Saturday, April 14, 2012

Choosing a Yagi

Choosing a Yagi

Tech Notes: Selecting Your Next Yagi 

Permit us to speak frankly for a moment about what the marketing realities are for some other antenna companies. 

Clever antenna designers and marketing departments know full well that the first test most amateurs test on a new antenna is for front to back ratio. It is an easy test to perform, and a good front to back ratio is sure to put a smile on a new owner's face. As marketing departments know, most "S" meters are inaccurate and tend to give inflated results, which inflate front to back ratios even more. So the most important design criterion given the engineer developing an antenna becomes front to back ratio, even though it may well come at the expense of front to side ratio, even front to rear quarter angles, and most certainly at the expense of gain. 

Designing an antenna for maximum front to back is a great deal easier than designing for maximum forward gain. Another factor favoring a designer's optimizing for front to back is that antennas using traps suffer less of a penalty in front to back measurements than they do when optimized for forward gain. The one inescapable truth is that optimizing front to back ratios is easier, and can give breathtaking results, but always at the expense of other considerations. 

We at Bencher think the most important design criteria should be gain. GAIN! Gain is why most amateurs buy yagis. Gain is what opens bands, digs weak signals out of the noise, and cracks pileups. When you turn your antenna to deepest Asia on a seemingly quiet band and DX signals suddenly appear, that is GAIN working for you, NOT front to back ratio. But there is no simple way the average amateur can test the gain of a new antenna, even if well equipped. So, the sad reality for most companies is that the designer optimizes front to back ratio first, and gain (ahem) dead last. 

But, while they can't measure it, most amateurs know very well that gain is what they really want, and what they are putting up towers for, and spending a lot of money for. Given the choice, knowledgeable amateurs would typically list gain as their number one criteria, NOT the last criteria. So what can a marketing department do to be sure that amateurs buy THEIR antennas when the gain of the antenna was in fact the least important criteria for the antenna design? The answer is simple. They cheat. 

Instead of telling the truth, their marketing department simply tells the prospective customer what he or she wants to hear - that their trapped tribander has, at the very least, gain equal to the finest monoband yagi of equal boom length. Indeed, the trapped antenna often claims to have better gain figures than the maximum possible gain for full sized monoband antenna. This is ludicrous. 

Besides simply inflating gain figures, most marketing departments use other tricks. Often they quote gain numbers based on having the antenna at a certain height so they can add the ground reflection to the gain of the antenna. Then they compare that gain against a unity gain theoretical antenna operating in free space where there is no ground at all, and thus no reflection enhancement. This marketing "technique" - and that is all it is - a MARKETING technique -gives terrific gain figures, but the comparison of an antenna in free space against an antenna over ground is absurd. 

On top of that, marketing departments know that gain figures quoted by antenna people are typically plus or minus a half dB. So to make sure things come out right they add that half dB to the number the antenna designer gave them as being the best possible gain for an antenna of that boom length. And then they round everything up to the next higher digit, confident that most amateurs have not yet mastered the complexities of the decimal point. 

But so what if the marketing people cheat? They know full well that if a ham buys their antenna and puts it up, he or she desperately wants it to work. As soon as the installation is complete, the proud new owner gets another amateur to run front to back tests with on the new antenna. If the results show good rear rejection, the smiles gets even broader. 

Often, thanks to vagaries of conditions etc. it is not until weeks later that the new owner realizes the antenna is not hearing as well as expected, not cracking the pileups as anticipated, and that the antenna simply is not performing as expected for the price paid. Not to mention the cost of the rotor, tower, cement, feedlines, erection time, etc. At that point the owner begins to realize maybe he or she has been had - that the weak link in the system that was so expensive is the antenna for which such inflated promises had been made. Don't let that happen to you! 

How is Bencher's approach to the Skyhawk any different? Skyhawk is an example of the most popular yagi style - the tribander. But unlike most tribanders, Skyhawk was designed and optimized right from the start as a trap free antenna. While much more difficult to design, and of course more expensive to manufacture, the benefits to be gained are substantial, especially in gain and bandwidth. Traps invariably narrow bandwidth, reduce gain, and are lossy. The ARRL Handbook states that traps cost between .5 and 1.5 db for each trap that the RF passes through. So, while this is not typically a factor on ten meters, where the RF goes through no traps, on twenty meters that can be a LOT of traps. 

Consider another point about traps, when used in parasitic elements: a trapped parasitic element (either a director or reflector) is a compromise right from the start. Not only is a trap a lossy component that reduces both received and transmitted signals, but it degrades potential performance in other ways. Let's say the trapped element is a director, trapped so it will work on both 20 and 15 meters. The position of the element on the boom is GUARANTEED to be a compromise. Clearly there is an optimal position for the director on 20 meters. Clearly, there is an optimal position for the director on 15 meters. And equally clearly, the optimal positions for 20 and 15 meters are NOT going to be the same. And guess what - the placement of the trapped element is going to be at neither position, but instead at the position that hopefully is the best compromise for the two spots. And that's what trapped antennas are all about - compromise. Compromise might be good in politics, but it certainly is not in the pile-ups. 

