[NLRS] Yagi Design (Was: Club Cheap Yagi Build Day Advice Needed)

[James Duffey]

Jerry - We have strayed fairly far from the original topic, so I have changed the subject accordingly. That way people who are only interested in the WA5VJB antenna build can skip over it if not interested. 

I use MMana-Gal as well as YagiCad and 4NEC2 in my modeling of the WA5VJB antenna. For serious work, not that Bill’s WA5VJB project isn’t serious, I use a couple of different tools to confirm modeled results, generating new input files as I go. Testing without a good antenna range makes it difficult to confirm results, but if the SWR or directivity is way off that is usually a clue that there is a problem in the design or modeling, and if one can wait a year for the CSVHF conference to roll around, the antenna range there is useful. A number of years ago when I had access to a licensed version of CST, a professional (read expensive) 3D electromagnetic simulator and NEC4 (controlled access only), I modeled the 3 element WA5VJB in those as well. Interestingly enough, the electrical properties of the antenna varied little with which model one used. I was aware of the closely spaced wires issue from W7EL’s work as well as conversations with Cebik at Dayton while he was still alive, so this good agreement was a bit of a surprise to me. However, when I modeled a dipole, a folded dipole, and Kent’s J shaped driven element (It is really more accurately described as a half a folded dipole) with the different models, they all showed the same gain and pattern. Only the input impedance and VSWR bandwidth were different, as one would expect. This makes sense as a dipole is a dipole, regardless of the physical form it takes. That gave me confidence that the WA5VJB antenna could be modeled well with any of those tools. This all makes sense in the context of how the Yagi works; the driven element serves to launch the electromagnetic wave in the antenna, and as long as the driven element geometry is efficient and balanced, then the particulars of the driven element are really only important as to matching to the feed line. With this in mind, the WA5VJB antennas can be modeled with simple dipoles or folded dipoles as the driven elements as far as things like element diameter correction, boom correction, and frequency scaling go. One just needs to account for the driven element impedance when looking at things like VSWR and matching or model the VJB driven element exact.

I have never heard the pejorative Crumcraft used when referring to CushCraft before. I have heard of them referred to as CushCrap though. They were more crippled by their marketing department than the performance of their antennas, although now that MFJ owns them, that may have changed. The CushCraft antennas based on NBS Tech Notes 688 perform as one would expect from the NBS data, which is not too surprising as those antennas were designed to be reproducible if one followed the directions in the Tech Note for designing and building the antennas. 

The K1FO designs were optimized not only for gain, but also for low side lobes and reasonably wide bandwidth, and he was the first to effectively optimize all three of those parameters. They were a big improvement over the NBS designs in that they emphasized important antenna properties in addition to gain. 

The NBS antennas lived on through the 90s not as marketable antennas, but often as starting points for the genetic algorithms that were becoming available then to optimize antenna performance. I think the generation of CushCraft antennas after the NBS clones, such as the 13B2, were done this way, and they were and are good antennas for the boom length. There are better antennas available now, but the 13B2 speaks to the advance in computing power applied to antenna design from the late 70s to the early 90s. 

YagiCad has no relation to Yagi Optimizer, it is a completely different program; essentially a Yagi specific front end interface to a NEC computational engine. Elements are entered in a spread sheet by position on the boom and full element lengths are used. It is much improved over generating a bare NEC card deck. You should try it out if you have not. It is a very useful piece of software. It has its limitations, as do most modeling tools, but it gets one pretty far up the design curve without a lot of problems. It has an optimizer and does scaling, both for frequency and element diameter and you can chose several techniques for doing this.  It has a genetic optimizer and a calculator for various types of matches. If you want to do something outside of its limitations, it generates .NEC input files which can be used in any NEC or miniNEC based tool; barebones NEC, MMana-Gal, EZNec, CocoaNEC, or 4NEC2; and it can generate 4NEC2 input files so you can use the extended features of 4NEC2 in your modeling. As with all modeling tools one needs to study the results of the model carefully and make sure that they make sense from a physics and physical point. But that is the case in any modeling. Try it, I think you will find it useful. 

