MHz region of 10 meters. The patterns show very little gain change (about 0.7 dB), and the front-to-back ratio holds up very well until we reach the very top of the band. The antenna uses a direct 50-Ohm feedpoint connection, and the lower part of Fig. 1 shows the SWR curve. Ideally, the SWR does not reach 1.4:1 anywhere within the 1 MHz of 10. In fact, we might redesign the antenna slightly and cover the entire 10-meter band with under 2:1 50-Ohm SWR.

The performance improvement over a standard 2-element wide-band Yagi with the same boom length comes at a price: an extra element and a phase-line. The array that we are exploring has 2 driver elements spaced 25" apart with a single director 39" forward of the first driver. The total length is 64" plus a few inches at the boom ends. A 6' boom would serve very well. Let's take the structure in small steps.

1. The Elements: We can build beams to be light or to withstand heavy winds. To give you a choice, I shall give 2 sets of dimensions. One set uses heavier elements for wind loads up to about 90-100 mph. The lighter version might be rated to the 60-70-mph level. Fig. 2 shows the way in which we construct the two types of elements using common aluminum tubing with 1/16" walls.

The following table of dimensions applies only to these two element-diameter taper schedules. If you change the material diameters or the interior lengths of wider tubing, the beam may not perform as advertised. Remember to add about 2-3 inches to the lengths of the smaller tubes to provide a secure overlap.

3-Element Wide-Band Short-Boom Yagi with Phased Drivers: Dimensions
Heavy-Duty Version using 0.75"/0.625"/0.6" elements
Element             Total Length        Tip (0.5") Length       Spacing from Rear Element
Rear Driver         205"                60.5"                   -----
Forward Driver      193"                54.5"                   25"
Director            190"                53"                     64"
Medium-Duty Version using 0.625"/0.5"/0.375" elements
Element             Total Length        Tip (0.5") Length       Spacing from Rear Element
Rear Driver         206"                35"                     -----
Forward Driver      194"                28"                     25"
Director            191"                26.5"                   64"

Both versions provide essentially identical performance across the band. The free-space gain varies from 6.4 dBi at 28 up to 7.1 dBi at 29 MHz. (Yagis with directors show a rising gain with frequency increases, while 2-element Yagis with only a reflector show a decreasing gain with rising frequency.) The front-to-back ratio peaks at almost 18 dB at mid-band. Its lowest value is about 14.5 dB at 29 MHz.

2. The Overall Design: The general layout of the beam appears in Fig. 3. The top portion shows the relative position of the elements. In this design, I have started with a simple narrow-band driver-director array (and all 2-element driver-director Yagis have a very narrow beamwidth). I then changed the driver system to a pair of phased drivers in order to broaden the antenna's operating bandwidth. By the judicious selection of element spacing, element length, and the phase-line characteristic impedance, I ended up with a beam that spread the relatively good driver-director performance across the entire 1st MHz of 10 meters.

The lower part of Fig. 3 shows the general layout of the interconnection of the 2 driver elements. The phase-line consists of a parallel transmission line with a 250-Ohm characteristic impedance. The line requires 1 (and only 1) half twist between the 2 drivers in order to provide the correct phasing for broadband service on 10. The coax connector--that is, the feedpoint for the main transmission line--goes on the forward driver. This position is convenient, since the position is fairly close to the mast.

3. Making Your Own Phase-Line: Since you need only 25" of phase-line (plus a bit of extra for connections to the elements), you likely should make your own. The following table lists the center-to-center spacing for 250-Ohm lines using some common bare copper wires, listed by AWG size.

250-Ohm Transmission Line Dimensions
AWG Wire Size          Wire Diameter          Center--to-Center Spacing
#14                      0.0641"                   0.262"
#12                      0.0808"                   0.330"
#10                      0.1019"                   0.416"
#12                      0.1285"                   0.525"

You will need spacers about every 3" to maintain the wire spacing accurately. The best way to make spacers is to drill the wire holes in a long strip of plastic (such as polycarbonate or similar). Then cut the spacers to size after you complete the drilling. Do not make the holes too large; you want a tight fit. If you do not de-burr the holes, the spacer will tend to stay in place through all kinds of weather.

You may already be tempted to substitute 300-Ohm TV twinlead for the specified homemade line. I do not recommend the substitution. Even high quality 300-Ohm line has a velocity factor of about 0.8. Using a taut line will make the TV phase-line about 25% longer electrically than the value needed to create the right conditions for the drivers to operate well. Two elements with a phase line use a fairly critical combination of element dimensions and spacing--along with a fairly critical phase-line characteristic impedance and electrical length--to get the job done. The job involves dividing the current at the feedpoint so that each driver element receives the correct current magnitude and phase angle for maximum gain from the pair (in the presence of the director element).

4. General Assembly: There are many ways to construct Yagis. In this design, all of the dimensions apply to elements that are well insulated and isolated from a conductive boom. If you use a 6' section of aluminum tubing as a boom, you will need polycarbonate or similar non-conductive plates for the boom-to-element junctions. I prefer to use stainless steel U-bolts with saddles to grip the boom and the elements without crushing them. Saddled U-bolts with solid or cast saddles are available by mail. I prefer them to the typical muffler-style fixture with a U-shaped saddle that contacts the tubing in 2 lines.

The director will be a continuous element, with no break in the center. So double the length of the largest tubing in Fig. 2 to arrive at the center-section length. Both drivers require a small gap (1/4" is fine) for connections. I tend to prefer fiberglass rod inside the largest tube and extending to the ends of the plate. This system has 2 advantages. First, it places an extra support under the element U-bolts and also aligns the whole element with only 2 element U-bolts near the outer edges of the plate. Second, the rod allows good support for #6 or #8 stainless-steel hardware for making the connections to the phase-line and to the coax connector leads. Fig. 4 shows the general scheme without the U-bolts.

The side view shows the element below the boom for best stability. As well, the boom helps to keep ice and snow off the phase lines. The bottom view shows the phase-line and coax connection points. Keep the phase line taut. The sharp twist in reality will become a gradual twist along the line length. Using ¼" thick insulation plates and saddle U-bolts will provide enough spacing from the phase-line to minimize interaction with the boom.

The coax connector can sit on a small metallic plate attached to the forward edge of the forward-driver plate. Just be sure that the screws you use to secure the coax connector plate do not contact the boom. Of course, all hardware should be stainless steel, which you can obtain from most home centers these days. For other construction ideas and methods, you can check any number of antenna books, along with past episodes in the series.

The short-boom wide-band 3-element Yagi with phased drivers is one more design in the arsenal of directive beams for the 10-meter operator. If you have followed these columns for the 13 years in which I have been producing them, your design notebooks should have a large collection of potential designs for the new sunspot cycle.

Updated 09-01-2006. © 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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