Building Yagis – Part 1 (70 cm)

M2 440-21 ATV Yagi

M2 440-21 ATV Yagi (Courtesy M2 Antenna Systems Website)

I decided to build the simplest of our four Yagi antenna first – the M2 Antenna Systems 440-21 ATV. This antenna features 21 elements on a 14 1/2 foot Boom and provides excellent gain and F/B performance. Here are the specifications for the  M2 440-21 ATV Yagi (Courtesy M2 Antenna Systems Website):

Model 440-21ATV
Frequency Range 420 To 440 MHz
Gain 18.04 dBi
Front to Back 23 dB Typical
Beam Width E=22° H=24°
Feed Type Folded Dipole
Feed Impedance 50 Ohms Unbalanced
Max VSWR 1.2:1 Typical
 Input Connector “N“ Female
 Boom Length / Dia 14’ 6“’ / 1“
 Max Element Len/Dia 13-7/8“
 Turning Radius 96“
 Stacking Distance 65“ High & 65“ Wide
 Mast Size 2“ Nom.
 Wind area/Survival 0.85 Sq.Ft. / 100 MPH
 Weight / Ship Wt 5 Lbs. / 6 Lbs.
    # of Elements 21

Our mast is a 3″ piece so M2 made up a custom mast plate for us (more on this in a bit). The first step in the assembly of the antenna was to layout and inventory all of the parts and to thoroughly review the assembly instructions.

70 cm Yagi Parts

70 cm Yagi Parts

The next step was to assemble the boom and to install the 21 elements using the supplied insulated bushings and lock rings. The element installation takes the quite a bit of time as most of the elements are of different lengths and they must be properly centered on the boom. To make this process a little easier, I used a marking pen to number each element location on the boom so that I could easily determine which length element went in each location along the boom.

70 cm Yagi Boom and Elements

70 cm Yagi Boom and Elements

The next step was the installation of the driven element, balun and associated matching system. The key to this step is to install the coupling bars at the proper location on the driven element/matching unit.

70 cm Yagi Driven Element

70 cm Yagi Driven Element

The final step in the assembly of the antenna was to install the mast clamp. We are using a 3″ mast on our tower and M2 Systems made a custom mast clamp plate to accommodate our mast. The mast clamp should be placed on the boom at the point where the boom including the weight of the feedline balances the antenna relative to the mast. It’s important to get the mast plate installed so that the elements of the antenna are at a right angle to mast. This is easily done by leveling the elements of the antenna in the vertical plane and then using a horizontal level to get the mast plate perfectly square with the antenna elements.

70 cm Yagi Mast Clamp Installation

70 cm Yagi Mast Clamp Installation

These steps complete the antenna assembly. The only step that remains is to test the antenna and rig it with a coax jumper cable before it does on the tower.

Completed 70 cm Yagi

Completed 70 cm Yagi

One can do a reasonable test of a Yagi of this type by elevating it 10 – 15 ft and doing an SWR test. I attached a 20 ft length of LMR400 UltraFlex feedline to the antenna, carefully fastening it to the boom of the antenna so that it did not couple to and interact with the antenna. I then used a RigExpert AA-520 Antenna Analyzer to verify that the Yagi’s SWR was as expected.

70cm Yagi Test

70cm Yagi Test

At this point, the first of the four Yagi antennas is complete and I plan to tackle the 2M Yagi next.

– Fred (AB1OC)

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