I think that there is merit to optimizing G/T, even for terrestrial use, although I realize that is not a universally held belief. It can make a big difference if your neighbor’s plasma TV is on a -10dB side lobe or on a -30dB side lobe. For the same boom length, there is a dB or more in SNR to be gained with the same length antenna by choosing an antenna with a clean design. That is a lot to disregard without some thought. I realize that you live in the country, so that may be a lesser consideration for you, but I think that relying on the station on the other end to have an antenna with less than optimum properties to work me is asking the station on the other end to give up a lot just for my advantage. I do agree that having 2 or more short antennas stacked vertically is a much better setup for contesting than a single long boom antenna. And much better for the rotor.

Thanks for pointing to me to your paper on stacking and the modeled performance of various antennas. It was very interesting. I don’t see the modeling of the 12 element M2 antenna you referred to in your earlier post in that article though; did I miss something obvious? You may want to revisit your modeling of the M2 9 element antenna. The gain you modeled is over a dB lower than the gain I modeled for that antenna when I was doing analysis for a friends stack that contains a 2M9SSB. It is also about that much lower than the gain for that antenna that VE7BQH quotes in his table of modeled Yagi data which he generates from several modeling programs and which is generally considered an accurate representation of the performance of existing antennas. I can dig out and send you my .NEC file for that antenna to compare with your input file and to check your model output. If you want to send me your input file, I would like to review that against mine as well to make sure that I haven’t made any mistakes. The modeling should be consistent, and when it isn’t it helps one’s future modeling efforts to find the cause. The modeled gain for the 9 element K1FO variant looks about where it should be though, so it looks like your Yagi modeling process with MMana-Gal is fine.

