Editing Category:NOAA APT (section)

== Antennas ==
The NOAA satellites that downlink APT signals transmit with a right-hand circularly polarized (RHCP) quadrifilar helical antenna (QFH). Using an RHCP QFH to receive these signals is a good idea, as the polarization will be the same and the antenna has a very wide beamwidth. Other popular choices that are circularly polarized are the turnstile and double cross dipole antenna.

Another option is a v-dipole. Though it is linearly polarized, the polarization mismatch loss is typically insignificant and easily ignored. This antenna uses two legs roughly 54cm long spread 120° apart at the feed point. This forms a 'V' shape, with the feed point (where the coaxial cable connects) at the bottom corner where both legs meet. This v-dipole is more directional than the previously mentioned antennas so it must be positioned properly. Point the antenna so that the legs are parallel to the ground, and a line bisecting the angle between the legs points North or South, it doesn't matter which. The height above the ground matters for this antenna. Heights that are multiples of half the wavelength should be avoided, so ~1m, ~2m, ~3m, etc due to nulls directly overhead. The best height is somewhere between 0.4m and 0.8m, anything higher will introduce nulls and dips in the radiation pattern. If the antenna needs to be placed higher, say above a tree line or buildings, then a reflector may be placed 0.4-0.8m below it for the same benefit. A reflector can be as simple as another v-dipole with slightly longer legs.

Permanent ground stations may wish to opt for the QFH or turnstile so that lower-elevation passes can be received. For single-pass captures or for mobile ground stations, v-dipoles are very good options as they are portable and offer more gain on high elevation passes.
[[Category:Satellites]][[Category:Modems]]