[NLRS] Club Cheap Yagi Build Day Advice Needed

Fri Nov 18 11:58:12 EST 2016

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.

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, … :^)=

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 Excellent read and reference, but the Yagi designs and empirical design methodology are now out of date.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.  

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.

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 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. 

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? 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. 

Interesting disucssion - Duffey KK6MC

On Nov 16, 2016, at 9:25 AM, Dr. Gerald N. Johnson <geraldj at netins.net> wrote:

> The wire diameter does affect the length of the elements so should match Kent's original.
> 
> Brazing rod from a welding shop I have used but it was often irregular in cross section, rarely round so its harder to fit in a drilled hole.
> 
> For more permanent antennas I like K1FO designs like were in ARRL handbooks for a long time. They do work well. K1FO didn't approve of shorter ones not having optimized them but I've found through modeling that tne 9 element gives more gain than a Msquared 12 element and does it with a shorter boom, like 10 feet at 2m or 1m boom at 432 and the 432 version measured more gain at CSVHF than the model showed.
> 
> Its important to correct element lengths for metal booms and for element diameter using the data from Pfisbecke's NBS report or G3SEK (now GM3SEK) program element.bas. I have it here as a windose command window element.exe from compiling the basic program, I believe.
> 
> 73, Jerry, K0CQ
> 
> On 11/15/2016 11:14 PM, James Duffey wrote:
>> Jerry - I never used #10, but I know that #8 and #6 copper wire works fine on the WA5VJB antennas with little or no sag. I used that size wire with no special treatment on my WA5VJB antennas that I used roving and they held up to Interstate Highway speeds or a bit more with few problems. Hitting low hanging trees and canopies did bend them, but they could be easily be bent back. To be honest, taking the antennas off and on the rover resulted in the elements looking a bit bedraggled, but that did not affect their electrical performance.
>> 
>> The original antennas by WA5VJB and the first ones I made were from #8 aluminum TV ground wire then available from Radio Shack in a 50ft roll pretty cheap. $8? Those elements were far less sturdy than the copper wire ones I built and really looked ragged. But they worked.
>> 
>> The three element VJB 2M antennas all require lengths greater than 3ft, which require splicing on an extension, which is where the exercise begins to deviate a bit from easy and may be a hassle if you are trying to help 8 or 10 people of varying mechanical and electrical skills make an antenna in an evening.
>> 
>> Having said that, I have made the antennas from 1/8in dia, 3ft long, bronze brazing (weld?) rod with small extensions on the end and they work well. I crimped the tubing on and then soldered it. The 3ft weld rod is suitable on higher frequencies, 220MHz and up, without the extensions and is probably the cheapest source of elements available. I bought the rod at Ace Hardware.
>> 
>> After Radio Shack discontinued the aluminum grounding wire I used 1/8 inch aluminum weld rod, again from Ace Hardware. Those antennas went together well and worked fine. The elements stayed straight even after hitting trees and canopies. They looked nice. But all those miles at speed took their toll. The aluminum work hardened and became brittle and even the slightest touch would break the elements. I switched those elements to the bronze weld rod as they broke. I suppose that this is not too surprising as the weld rod is not made from a structural alloy.
>> 
>> I have moved on from the VJB antennas for my rover as W7QQ designed and built some 8ft rover antennas. Still I have a few in the spare antenna pile and I always recommend them to beginners, particularly if they don’t want to invest a lot of money to see if they are interested in weak signal VHF/UHF operation.
>> 
>> I never thought of the VJB antennas in terms of longevity. By the time they start to deteriorate most operators will want to move up to longer boom antennas, either longer VJB antennas or aluminum antennas with a longer boom. - Duffey KK6MC
>> 
>> 
>> On Nov 15, 2016, at 9:29 PM, Dr. Gerald N. Johnson<geraldj at netins.net>  wrote:
>> 
>>> 
>>> 
>>> 3/16" brass rod 3 feet long can be found at some farm stores, good hardware stores, and hobby shops. Maybe even big box home stores, at Menard's in the hardware department next to steel and aluminum bars and rods, not the eletrrical department. The middle two places can carry K&S Engineering products. And the hobby shop brass (also available on line without the K&S numbers but the same sizes at McMaster-Carr, www.mcmaster.com) has tubing in 12" and 36" lengths with 1/64th wall in telescoping sizes. So 3/16" tubing is easily spliced with a bit of 5/32" tubing inside at the soldered splice. The tubing won't bend in the U shape though. I'm sure Kent lists the wire sizes and the wood sizes he has used.
>>> 
>>> 3/16" ID brass tubing, 7/32" OD can extend 3/16" tubing or solid rod on the outer ends of the slements.
>>> 
>>> #10 copper wire will probably sag on horizontal elements not being hardened like brass tubing. Of course the director and reflector can be aluminum rod or tubing probably readily available at the sources mentioned by Donn. Copper wire can be work hardened by stretching it.
>>> 
>>> Varnish is important to longevity of wooden boom antennas. Bare brass and copper elements survive longer if painted with something like clear Krylon or Rustoleum or clear epoxy paint if it can be found.
>>> 
>>> 73, Jerry, K0CQ
>>> 
>>> On 11/15/2016 7:14 PM, W. S. Mitchell wrote:
>>>> 
>>>> 
>>>> Greetings,
>>>>   I am organizing a WA5VJB Cheap Yagi build day for the Bloomington
>>>> (MN) Amateur Radio Association, which will happen at some point in the
>>>> next few months.  However, having never built these antennas before, I
>>>> could use some advice.
>>>> 
>>>> Assume for a moment that there are 8 participants, each of whom is
>>>> interested in building a 3-element 2 m yagi.  Each antenna requires
>>>> roughly 12' of conductor, with lengths of 37" (director), 41"
>>>> (reflector), and 60" (driven).
>>>> 
>>>> Am I correct to think that the best way to approach this is to use #10
>>>> solid copper wire for the whole thing?  I've looked for welding rod,
>>>> and it seems like it's more suited for 222 MHz and above (hard to find
>>>>> 36" pieces).  From my searching, 1/8" or 3/16" copper tubing are a
>>>> bit tricky to find as well, and is generally more expensive (and less
>>>> robust in the field) than the (admittedly thinner) solid stuff.
>>>> 
>>>> My hardware store has 1/2"x3/4" pine in 8' sections, which should be
>>>> good for a few antennas.  I already have coax that can be used, and
>>>> tracking down crimp-type connectors (BNC, PL-259, probably not N
>>>> because the club members are unlikely to have any N gear) shouldn't be
>>>> too difficult.
>>>> 
>>>> Any advice or suggestions on sourcing materials would be greatly appreciated.
>>>> 
>>>> Thanks,
>>>>   Bill
>>>> AE0EE
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>> 
>> 
>> 
> 