Being trap free allows the Skyhawk to be much more broadbanded than any trapped yagi. Skyhawk delivers significant gain across the entire 20 meter, 15 meter and first 900 kHz of the 10 meter band (the part where serious DX chasing goes on), and offers good front to back ratios across those same frequencies. 

Optimizing for gain carries a penalty for front to back ratios. However, the penalty is small, and the trade off not considered significant. On the other hand, optimizing for front to back carries a significant penalty for both gain and gain/bandwidth. Also, optimizing for pure front to back actually yields poorer performance and rejection of undesired signals off the sides and rear quarter angles of the antenna. But it makes for breathtaking results for signals directly behind the antenna. Knowing full well that amateurs can't effectively test for gain, most yagi manufacturers optimize for f/b. Also, the overall design is far simpler to achieve. 

The Skyhawk is optimized for gain/bandwidth. We believe you would prefer significantly more gain at the expense of a small penalty in the front to back specs. Isn't gain is what you really want? 

Bencher's marketing of the Skyhawk is different from some antenna manufacturers in one significant respect - we tell the truth. Our specifications are honest, not inflated claims. There is another very subtle but important point in Bencher's specifications - on each band, we quote the WORST gain and f/b numbers within the entire design bandwidth, as well as numbers for the BEST frequency. 

For the sake of comparison, here are the gain specs for Skyhawk using the ground reflection trick other manufacturers use to magnify their product: 

Gain:20 meters - 13.4 dB15 meters - 13.6 dB10 meters - 15.3 dB

And even on the unrealistic basis that those numbers are derived from, at least they are honest within the limitations of that specification - which is still more than you can say for some yagi manufacturers. We didn't take theoretical maximums for the boom length, nor did we add a half dB to cover that "Plus or minus 1/2 dB" tolerance. We did not even round up to the next full digit. 

Just remember - no yagi antenna with traps in any part of it can be as good, let alone better, regardless of the number of elements, as a properly designed full sized antenna of the same boom length. 

The above numbers reflect gain of an antenna one wavelength above ground over an isotropic antenna. (But that also means that the antenna has to be raised or lowered for each band to achieve that one wavelength specification. Just how stupid do they think you are?) Also, the above specs reflect the gain at the antenna's BEST frequency in each band. invariably, the worst gain figure within the band is suppressed. Want to guess why? But Bencher tells you the gain at the antenna's WORST frequency too. So far as we know, no one else has the courage to do that. 

Now let's add the rest of the tricks to the Skyhawk gain - add 1/2 dB, then round up: 

Gain:20 meters - 14 dB15 meters - 15 dB10 meters - 16 dBImpressive, huh? 

Some so-called tribanders have been offered in years past which were trap free - and promptly achieved a terrible reputation for performance. In fact, they really were simply monoband antennas mixed together willy nilly on a common boom. The interactions between elements for different bands killed them. One of the attractions of designing a trapped tribander was that element interactions are greatly reduced, since there are fewer elements. The Skyhawk was designed using recently developed antenna design computer programs which recognize interaction as a potential problem and deal with such problems. 

Another area you will want to pay close attention to is the hardware offered with any antenna you might be considering buying. Here we are talking about the nuts and bolts and screws. In years gone by most, antenna manufacturers used galvanized fasteners for antenna assembly but eventually went over to stainless steel for the smaller fasteners. However, most manufacturers still use plated steel U-bolts for fastening the antenna to the mast. The justification offered is that stainless steel nuts will gall on stainless U-bolts. Their elegant solution to this is to use a plated steel U-bolt, so that there won't be any galling. By a remarkable coincidence, the plated steel U-bolt costs a lot less money. However, the threads of the plated U-bolt quickly start to rust. And rust causes far worse problems than galling, in the short run when you want to remove the nut, and even more so in the long run when the bolt rusts through. 

It is true that there is a galling problem with stainless steel hardware. The cost of a stainless U-bolt is guaranteed to gall any bean counter. Stainless U-bolts and non-rusting saddles cost a lot more. Mechanical galling is a legitimate potential problem, but it is simple to overcome - putting an anti-seize compound over the threads eliminates the problem. Bencher supplies ONLY stainless U-bolts and nuts, and includes anti-seize compound for them. If you are considering an antenna from another supplier, find out what they think of you, the customer. Ask them what their boom to mast fasteners are made out of. 

All threaded hardware supplied by Bencher is stainless steel. The U-bolt saddles are aluminum. The Skyhawk is designed to go up and stay up. 

