STL Antennas — Too Much Height?

Usually height is a good thing with STL antennas. But not always.
Publish date:
Social count:
Image placeholder title

Fig. 1:An STL path with water reflection. Did you know that microwave STL antennas can be too high?

A microwave signal can reflect off of water, as shown in the exaggerated drawing in Fig. 1.

If both antennas are within line of sight of water, and the water is in an unfortunate location, two signals can arrive at the receiving antenna — one from the transmitting antenna and one from the reflection off the water. If the reflected signal arrives out of phase with the direct signal, it can cause the received signal to be severely degraded or even unusable.

This problem can be solved by intentionally blocking one of the antennas from the over-water reflecting path. Trees are often a good source for intentionally blocking a path, but terrain and taller buildings can also serve the purpose. In Fig. 2, we simply lowered the height of the transmit antenna to cause the trees to block the path towards the water, which will eliminate the reflected signal.

Image placeholder title

Fig. 2:STL path using terrain to block reflections.
The reflective medium does not have to be water. A swamp, a marsh or even a flat pasture might cause a reflection. Make sure you have enough height to clear the Fresnel zone, but don’t install the antennas higher than necessary without investigating reflections.

Jeffrey Gehman is engineering associate at Kessler and Gehman Associates in Gainsville, Fla.

Send your story ideas or tips to


STL on a Budget promo image

STL on a Budget

It is possible to create a broadcast-quality STL using readily-available tools and an internet connection

Antenna Tests and OEM Practices

How can you get the most out of your FM antenna system? To provide some insight into this subject, three prominent broadcast consulting firms were asked to respond to 15 topics about FM antenna performance.