Dish and mount

In the A.R.G.H. project

• Prime-focus parabolic antenna, 2.4 m diameter.
• Operating band 1–2 GHz, optimised for the HI line at 1.42 GHz; estimated gain of about 30 dBi.
• Material: cast aluminium, with mesh or solid sheet.
• Equatorial mount on a pole with a base.

180 cm or 240 cm?

Both sizes were evaluated during the design phase.

	180 cm	240 cm
Beam at 1.42 GHz	about 8°	5–6°
Estimated gain	26–28 dBi	30–31 dBi
Pointing	Very tolerant, simple tracking	Still tolerant, no high-precision tracking needed
Radio maps	Very coarse	Better defined, better signal-to-noise ratio

For reference, the full Moon covers about 0.5° of the sky. Both work for the HI line, the distribution of galactic hydrogen and Doppler measurements. The 240 cm dish is the preferred configuration, subject to the space available at the site.

A wide beam is an advantage: it tolerates small mechanical errors and allows relatively simple polar mounts.

What the SRT teaches us

• The focus. The feed must be placed so that the focus falls on its axis, half way along its length. To find it, measure the diameter D and the depth d of the dish (distance between a string stretched across the rim and the centre); the focal length is f = D² / (16 · d).
• Weight. For weight reasons, mesh dishes are the best.
• Custom bracket. Attaching the dish to the mount axis almost always needs a purpose-built bracket: it should be designed with a technician or an engineer.
• Concrete base. The SRT prototype uses a pier with a hole of about 76 cm in diameter and 1.2 m deep, below the frost line, with sides tapering towards the bottom, gravel for drainage, and the pipe kept plumb (checked at two points 90° apart) until the concrete sets. These are example figures for a 2.3 m dish: the sizing must be decided with a technician, following local regulations and the soil conditions.
• Alternative without digging. A non-penetrating base with ballast is useful on flat ground or a roof, but on a roof the load capacity must be checked.