My requirements for the base include:
- Do not interfere with the septic system tile bed under the tower.
- Support the dead weight of the tower and antennas (and ice), plus the vertical force due to guy tension from preload and wind load.
- Secure the base from lateral forces due to wind and from accidental or deliberate interference.
- Easy to remove and restore the lawn when the tower is dismantled.
- Survivability of at least 5 years.
Each piece of lumber is cut into 3 sections: 3' and 2 x 2.5'. The longer sections form the bottom of the base, and the 4 shorter ones are secured across those. The depth of the base is about 6", leaving approximately 1" above grade (2 x 3.5" = 7"). There is just enough space between the upper sections to allow water to drain, so that is does not pool for long between the base plate and base.
The tricky part of the installation is getting everything level while not leaving any air gaps beneath any part of the base. The picture shows the base centred, levelled and ready for final hammering and packing of soil. The unrestored tower base plate is included in the picture for perspective.
The base gets its strength by coupling to the soil, both downward and laterally. This is fine for wire antennas. If you plan to install a yagi or similar rotatable antenna it is important to add diagonal strength to the base so that it better resists twisting. Unlike a concrete base there is not enough mass or overburden to resist much uplift force so I'll be out of luck if a tornado strikes. These are happily rare in the Ottawa valley. Frost heave during spring thaw is modest in my experience here (~1 cm), which I judge to be negligible in this application.
Once installed and the sod replaced it makes for a tidy installation. Even the local waterfowl seem unperturbed by its presence. The trees that will serve as guy anchors bracket the picture. Notice that the base and lawn are not on the same plane. The base is level, the lawn is not. Never assume that the ground is level. Measure!
The result is quite good considering that I intended to not adjust for minor asymmetries. About 1° of the 3° error is because I hadn't taken into account that the southwest tree bends a bit towards the south and the anchor point is above head height. At a distance of ~9 meters from the base a 1° lateral offset is equal to ~16 cm (6").
With all the ground prepared I now have some finishing work to do on the tower, after which it will be ready to be raised.