HGA (High Gain Antenna)

HGA (High Gain Antenna)
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The HGA was stowed during launch, folded behind the Service Module alongside the SPS engine bell. After separation from the S-IVB, it is deployed to the side of the SM. It consists of four 79-cm parabolic dishes clustered around a 28-cm square feedhorn. The dish assembly is mounted on an articulated joint at the end of the support arm and can be pointed at Earth under manual or automatic control. The antenna works in the 2 gigahertz range (within what was known at the time as the S-band) and has three modes of operation: wide, for near-Earth operation; medium, for distances up to halfway to the Moon; narrow, for up to lunar distances.

The choice of beamwidth is a compromise between signal to noise ratio, antenna pointing accuracy and distance. Additionally, there are occasions when the narrow mode of the HGA locks onto a side-lobe of its radiation pattern, usually when reflections from the spacecraft's skin interfere with reception. When this occurs, a fix is to switch to wide beam, let the antenna repoint, then return to narrow.

A switch on the right-hand side of the Main Display Console selects which signal will use the auxiliary channel of the S-band radio system. If not set to 'off,' it can carry science data from the SIM bay, or TV from the CM's colour camera. (ap15fj)

    This article is based on material extracted, with permission, from the [Apollo Flight Journal] by David Woods and Frank O'Brien. The NASA Apollo Flight Journal files are copyright © 1998 - 2004. W. David Woods and Frank O'Brien.(more...)
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