12 and 17 Meter Dual Band 2-element Yagi

I have no antennas for the WARC bands: 30, 17 and 12 meters. They've been low priority since I began building this station in 2016 because they are not contest bands. However, I also love DXing so I have never lost sight on this objective. I have been reasonably successful on those bands by using antennas for other bands, judiciously selected for their performance rather than match. You can do well with compromised antennas when they're up high, really high.

For 30 meters, I will be happy with a single element antenna, as long as it's resonant with a reasonably omni-directional pattern and does not interact with contest band antennas. I have a plan but I won't talk about that in this article. Yagis for 17 and 12 meters are desirable for their gain. That's the subject of this article.

My objectives:

• Efficient: no coils or other loading devices
• Gain and pattern: "good enough" performance across both bands
• Small and uncomplicated: I don't need or want to deal with of large and complex antennas for non-contest bands
  
• Match: low SWR without an elaborate matching network

A dual band 2-element yagi seems to meet my objectives. Although the gain bandwidth for any 2-element yagi (and that includes Moxons) is narrow, so are these 100 kHz wide bands. F/B is similarly adequate since high performance on that metric is not that important to me. Gain and match matter more to me, though you may have different requirements. Cebik noted how a 2-element yagi is suited for the WARC bands. There are commercial 2-element yagis -- examples one and two -- for those that prefer to buy rather than build.

Developing a model of the antenna is quite easy. I began with constant diameter elements since I was not yet decided on the physical design. I used EZNEC, as I usually do. The included NEC2 engine is adequate to model the antenna, and its built-in SDC (stepped diameter correction) will be useful when I settle on the physical element design. Since the DE (driven elements) will be close together, the segment alignment guidance in the EZNEC manual should be carefully studied.

There were several decisions that needed to be made even for a small antenna like this:

• Dual driven elements versus one driven element (coupled resonator for the other DE)
• 12 meter DE on the inside or outside of the 17 meter DE
• Matching network, if any

Yagis are peculiar antennas when you stop to think about it. They rely on opportunistic coupling to create beneficial currents (phase and amplitude) in parasitic elements to create the antenna pattern. Beyond a few elements the relationships are so complex that they defy analytic solutions. Optimized designs had to wait for fast computers and algorithms to power numerical analysis and optimization. Even with just two elements there is much insight to be gained by playing with computer models.

The parasitic elements of a yagi determine the performance and frequency range of operation. It may surprise some to learn that the DE doesn't play a significant role in that. The role of the DE is to effectively couple energy to (and from) the parasites and to set the feed point impedance. 

For a 2-element yagi we therefore need to first focus on the one parasitic element. It is usually chosen to be a reflector since that typically makes for a better antenna. However, the difference between it and the director version is modest. 

As an aside, it should be evident that since the parasite determines the pattern that in a 2-element yagi there is a "mirror image" frequency range where the reflector behaves as a director. For example, for the 12 meter 2-element yagi, the gain and F/B peak in the reverse direction at 23.85 MHz (shown above). That attribute can be ignored since that occurs outside of any amateur band and a DE designed for 12 meters won't be provide a good match at the lower frequency.

Returning to the practical design of a 2-element dual-band yagi, the necessary element arrangement imposes constraints. The first is that the two DE must be adjacent. This is because they are either driven by one transmission line or one of the DE (the smaller) is excited by coupling from the other DE. That adjacency determines the placement of the two reflector elements.

Spacing between the 17 meters elements is 2.8 meters (0.17 λ). The 12 meter reflector is 0.7 meters inboard of the 17 meter reflector. For this configuration -- there are alternatives that we'll discuss -- the 12 meter element spacing is 2.3 meters (0.19 λ). Boom length is 3 meters (10'). DE spacing is 20 cm.

