Feed and LNA

In the A.R.G.H. project

• Feed optimised for 1.42 GHz, placed at the focus of the dish.
• LNA (low-noise amplifier) with a noise figure below 0.6 dB, mounted close to the feed.

The feed collects the signal concentrated by the dish; the LNA amplifies it immediately, before the cables can degrade its signal-to-noise ratio. Every loss before the LNA worsens the noise of the whole system.

What the SRT teaches us

In the MIT telescope the feed/LNA assembly is the most characteristic part and a good reference model:

• Feed. A low-profile helical antenna (copper tape wound on a foam core) mounted on an aluminium cavity. It is the only component built from scratch; the others need little machining.
• LNA. It sits in a watertight box mounted on the back of the cavity, so it blocks very little of the dish aperture. It contains two ultra-low-noise amplifier modules, a band-pass filter and a bias-tee that supplies power to the amplifiers.
• Checking the feed. A network analyser measures the return loss (S11): the expected value is about −20 dB.
• Protection. A shorted quarter-wavelength stub on the feed provides a DC path that protects the amplifiers from lightning and static electricity.
• Labels. It is wise to label connectors with weatherproof labels, to avoid wrong connections.

Open choices

• Home-built feed following the SRT model, or a commercial one.
• LNA model and gain level.
• How to power the LNA (bias-tee) and compatibility with the RSP2pro input.
• Position and size of the feed mounting holes, depending on the chosen dish.