A QuackBeam is simply a yagi antenna that I (AD5Q, aka “Quack”) have optimized using YO (by K6STI) software. I don't think there is very much unusual about them, as many have used software to optimize and design antennas according to their own preferences. I even doubt that I am the only one still using DOS software to do this, but note that most of the many nice Windows modeling programs do not have optimizing routines. After designing and building several antennas for local hams or Field Day use, they developed a reputation because they were always remarkably loud. Then somebody coined the name “QuackBeam” and it stuck. It hasn't actually been trademarked.
As for those design priorities, YO gives us three tradeoffs: SWR, Gain and F/B Ratio. We can weight these tradeoffs anyway we want. I use priorities that are the opposite of those used by most commercial antenna manufacturers. By placing high importance to a broadbanded low SWR, I can usually produce a yagi with a very low SWR on both the CW and Phone portion of an HF band. Since I also emphasize forward gain, a QuackBeam will always outperform a commercial antenna of comparable size. These are usually designed for a high F/B ratio, which is always my 3rd priority. A QuackBeam is typically built from the parts of a commercial yagi – sometimes even a new one right out of the box.
Why Front-to-Back Ratio?
This is a good question. I remember my teenage years as a ham (in the mid 60's). It was common to find people on the phone bands (AM, BTW) spinning their beams around to show off their F/B ratio. I did this with my 2 element quad, and another local from my high school had a TH3 – both at about 30 feet. This was the way we could tell our antennas worked. We could also measure the SWR. The other way we knew they worked was to compare them against the old dipole or vertical – obviously superior. Forward gain, however, was not easily measurable – and still isn't. So an antenna manufacturer could advertise the SWR and F/B of an antenna and we could verify it. Since this is not true of forward gain, the manufacturers can use any number they choose. It certainly appears that they do. Thus, advertised specifications of forward gain often seem ridiculously inflated. In any case, it's not a good idea to buy a commercial antenna from one manufacturer solely because their claimed forward gain is 3 db higher than a competitor. Another item that has always impressed us hams is the number of elements on an antenna. More = better. In other words, many commercial antennas have more elements on a boom than are actually optimum. Few are wide-spaced.
Antenna modeling software has given us a benchmark. We can measure all the dimensions of any antenna and input them into the software to get at the truth. Files typically come with the software for many of the commercial designs, so we sometimes don't even have to do this. We can take their stock dimensions and simply run the optimizer. It's astonishing how much more gain we can get. Is F/B ratio really that important?
The most widely touted reason for a high F/B ratio is the ability to reject QRM off the back of the antenna. This might make sense if you live on the U.S. East Coast, but here in Texas we don't have a problem with QRM from Mexico and the Pacific while running Europe. Most of the QRM is actually in the same direction as Europe, and when running JA's it is GOOD to have South Americans call in from the reverse direction. So why are antennas seemingly manufactured according to the desires of hams that have a superb advantage into Europe to begin with? That's UNFAIR, especially since most of the serious QRM is not only in their skip zone, but in our main lobe.
This used to be my thinking. My first few HF yagis were optimized with ZERO emphasis on F/B ratio. They were loud and flat. There are, however, drawbacks to designing antennas this way. The need to reject QRM is not the only reason that F/B ratio is a good thing. Band noise seems to be louder in directions other than the DX opening, especially on 10 Meters. So an antenna with good rejection to the rear should be quieter. The main reason we need F/B has to do with pattern. A yagi maxed out for forward gain has an elongated pattern, with a poor F/B ratio and a narrow front lobe – too narrow. This means you have less coverage of populated areas during DX openings, and less coverage of the USA in stateside contests. Thus, the primary reason to consider F/B ratio as a design priority is to get a better pattern: a fatter main lobe with somewhat less band noise.
Disadvantages of Front-to-Back Ratio:
Is SWR important to you? Well, antenna modeling has proven that it's impossible to design a yagi with a high F/B ratio that also covers a lot of bandwidth. Most manufacturers advertise where their SWRs are 2:1 or better. I think 2:1 is too high, and the amps don't like it. Try designing a yagi with a high F/B that covers an entire HF band with 1.5:1 or better on 10, 15 or 20. If you succeed, you will give up something else: forward gain. SWR Bandwidth, F/B Ratio and Forward Gain are conflicting tradeoffs, and an emphasis on F/B has an especially negative impact on the other two factors. This is why some commercial yagis (notably HyGain) come with assembly instructions for monobanders to build the antenna for either CW or Phone – but never both. An SWR of 2:1 or better across just the Phone band is considered acceptable? I disagree.
