RF chain and cables
In the A.R.G.H. project
| Element | Specification |
|---|---|
| LNA | NF < 0.6 dB, close to the feed |
| Band-pass filter | Centred at 1420 MHz, 20–30 MHz bandwidth, to reduce interference (RFI) |
| Cables | Low-loss coaxial (LMR-400 or equivalent) from the dish to the SDR |
| Connectors | Professional-grade SMA |
The filter narrows the bandwidth around the hydrogen line and protects the receiver from strong out-of-band signals; a low-loss cable carries the signal indoors without degrading it.
What the SRT teaches us
- Short flexible run + long run. On the SRT a short run of thin cable (LMR-240, about 3 m) runs along the dish structure to the mount, and a long run of LMR-400 (about 15 m) goes indoors.
- Fixing. The cable is secured to the dish every 15–20 cm with UV-resistant ties.
- Rotation. A loose section is left at the mount to allow the dish to rotate fully.
- Outdoor joint. The connection between the two cables must be protected from the weather (the SRT uses an improvised container with a taped hole: for A.R.G.H. a proper watertight solution is preferable).
- Testing the chain. The SRT checks the gain by injecting a test signal at 1420 MHz (about −80 dBm) at the LNA input and reading the output: the SRT receiver + LNA system reaches about 71 dB. It is a useful order of magnitude when planning the gain ahead of the SDR.
Outdoor cables and connectors are among the first parts to deteriorate: periodic checks should be part of routine maintenance.