Keep in touch. - Duffey KK6MC 


> On 11/18/2016 10:58 AM, James Duffey wrote:
>> Jerry - We are getting a bit far afield from Bill’s original question
>> on the WA5VJB antennas with discussions of permanent antennas and
>> boom correction factors which are  really not needed for the VJB
>> antennas at 2M as they have an insulated boom, but I find the
>> discussion useful. Others may too.
> 
> The element diameter still has an effect while the wooden boom effect is small its probably not zero.
>> 
>> Modeling shows that replacing the 3/16 inch elements that Kent
>> specifies with #8 gauge copper wire results in a decrease in gain of<
>> 0.2dB, an increase in the SWR minimum of 0.6MHz, and an increase in
>> the Front to Back of about 4dB. None of these are show stoppers and
>> may well be worth accepting as a tradeoff with simplicity of not
>> doing the corrections and coming up with dimensions not on Kent’s
>> favorite English scale of 1, 1/2, 1/4, 1/8, 1/16, … :^)=
> 
> It is important to be careful of modeling with his J driven element. Modeling with NEC versions works poorer than MININEC for close spaced wires according to one of the creators of MININEC and the late Cebik. I have published papers from QEX on line at:
> http://www.geraldj.networkiowa.com/papers/nec/Cebik4s.pdf
> and http://www.geraldj.networkiowa.com/papers/nec/Mininecs.pdf
> 
> I like Manna-Gal for modeling because it has a handier user interface that the Vintage EZNEC that I have and it uses MININEC. Its all free with more than 4 times the segment count as the basic EZNEC on my DOS computer. It also keeps the antenna description files in a people readable format which allows for editing outside its spreadsheet style data entry. I've developed lots of weather data software in use at weather.net and making data files people readable is one of my basic rules that makes trouble shooting tons easier.
>> 
>> I assume that you are referring to Peter Viezbicke, author of NBS
>> Technical Note 688, available from the NIST (formerly NBS) archives
>> at<http://tf.nist.gov/general/pdf/451.pdf>  ? Excellent read and
>> reference, but the Yagi designs and empirical design methodology are
>> now out of date. The boom correction factors, which were determined
>> experimentally, are still good and recent improvements on boom
>> correction factors are still largely empirical and are really second
>> and third order corrections. DG7YBN has collected together a nice
>> discussion on boom correction factors on his web page:<
>> http://dg7ybn.de/BC_numbers/BC.htm>. That whole website is full of
>> good stuff on Yagi design and well worth a look. Among the
>> interesting information there is the work of UA3TZ on boom correction
>> factors who has discovered that the boom correction factor depends on
>> the location of the element on the boom and position relative to
>> other elements in the Yagi. He has a computer program, available free
>> on DG7YBN’s web site that does the corrections. To be honest, all of
>> this is overkill for WA5VJB antennas, which, if built to Kent’s
>> plans, will work very good as built. And the UA3TZ program is really
>> second or third order corrections.
> 
> Before NBS 688 VHF and UHF antennas were rarely duplicated. Many designs were published that were junk. My friend K0DOK built a 432 yagi from one of the VHF handbooks and found it had less gain than the dipole in front of a sheet reflector he had been using. He had access to a commercial antenna lab and found it actually was tuned to 420 instead of 432. When he trimmed it to 432 elements it worked like claimed and was able to copy EME from Arecibo that year.
> 
> The effects of boom were not known nor much of the effects of element diameter. That report is based on several years of experiments and after it came out the antennas detailed proved to be reliably replicated. They worked as he claimed. Crumcraft copied some of them. Crumcraft inspired CSVHF conferences to always include antenna ranges because CC claimed gains as much as 5 dB greater than their antennas provided and their NBS copy they claimed over 3 dB more gain, though none of them measured that good at CSVHF.
> 
> The next step in yagi design came from DL6WU who designed by his theory that the directors should have a particular phase angle set by their length to match their distance from the driven element. He also wanted the first director to not be really close to the driven element so the plain driven element had a 50 ohm impedance or 200 ohms for the folded dipole fed element. His basics were published in VHF Communications in the 3/1977 issue pages 157-166 and then 4/1977 pages 204 to 211. His next treatise was on extremely long yagis in VHF Communications 3/1982 pages 132 to 138. Those should be available on line now.
> 
> Somewhere in there he suggested a method of measuring boom effect. He said build a two element beam with a gamma matched driven element with adjustable gamma length and series capacitor. Then with a completely insulating front boom mount a proposed driven element based on his phase angle theory fairly close to the driven element. With nothing to reflect to the antenna adjust the gamma match for an impedance match. Then replace the test director with one mounted to its metal boom and adjust the length to get the impedance match on the driven element without changing the gamma matching settings.
> 
> His yagis became a standard of performance and have the characteristic of continuing tapering directors, unlike NBS and CC.
> 
> K1FO spent thousands of computer hours optimizing DL6WU designs for gain and G/T and gained generally less than 1 dB. He also allowed for changes from water and ice.
>> 
>> YagiCad,<  http://www.yagicad.com/yagicad/YagiCAD.htm>  , is a good
>> and easy, not to mention free, way to design, scale, and optimize