Another area antenna buyers should carefully consider are the plastics used in an antenna. For example, some antennas offered today use white PVC to insulate elements from the boom. But white PVC has a terrible reputation in solar ultraviolet light, and will typically disintegrate within a few years. For this reason Bencher has selected for its element insulation a special agricultural grade of low-density polyethylene tubing that incorporates several UV inhibitors. This material is designed to offer many years of service in areas of strong sunlight and is, we believe, the best material available for the application, regardless of cost. 

Bencher engineers had decided to use stainless steel lock nuts with nylon inserts in several points of the antenna, as being an an elegant way to insure a secure fastening. However, it was discovered that the nylon plastic used for the inserts has no anti-UV characteristics at all. A check with possible suppliers confirmed that these nuts make a poor choice in outdoor environments, as the nylon disintegrates from UV exposure and falls out of the nut. Then, the nut that was relying on the nylon to lock itself becomes a timebomb. 

Of course, Bencher then chose alternative options, like stainless steel lockwashers. But we note with interest the use of these same stainless locknuts with elastic inserts on several competitive products. 

Let's face it - the average age of the typical DX'er and Contester is rising. Most of us can foresee a day when we will be unable to climb and maintain our towers anymore, and instead will have to search for and pay competent people to do this for us. Clearly, money spent for an antenna that has no traps to blow up, no steel bolts to rust through, and which utilizes premium materials engineered to withstand the elements, is money well spent. 

The Skyhawk is, so far as we know, the first production amateur radio antenna ever offered that is wind-compensated. This means that an additional UV-resistant non-metalic element has been added to the boom, and a UV-resistant deflector plate also added to the boom, so that the antenna has no tendency to turn no matter which way the wind is blowing. 

The benefits of this may not seem instantly apparent, but they are very real. Right at the start, when putting the antenna up on top of the tower, the climber notices that when the breeze blows he has a lot less fight on his hands. True, the wind want to make the antenna move over, but at least it is not also trying to twist on him. 

There are other benefits. Being wind-compensated means that life is much easier for the tower and rotator the Skyhawk sits atop. One thing few hams seem to realize is that most weather related tower failures (not related to component failure like a guy wire or hoisting cable) are caused by or greatly abetted by TWISTING of the tower - not by simply blowing over in a wind. This is especially true of guyed towers, but is also true of crank-ups and other self supporting towers. When the tower is twisted the vertical structure is greatly weakened and the tower far more likely to buckle at a twist point and come tumbling down. Towers come down - they rarely fall over. And twisting is the most common factor is such failures. 

Antenna rotors too are severely tried by antennas that twist in the wind. What is especially hard on a rotator is the working caused by varying levels of winds, as well as varying wind directions, so that antennas that twist in the wind are constantly working against the rotator and its gears and brake. The wind blows, the antenna strains against the rotator, the wind lessens, the load eases up, then the winds blows again. The variable working is very hard on the rotator. 

Twisting abuses the antenna itself. The constant movement, then bumping into the rotator brake, then easing back up, then back, in the long run cause wear and fatigue, shortening the life of antenna components. The Skyhawk, having been wind compensated, does not fight the wind, the tower, or the rotator. 

Most longer boom tribanders require one form or another of strut to support the boom. Bean counters figured out long ago that a few feet of galvanized steel wire and a couple plated eye-bolts are a lot cheaper than the extra aluminum tubing needed to beef up the boom so it can be self supporting. The trouble is, the most expensive real-estate in your antenna system is the space on the mast sticking up from your tower. You would love to stack several antennas. (and don't worry - we have more coming that will surely tempt you) but a steel strut wire that has to stick up well above your tribander boom is a major impediment. But the bean counter can show his boss that he saved a few bucks on every antenna. 

Skyhawk needs no strut, nor is one supplied. This means your mast space is ready for additional antennas. 

One last point - the Skyhawk has a new mast to boom assembly that was designed by a tower climber for tower climbers. Have you ever been strapped in atop your tower, arms full of antenna, trying to somehow fish a lockwasher and a nut out of your shirt pocket without dropping them so you can get the first bolt fastened and get rid of some of the weight on your arms? And your nose is itching like crazy? 

That is one situation you will never face with the Skyhawk. The Skyhawk uses two mounting plates, one on the mast, and one on the boom, each easily assembled by themselves before the antenna is put up. Then, the antenna is lifted into place, and the climber simply guides the antenna mounting plate over two bolts already sticking out of the mast plate. The bolts go through two keyholes and the antenna drops into place. It is as simple as that! Then the climber only need tighten the two bolts, fit two more bolts in but with no load on him, tighten all nuts and he is done. It is far the simplest arrangement ever offered on an HF yagi. 

Examine other manufacturer's products and claims against ours. We think, in spite of the inflated claims you will surely find elsewhere, you will realize that the Bencher Skyhawk is without question the finest product available in its class. Skyhawk represents the first product in a new line of antennas from Bencher, with other revolutionary antenna and accessory designs to follow.

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