The few commercial designs I've looked at place the 12 meter DE on the outside and make it a coupled resonator. It seems reasonable to me to shorten the boom by putting it on the inside and to slide the 12 meter reflector back a little to compensate. Dual driven elements, connected by a short transmission line, are not difficult and would seem to remove the problem of fine tuning the DE spacing and length for proper excitation on 12 meters; coupled resonators (they go by other names as well) can be finicky.

When I placed the 12 meter DE inside the 17 meter DE, although the gain and F/B were fine it was difficult to achieve a good 50 Ω SWR on 12 meters. This was true with both dual driven DE or the 12 meter DE as a coupled resonator. When placed on the outside of the 17 meter DE, a good match was easy to achieve with either feed method.  As already mentioned, the "found" attributes of yagis have to be worked with or worked around as determined by the mutual coupling of elements in any particular design.

When dual driven, a short transmission line connects one DE to the other. While it doesn't matter which is connected to the feed line there are two differences to consider. One is tuning. The short transmission line inductively loads the second DE because its nominal impedance is typically 150 Ω to 300 Ω. The DE at the far end of the line must be shortened to compensate. The second is that it is mechanically and electrically awkward to connect coax to the outer DE. You can see his in the Bencher (DXE) Skyhawk yagi where the coax has to loop backward to connect to the outer DE.

Since the coupled resonator version of the antenna works so well it is no surprise that the commercial designs chose a coupled resonator. Only one DE needs to be isolated from the boom, simplifying construction. I recommend spacers on each side of the DE to prevent the close-spaced DE from touching in windy conditions.

Achieving the excellent SWR in the model required careful adjustment of the DE spacing and the length of the 12 meter DE. In practice it will be necessary to make physical adjustments during construction and tuning. Although I achieved the best 12 meter SWR with a DE spacing of 20 cm, I strongly suspect that they will be closer after tuning.

I have deliberately avoided reporting the element lengths since that depends on tube diameters, lengths and steps, and the method of mounting them to the boom. If you are not comfortable with computer modelling you may be able to adjust lengths during initial tuning since the antenna isn't complex. However, modelling is strongly recommended since it can be extraordinarily accurate using EZNEC with NEC2, its built-in SDC, W6NL boom coupling adjustment and segment alignment of the close spaced DE as documented in the EZNEC manual. If and when I build this antenna, I will create a precise model of my chosen physical design.

It is worth noting that the SDC algorithm in EZNEC can only be applied with a narrow frequency range. When setting the frequency in the model to one band, the SDC cannot be done for elements of the other band. That restriction does not apply for the NEC4 and NEC5 engines. However, the SDC constraint has little effect on a dual-band yagi design of this type in which the non-resonant element lengths of the other 2 elements have a limited role. But be aware of the restriction when you model a multi-band antenna.

6.6 dbi free space gain for both bands is quite good. Although the theoretical maximum gain of a 2-element yagi exceeds 7 dbi it is very difficult to achieve in practice. It is more difficult for a dual-band design. F/B is not great but that is not unusual for a 2-element yagi since the frequencies of maximum gain and F/B do not coincide. Improved F/B is possible with nested Moxon rectangles at the cost of greater complexity. I don't require a high F/B so I am happy with these azimuth patterns.

I didn't bother to plot gain and F/B across the 100 kHz of both bands since there is so little variation. F/B varies about 2 db and gain less than 0.2 db. Tune these antennas at the centre of the bands and they will work well over the full bands. The same is true of the 30 meter band which is twice the wavelength but half the size (50 kHz).

Now we come to the big question: will I build it? My difficulty is less with the construction than where to put it. My towers are crowded and I want to avoid destruction interactions with my large contest band antennas. Side mounting is not an option since this directional antenna has to rotate. It will be necessary to develop models of it next to my other antennas to see where they may be problems. I am no willing to build and maintain another tower for this antenna.

Even without a placement plan I may still go ahead and build it. It's a small project and offers an opportunity to compare it to the model and see how it tunes and performs. Until then, I will continue to use my non-WARC band antennas on the WARC bands