The next step in our EME project is to assemble the four M2 Antenna Systems 2MXP28 Yagis. These antennas are large, cross-polarized yagis. They feature 28 elements each on 34 1/2 foot booms. The design operates as an independent horizontal and vertical Yagi on a shared boom and each plane has an independent feed point.

We are building four of these antennas to be mounted on an M2 Antennas 2X2 H-Frame. It is important that the four antennas be identical so they operate properly as an array. This includes things like symmetrical mounting and alignment of each antenna’s vertical and horizontal elements and the associated feed points. We will cover the assembly of the H-frame and Elevation Rotator systems in the next article.

Preparation

M2 Antenna Systems instruction manuals are very good and they specify the tools and procedures to properly assemble the associated antennas. A few additional items were helpful in our project. These included:

The assembly steps and procedures are similar for most M2 Antennas 2m and UHF Yagi antennas so I’m going to share some details and a few tricks that we’ve used successfully to build a number of their Yagis. You can see some of these other projects via the following links:

We successfully built all of these antennas using similar components and techniques.

The first step in assembling each antenna was to inventory and arrange all of the parts. I also took the time to wipe the boom elements with a solvent soaked cloth to remove dirt and aluminum dust that results from the manufacturing process. This makes assembling the antenna a much cleaner process.

M2 supplies detailed dimension sheets and boom layout diagrams with their antennas and we took the time to carefully identify each element and boom component according to the diagrams.

This step included careful measuring, sorting, and marking each element with its location and polarity (horizontal or vertical). This step makes the somewhat difficult step of getting all of the elements in the correct polarity and orders much easier. The marks allowed me to check and confirm the correct installation of all of the elements on the antenna boom before locking them in place.

We also pre-assembled things like the Mast Clamp and the Boom Truss Clamp during the parts inventory process.

Boom Assembly

The first step in assembling the antenna boom was to arrange all of the boom segments in the correct order and confirm their front/back orientation. This took some time to get right on the first of the four antennas. Each Boom segment was marked with the Sharpie to indicate its location and orientation in the final assembly.

We also installed the T-Brace Clamp to attach the rear of the antenna to the H-Frame’s T-Brace. It’s essential to do this step before assembling the Boom as the clamp cannot be attached once the antenna’s elements are in place. The correct location for the clamp was established via a careful measurement and the location was marked on the boom using the Sharpie.

The next step was to assemble the boom sections paying careful attention to the markings made earlier. We did not tighten any of the bolts that hold the boom sections together at this stage to allow us to re-clock each boom section for the best alignment of the elements later. A generous coat of NOALOX was used at the joint of the two largest diameter Boom sections to facilitate easier assembly and potential re-clocking later. NOALOX was also used on all bolts to provide anti-seize lubrication.

Once the boom is assembled, a 40-foot tape measure is used to carefully confirm that all of the holes for the elements are in the correct location. The Dimension Sheet is used as a reference to check and confirm that all measurements are correct before installing the Elements. This is also a good time to measure and carefully mark the location of the center of the Mast Clamp on the Boom.

The eye bolts that attach the Boom truss cable are also installed at this time.

Element Installation

Next came the installation of the elements. We began with the Horizontal reflector and worked towards the front of the antenna. The elements are held in place with insulated buttons and stainless locks. The elements are first installed in the correct location and carefully centered using a steel ruler. Vise-grip pliers are then used to hold the element in its centered position while the M2 supplied tool is used to push the lock on the opposite side of the element. The center is next checked again and if all looks good, the second lock is installed. This process is continued until all of the elements are in place. We pay careful attention to the markings on each element as part of the installation procedure to ensure that all of the elements are in the correct location on the boom.

Once all of the elements are in place, the antenna is rotated 90 degrees to enable boom adjustments to align the elements. It is common for the boom sections to be misaligned a bit after the initial assembly. A combination of clocking each boom section either a bit one way or the other or sometimes removing the bolts holding two sections together and turning them 180 degrees relative to each other will create a perfect alignment of the elements. Once this is done, all of the bolts that hold the Boom sections together are fully tightened taking care not to distort or crush the Boom tubes.

The same installation process is repeated for all of the vertical elements.

Driven Element Assembly

The Driven Element feed point blocks are installed next. The mounting screw and the Allen screws in the Shorting Bars all receive a light coat of Blue Locktite thread locker prior to installation.

Next, we loosely install the blocks in their correct location on the Boom and then install the Shorting Bars loosely on the Feed Point Block and Driven Element. Once these parts are in place, the screen that holds the Block to the Boom can be tightened fully, guaranteeing a perfect alignment of all of the parts.

The next step was to accurately set the spacing between the Feed Point Block and each shorting bar. I used a dial caliper to do this accurately but it can also be done with the careful use of a metal machinist’s or similar ruler.

The final step for each feed point was to install the 1/2 wave Coax Balun to the Feed Point Block. Be careful not to overtighten the coax connectors. Just make them snug and you are set. The supplied cable ties are used to secure the Balun to the boom.

The same steps are repeated for the Vertical feed point. It’s a good idea to install connector dust caps on the feed point Block connectors to keep them clean and dry prior to installation.

It is critical that the relative orientation between the Horizontal and Vertical Feedpoint Blocks be the same on all four of the antennas in the array. If this is not the case, the pattern of the array will be upset which will have a major negative effect on the array’s performance.

Mast Clamp and Boom Truss

The Mast Clamp assembly is installed next using the center mark placed on the Boom earlier. I also marked the backside of the Mast Clamp plate to show its center to make lining things up easy. The clamps should be oriented according to the H-frame mounting diagram (show at the front of this article).

The final step in the assembly process is the assembly of the Boom Truss. The 2MXP28 Yagi is supplied with a Phillystran cable. The height of the Boom Truss will be set later when the antennas are attached to the H-frame so we just installed both ends of the Phillystran cable to the Eye Hooks installed in the Boom. The connections are made using the supplied Strain Relief Loops and small cable clamps. A drop of oil on each nut helps things go together smoothly. We had some Phillstran cable end caps so I installed them on the Phillystran cable ends to protect against water ingress. The turnbuckles, remaining clamps/strain reliefs, end caps, and truss clamp assembly were stored in a plastic Ziploc bag and cable-tied to the Boom to be installed later when the antennas are attached to the H-frame.

Final Details…

It’s a good idea to give everything one last go-over now that the antenna is complete. All bolts and screws are checked for tightness, the Elements are all confirmed to be in the right locations, and the feed point assemblies are given a final check.

Our EME project involves the assembly of four of these antennas with a total of 112 elements! It took me about 3 days to assemble each antenna (working about 3-4 hours each day). We stored the antennas on our deck to make space in our shop as we went. The antennas are well supported using low saw horses and woodblocks so as not the bend the Booms or the Elements.

The next step in our project will be to assemble and test the Elevation Rotator system. You can read more about our EME station project via the links that follow:

If you’d like to learn more about How To Get Started in EME, check out the Nashua Area Radio Society Teach Night on this topic. You can find the EME Tech Night here.

Fred, AB1OC