[NLRS] Club Cheap Yagi Build Day Advice Needed

[Dr. Gerald N. Johnson]

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 now out of date.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