Antenna Heights
Antenna Heights Overview
You Need to Know
- Antenna heights are always measured from ground level even if a building has been specified at the end of a path
- Antenna heights are measured in feet or meters as specified on the units box on the status. To change the units at any time, click on the units box. This changes the units throughout the program
- Any changes in the antenna heights are not finalized until, the wrench button or the F2 key is clicked.
- To reset the antenna heights to their original values, click Configure - Set antenna heights to bring up the antenna data entry form and then click the OK button.
- A reflective path with 2nd Fresnel zone clearance will experience signal cancellation. 2nd Fresnel zone clearance is equal to 1.414 F1 or 141.4% of the first Fresnel zone radius.
Microwave Radio Links
On point to point microwave radio links, the antenna heights are calculated using a set of clearance criteria consisting of the following components:
- The earth bulge for a specified value of the earth radius factor (K)
- A specified percentage of the first Fresnel zone radius
- An arbitrary fixed height
- A minimum clearance value
On a microwave radio link, the first step in the design is to determine the feasibility of the path based on the antenna heights required for free space loss line of sight. The clearance criteria for this condition in a temperate climate is 60% first Fresnel zone at K = 1.33. A 100% first Fresnel zone is often used to provide some margin.
Microwave availability calculations do not usually take into account the outage probability due to diffraction fading at very low values of K. It has been assumed that the antenna heights are sufficient to provide a usable signal at the lowest expected value of K. Traditionally this has been carried out by imposing two sets of clearance criteria. The first criteria establishes free space line of sight heights - the second criteria deals with low values of K. The highest antenna heights are used, Typical values for this traditional approach are:
- First criteria 100%F1 at K = 4/3
- Second criteria 30%F1 at K = 2/3 for heavy route or highest reliability systems
- 60% F1 + 3 meters at K = 1 for light route or medium reliability systems
This analysis can be carried out as two sets of clearance criteria are provided; however this approach is not recommended as excess clearance will usually result and on some path geometry, second Fresnel zone clearance can occur. On short high frequency paths, low values of K have a negligible effect, On long paths, the following procedure is recommended:
Determine the antenna heights required for 100% first Fresnel zone clearance at the median value of K (4/3 in temperate climates). Calculate the diffraction loss that will occur at the minimum expected value of K. If this loss is greater than 50% of the available fade margin, then increase the antenna heights. This will virtually guarantee negligible diffraction fading on the path.
On reflective paths, the antenna heights may require adjustments to avoid signal nulls. If space diversity is used to deal with a specular reflection, both the main and diversity antenna heights will be adjusted to avoid simultaneous nulls on the main and diversity antennas.
VHF-UHF Radio Links
Clearance criteria are not used to determine antenna heights at the lower frequencies. The large Fresnel zone radius would result in prohibitively high towers. As a result, these links generally operate under less than free space line of sight conditions.
In these cases, the link viability is determined by the diffraction loss. This can be carried out in the Antenna Heights or Diffraction modules.
Antenna Configurations
Antenna height calculations are based on the selected antenna configuration. Setting the antenna configuration is the first step in a design. If space diversity is an option, then start with the space diversity configuration and calculate the main and diversity heights. You can switch to the non diversity configuration later, if required.