>> Yagis. In practice, on 2 meters, for the VJB Yagis, the change in
>> length of an element between using #6 copper wire and the 3/16” that
>> Keith originally specified isn’t much and one can stick with the
>> original dimensions for the three element 2M design with not much, if
>> any deterioration in performance. I would do the length adjustment
>> with element diameter for the higher frequency antennas, but then the
>> materials for those, 1/8inch, are more commonly available in the
>> useful lengths, so they can be built to plan without material
>> availability problems. The resonant frequency of the VJB antenna
>> changes a bit with substitutions of different diameter elements, but
>> the properties are pretty broadband as is. A well designed antenna,
>> which Kent’s are, should be pretty forgiving to tolerances in this
>> day and age. If you download YagiCad you will also get an excel
>> spreadsheet that will do element length adjustment and
>> transformations from tapered elements to linear elements and back.
> 
> I don't know about YagiCad, but an early program YagiOptimizer did not work well for all cases, because it analyzed the antenna using a coupling formula from W8JK's first edition that was only accurate according to W8JK when the elements were near to halfwave resonance. It worked really bad when thinking multi band yagis.
>> 
>> Yes, the brazing rod is not manufactured to hold tolerance in
>> diameter, but I have found it pretty good none the less and have had
>> no problem fitting it in a drilled hole in the wood booms of Kent’s
>> antennas. As the elements are glued in the boom, the hole can be
>> drilled a bit oversize in the wood boom to accommodate any errors in
>> roundness of the weld rod. In practice the wood will compress enough
>> to accommodate the out of round rod with a hole the same size as the
>> nominal diameter of the element. All this is for a wood boom,
>> aluminum booms do not compress and hence a clearance hole must be
>> drilled, but that is not a problem with the K1FO antennas you suggest
>> as the elements are insulated with the plastic insulators that are
>> press fit in the boom and give enough to accommodate the out of round
>> welding rod. The holes in these insulators are probably have a
>> similar or worse tolerance in roundness than the brazing rod has.
> 
> The shoulder washers I have used seemed to be very round.
>> 
>> The K1FO antennas are getting a bit long in the tooth and more recent
>> designs are probably better choices for building permanent antennas
>> from scratch. Recent offerings from YU7EF and others in Europe have
>> emphasized the gain over temperature, G/T, rather than simply gain
>> and low side lobes alone. The YU7EF designs are very good and are
>> optimized for each design, including the shorter boom lengths. He
>> presents designs for free space, one can easily adopt them to
>> whatever construction technique that is being used. For example, I
>> recently finished designing a 10ft boom 222MHz antenna, 9 elements,
>> based on a 2M YU7EF design for N7KA with the requirement to build
>> using the leftover parts from a couple of old Cushcraft 2M antennas.
>> The design was straight forward with YagiCad. The antenna models very
>> well, and I am eagerly awaiting the results of the finished product.
>> It looks like it will be the equal or better of the K1FO and M2
>> designs of the same boom length, and much better than the Cushcraft
>> 222MHz antenna of the same vintage. Like I said, I am eagerly
>> awaiting Arne’s build of the antenna to see how it performs.
> 
> I am not in favor of optimizing terrestrial antennas for G/T as spplied for EME. When operating from near the Boondocks the fancy G/T antennas don't hear me off the side and I can have to run high power to be heard in the nearest population centers of the cities and Chicago. I don't think side lobes down 40 dB or 25 dB affect terrestrial noise much, but do affect noticing me off the side.
>> 
>> Is your 9 element on a 10 ft boom optimization of the K1FO design
>> available to study? I couldn’t find details after a quick look on
>> your web page. After optimizing the gain, does your design have
>> adequate bandwidth with the low side lobes that K1FO designed to?
> 
> I used K1FO dimensions, no optimization. Side lobes are not way down, but after having used an M2 5 wavelength on 2m a stack of these will be a lot easier to aim. Lots of plots are in my paper from 2011 on How Close Can they be stacked. Pressented at Aurora that year and at CSVHF.
> http://www.geraldj.networkiowa.com/papers/CSVHF2011/HowCloseB.pdf
> 
> > The
>> reason I ask is that the M2 12 element Yagi, on a 20 ft boom, has
>> 12.7dB gain, per VE7BQH’s tables, with a G/T of -2.96. As gain is
>> primarily a function of boom length; the 10ft boom length antennas
>> modeled by VE7BQH all have gains between 10.5dB and 11dB. The 10
>> element K1FO design, at 12 ft on 2M, the shortest boom length he
>> optimized a design for, comes in at 11.33 dB with a G/T of -4.7dB. I
>> realize that it is possible to achieve very high gain for a given
>> boom length, but that usually occurs at design points of very narrow
>> bandwidth to the point of almost being unusable and uncontrolled side
>> lobes.
> 
> The 432 version has measured more gain at CSVHF twice than the computer modeling. That may be a sign that the K2RIW reference antenna used on the range is going bad from being around for so long.
> Measuring G/T and side and back lobes is beyond most antenna labs because reflections from the main lobe are too strong unless the antenna lab is isolated from everything for miles in all directions (including up and down).
>> 
>> Interesting discussion - Duffey KK6MC
>> 
> I have built and tuned a 2m version but haven't taken it to a conference yet.
> 
> 73, Jerry, K0CQ