Weighing Yagis

As I get closer to raising the stacks of 15 and 20 meters yagis onto the new tower I am increasingly sensitive to the mechanical challenges. Not only are they large size they weigh a lot. That is a primary consideration in the decision of how to accomplish the lift. Since I am far enough along in construction to weigh the antennas, albeit as a collection of pieces, I have done so.

Not surprisingly the booms comprise approximately half the weight of each yagi, more for the 20 meter yagis than for 15. The longer booms for the 20 meter yagis (12 meters or 40') and the heavier elements need to be stronger than the shorter 15 meter yagis (9.5 meters or 32'). Similarly the rotatable yagis at the top of the tower need to be stronger than the lower fixed yagis. The elements and element-to-boom clamps for each band are identical for both yagis.

Their measured weights are as follows:

• 20 meter side mount yagi: 36 lb (16.5 kg)
• 20 meter rotatable yagi: 60 lb (27 kg)
• 15 meter side mount yagi: 25 lb (11.5 kg)
• 15 meter rotatable yagi: 45 lb (20.5 kg)

I weighed them by subtracting my weight from the combined weight of me holding the boom. It is important to provide a solid and level support for the scale to achieve reliable accuracy. I took at least two measurements to eliminate method errors.

It is no accident that the booms for the side mount yagis are lighter. They are lower and thus subject to less wind stress, a concern since they have a larger surface area. In this case larger diameter does not mean stronger because for most of their lengths the wall thickness is less.

Next up were the elements, which I weighed in the same manner. There is some uncertainty in these quantities because of their low weight. Element-to-boom clamps are included in the weights.

• 20 meters: 8 lb (3.5 kg)
• 15 meters: 5.5 lb (2.5 kg)

The ratio is about what you'd expect for the wavelength ratio. Tube sizes are the same, following the same taper schedule, with shorter lengths of each on 15 meters. They taper from 1" down to ½". I'll have more to say about the element design and construction in a future article.

Putting it all together I can estimate the total weight of each yagi:

• 20 meter side mount yagi: 115 lb (52 kg)
• 20 meter rotatable yagi: 80 lb (36 kg)
• 15 meter side mount yagi: 58 lb (26 kg)
• 15 meter rotatable yagi: 85 lb (38 kg)

The weights include an allowance for gamma matches, boom trusses and boom-to-mast clamps which are not complete. The side mount yagis do not include boom-to-mast clamps since tower brackets are separately lifted and installed.

Lifting options

Antenna weight is the major consideration for choosing a lift method. Both side mount yagis are light enough to be trammed using my existing hardware. The 15 meter yagi is moderately heavy and the 20 meter yagi, although very heavy, is being lifted only halfway up the tower (20 meters high). The rotatable yagis are another matter. At 85 lb the 15 meter yagi is close to the limit I'd want to lift to 140' (43 meters) with my existing tram, and the 20 meter yagi is substantially heavier.

My choices are to make a stronger tram or to lift the antennas in pieces and assemble them on top of the tower. The latter is not as daunting as it sounds. It's a method used by many hams. I have the mechanics of the process thought through since when I eventually build my full size 40 meter yagi it will be lifted in this fashion. That antenna design is not yet complete but I have enough of the components to estimate a weight in excess of 200 lb (90 kg). A crane can do the job but at substantial expense.

Wind load and yagi survival

I have not done detailed engineering calculations to determine wind and ice survivability of these yagis. Instead I interpolated boom and element strength from documented designs to achieve my strength objectives. The only calculation I did was to estimate boom strength using software for simple beams with a load on the extremity.

The lighter side mount yagis have the greatest wind load due to being 3" diameter end-to-end. The booms of the rotatable yagis are lower diameter but with thicker wall to compensate. In general there is a greater strength benefit from increasing tube diameter than increasing wall thickness. However, the strength is not only adequate the lower surface area reduces the wind force and ice load.

Projected cylindrical surface areas for the yagis are as follows:

• 20 meter side mount yagi: 10 ft²
• 20 meter rotatable yagi: 8 ft²
• 15 meter side mount yagi: 8 ft²
• 15 meter rotatable yagi: 6 ft²

The elements are identical for side mount and rotatable yagis. Their wind surface areas are estimated as follows, representing an average since each element has a different ½" tip length.

• 20 meters: 2.0 ft²
• 15 meters: 1.25 ft²

Multiplying by 5 (each antenna has 5 elements) we get 10 ft² and 6.25 ft² for 20 meters and 15 meters, respectively.

Maximum wind load occurs when either the boom or elements are orthogonal to the wind direction, with a smooth and shallow dip between those extremes. Obviously the side mount yagis have to contend with whatever weather occurs while the rotatable yagis can be rotated to lessen the impact. Boom and element strengths must also be considered but I don't have those calculations yet.

I live in a 135 kph (85 mph) wind zone. Over the decades I've lived in this region the strongest wind I can recall peaked at 120 to 130 kph. At 85 mph the wind force on a long cylinder is approximately 20 lb per ft². Therefore the booms must withstand from 120 lb to 200 lb wind force (shortest and thinnest to longest and fattest) plus gusts and turbulence. Each element must similarly withstand 25 lb or 40 lb wind force, which sums to 125 lb or 200 lb per 15 meter and 20 meter yagi, respectively.

The booms carry the force from the elements and the sum appears at the mast and tower. For the side mount yagis these forces are well within the capability of my tower. The only significant concern is the rotatable 15 meter yagi mounted 3 meters above the tower on the mast. For my selected mast (diameter, wall thickness and tensile strength) initial calculations are favourable.

However you can never say never. Although I intend to pin down the survivability calculations that won't delay the project. It is more important that the yagis go up this fall.

Moving forward

With this data in hand I can confidently move forward with my plans to lift the antennas. The rotatable antennas will be turned with a prop pitch motor which is more than adequate to handle these behemoths. Tower plates for the motor and bearings are currently being machined in my workshop.

One difficulty is tuning the antennas. With the driven elements so far from the mast and tower that job could be more difficult than the lift alone. Tuning will require multiple lifts to a low height or a mechanism to rotate the antenna, without tangling guys, to repeatedly access the feed point. This, too, is being planned.

Big yagis look impressive up in the air but getting to that point is a lot of work! I still think it's worth the effort. That puts me in a small minority of hams, even among committed contesters.