K1KP Scratch built W6NL 40m Moxon
W6NL design modified/adapted by K1KP
6/2011
Initially the plan was to modify a 40-2CD 2 element 40m beam into the W6NL Moxon configuration which offers better efficiency, gain and bandwidth than the original design. After consideration of my specific project plans, it was decided to build the antenna completely from scratch. This was for the following reasons:
-By having the new antenna fully assembled and ready to install on the same day as the old one was being taken down, only one day of tower work with professional help was required instead of two.
-By saving the original antenna, it could be used as a backup should the new antenna fail, or it could be sold for more than the salvage value of the reusable parts.
-Working with new aluminum tubes avoids any issues with tubes from the old antenna that had bends, holes, or other issues.
Overall
The design goal is to provide survivability to approximately 80 MPH (no ice) for the elements and the boom, and yield an antenna with essentially the same performance electrically as the W6NL Moxon conversion of the XM-240. The dimensions for new tubing have been arranged to make efficient use of commonly available tubing lengths (3’ and 6’) as much as possible.
Modeling
The design has been modeled electrically using 4nec2 and mechanically using Yagi Mechanical. Both modeling programs have their limitations. Electrically, the antenna modeling does not agree with the real world. This applies to both the original W6NL version and the K1KP version. Specifically, the minimum SWR frequency comes out low and the peak gain comes out even lower. I have had the design modeled on EZNEC4/PRO yielding essentially the same results. Others have reported the same problems modeling this design in antenna forums on the web. As for real world experience, I have reports from Ned K1GU and Tim K3LR that the W6NL version performs beautifully and comes out spot on as far as SWR. So at this point my goal as far as modeling is to make the behavior of the modified design come out to match the modeling results of the W6NL design. That’s the best I can do given the limitations.
Mechanically the design presents modeling difficulties as well, as the Tee loading elements make the antenna look like a beam-within-a-beam. So that is exactly the approach that was taken to model all the parts of the antenna. First, the Tee elements themselves are modeled as elements of a simple 2-element beam. This gives confidence in the strength of the Tees themselves. Then each element is modeled without the Tees as a single element of a two-element beam. This is probably the most accurate modeling of the elements but does not include the wind loading or weight of the Tees. Then each element assembly with Tees is modeled as a two-element beam with the Tees as elements and the element as the boom. This provides the opportunity to consider trussing. In actuality, the wind loading of the Tees probably does not affect the element strength as they are perpendicular to the elements. Finally the overall antenna, without the Tees, is modeled as a two element beam to study the boom strength. Each of these steps yields a design which is 80 MPH or better. At this point (not having actually built it yet) it looks like trussing the elements will not be required.
The latest design for the antenna is described in ‘W6NL_Mox_100.pdf’ available on the web. This version was designed in 2010 by W6NL in response to mechanical failures experienced at K3LR. It uses 1.66 OD schedule 40 aluminum pipe for the element center sections. Instead of using this nonstandard size, two sections of double-wall tubing (1.5” /1.375” and 1.25”/1.125”ODs), were chosen for the center sections to achieve reasonable strength. The element tip lengths are the same as the 100MPH version. The reflector element tips are single sections of .5” OD tubing as in the 40-2CD, instead of telescoped combinations of .5” and .375” as in the XM-240. This tapering change did not show an effect in modeling.
There are two options for building the tees. The first option is to build them exactly as per W6NL, which requires 4' tubing lengths. The taper schedule of the tees actually has a large impact on the resonant frequency of the antenna. The cost difference is only about $10, so it was decided to build the tees exactly as per the W6NL design and simply cut down longer lengths of tubing.
Driven Element
The element mounting is changed to be similar to the XM-240 style.The center of the driven element is supported on a 4” wide 24” long aluminum channel with insulated clamps (DXE). The driven element is split in the center of course and direct fed with a balun. The elements start at the center as a double-wall nesting of 1.5 and 1.375 inch OD tubing. The tubing schedule is listed below. The locations of the ends of each piece of tubing are the last two numbers in the row, measured in inches from the center of the boom.
Driven
Length Exposed
1.500 6' / side 72.00 72.00 0.00 72.00
1.375 insert 72.00 0.00 0.00 72.00
1.250 72.00 60.00 60.00 132.00
1.125 insert 72.00 60.00 132.00
1.000 72.00 69.00 129.00 201.00
0.875 insert 36.00 129.00 165.00
0.875 9.50 6.00 197.50 207.00
0.750 6.00 3.00 204.00 210.00
0.625 12.00 7.00 205.00 217.00
0.500 72.00 67.50 210.00 284.50
284.50
The Driven Tee tubing schedule is:
Driven Tee – W6NL version
0.750 48.00 24.00 0.00 24.00
0.625 insert 24.00 0.00 12.00
0.625 24.00 21.00 21.00 45.00
0.500 48.00 45.00 42.00 90.00
0.375 36.00 28.75 82.75 118.75
The driven element Tee sections are mounted with an aluminum plate and clamps at 208.5” from the inside end of the driven element as per the original W6NL design. They are mounted on the .75” diameter tubing such that the edge of the 3” x 3” plate is flush with the end of the .75” tubing.
Reflector
The reflector design is similar to the driven element. The reflector halves must be electrically connected at the center, which could be done using a scrap 1.25” diameter piece between the element halves. The reflector is mounted the same way as the driven element, using aluminum channel and insulated clamps. The Reflector and Tee schedule is listed below:
Reflector
Length Exposed
1.500 6' / side 72.00 72.00 0.00 72.00
1.375 insert 72.00 0.00 0.00 72.00
1.250 72.00 62.00 62.00 134.00
1.125 insert 72.00 62.00 134.00
1.000 72.00 69.00 131.00 203.00
0.875 insert 36.00 131.00 167.00
0.875 28.50 25.00 199.50 228.00
0.750 6.00 3.00 225.00 231.00
0.625 6.00 3.00 228.00 234.00
0.500 72.00 54.25 228.00 288.25
288.25
Reflector Tee – W6NL version
Dia.
0.750 48.00 24.00 0.00 24.00
0.625 insert 24.00 0.00 12.00
0.625 24.00 21.00 21.00 45.00
0.500 48.00 45.00 42.00 90.00
0.375 24.00 34.75 88.75 124.75
The Reflector Tee sections are mounted with an aluminum plate and clamps at 229.5” from the inside end of the reflector element. They are mounted on the .75” diameter tubing such that the edge of the 3” x 3” plate is flush with the end.
Boom
After one year in the air, the boom design below has survived, but if I had it to do over again, I would go with a larger diameter boom design.
The boom is made of 6' sections 2.125” diameter either side of center. The ends are extended with 2” OD tubing with 12” of overlap. The boom gets a new center reinforcement section of 2” tubing, 6’ long, with a second insert of 1.875” tubing 3' long Additionally, the ends get an inner 12” long section of 1.875” diameter tubing. The boom should be trussed using any method you choose.