I think the SWR curves of most manufactured antennas are HORRIBLE, and they also sacrifice up to 1.5 or 2 db of forward gain. Where it is impossible to design a high F/B antenna with good SWR and performance on both CW and Phone, it is actually EASY to design one with nearly maximum forward gain AND a nearly flat SWR across a broad range. The F/B will be poor, but some improvement is possible. And so the design priorities on a QuackBeam are different. I start with a very flat SWR, then maximize for forward gain, and finally try to get as much F/B as I can – without giving up much in the way of bandwidth and gain. I try to get 20 db of F/B, but sometimes settle for around 15 db. I always keep the SWR below 1.5:1 across the whole band on 20 Meters and above. 40 is more of a challenge. Nearly always, the SWRs of the completed antennas are better than the models, probably due to loss in the transmission lines.
The New Stacks at WX0B:
It was my pleasure to do the optimizations for the rebuilt 10, 15 & 20 Meter stacks at WX0B. The antennas on 10 & 20 are the old antennas, but on 10 we added an extra element. So they are five elements instead of four. For 15M, we started with a brand new 6 Element KLM Big Sticker right out of the box (36 ft boom) and built it as a 5 element QuackBeam with one driven element instead of two. We built a second beam to match it, with a new boom (custom built by Force-12) and elements from the old 15M antennas. The 20's are still 4 elements on 33ft booms, but remodeled for a huge difference in performance. The boomlength on the 5 element 10's is 25ft. All these antennas are matched with hairpin coils.
On 20, SWR is 1.1:1 or better across the entire band from 14.000 to 14.350, and usually under 1.05:1. On 15, it's 1.13 at 21.000 and goes flat across the middle of the CW band all the way into the phone band, then rises to 1.2:1 at 21.300 and 1.35:1 at 21.450. On 10, we are also high (1.2:1) at the low end of CW and flat across a wide range of the band, finally rising to 1.5:1 at 28.990 and 2.0 at 29.065. So we can't use the FM repeaters on 10 (duh). There is also a 6 element 6M Quack (18ft boom) that is flat everywhere. All measurements were taken in the shack using PowerMaster's by Array Solutions.
Comparison With The OWA Design:
Antennas that cover both phone and CW with a low SWR and high performance are becoming very popular, though most manufacturers are still designing for high F/B ratio. The performance of an OWA antenna is comparable to a QuackBeam in every way. Their forward gain is excellent, and the F/B ratio is not so high. There is, however, an additional design priority: a direct match to a 50 Ohm balun. This constitutes an additional design tradeoff that can only have a detrimental effect on SWR, gain and F/B as compared with a QuackBeam on the same boom. The difference, however, is very slight. It is easy to design an OWA yagi using YO. Simply configure it for a direct 50 Ohm feed and let it optimize for everything else.
My point here is that if you remove the 50 Ohm restriction, an optimizer will produce a design under the same other priorities with slightly more gain, flatter SWR, and a lower feedpoint impedance – usually 40 Ohms or less. This depends on the number of elements, as it is easier to achieve the goal of a 50 Ohm match when there are 5 elements or more. With 3 or 4 elements, forcing the antenna match a 50 Ohm load will often cost more bandwidth and gain. If the ideal solution to your design priorities were a 50 ohm antenna, your optimizer would produce it.
The stated advantage of feeding the antenna without a matching arrangement is that there is less loss, but I truly feel that the loss on a properly tuned hairpin match is negligible. When allowed to optimize to the lower impedance, the extra gain comes to about 0.2 db at the most for 5 or more elements, and usually more if there are fewer. You will, however, need to match the lower impedance to the coax and balun, which is where we use a hairpin coil across the terminals of the balun. An additional advantage of using the coil is that minor variations of SWR from the model can be tuned out by goosing the coil turns. The principal disadvantage of optimizing as a QuackBeam instead of an OWA is that it will cost an extra dollar to make the coil. The antennas are very similar, because the priorities of bandwidth and forward gain are reflected in both optimizations. The computer does all the work.
Roy (aka “Quack”) – AD5Q