As mentioned in the first of these articles, you don't need a high-gain tracking antenna to receive the 137.5MHz or 137.62MHz APT (automatic picture transmission) signals from the polar orbiting weather satellites. A fixed antenna will do the job but you do need one with an approximately hemispherical reception pattern. It also needs to be capable of receiving circularly-polarised signals, because the signals from the weather satellites use this format.

There are three main antenna types that meet these requirements but two of them - the quadrifilar helical (QFH) antenna and the Lindenblad - are not at all easy to build. The antenna we're describing here is the third type which is usually described as a "turnstile/reflector" (T/R) or "crossed dipoles with reflector" antenna. In fact, we decided to go with this type after building a Lindenblad and getting quite disappointing results.

As a bonus, the T/R antenna is much easier to build than the other two types and is also less critical about the type of roof it's mounted above - although it should still be mounted as high as possible, so that it has the largest possible unobstructed view of the sky in your location.

As you can see from the photo, the antenna is fairly simple. The "active" elements consist of two horizontal half-wave dipoles which are crossed (ie, at right angles to each other), with their feed points connected together via an electrical quarter-wave length of 75Ω coaxial cable. This introduces a 90° phase shift at the reception frequency and it's this phase shift that allows the antenna to receive circularly-polarised signals.