RG142 is a high-performance coaxial cable that works very well at 1420 MHz (the neutral hydrogen line) for radio astronomy applications.

Here’s the key info:

Parameter	RG142
Type	Coaxial, silver-plated copper conductors, PTFE dielectric
Impedance	50 Ω
Velocity factor	~0.69
Attenuation at 1 GHz	≈ 12.2 dB/100 m
Attenuation at 1.42 GHz	≈ 14–15 dB/100 m
Power rating (at 1.4 GHz)	> 200 W (continuous)
Shielding	Double braid (excellent shielding effectiveness, typically > 90 dB)
Temperature range	−55 °C to +200 °C

Practical notes for 1420 MHz use in radio astronomy:

• Keep runs short — At 14–15 dB loss per 100 m, you want your cable to be as short as possible between the feed and LNA.
• LNA placement — Put a low-noise amplifier right at the antenna feedpoint to overcome cable loss.
• Durability — PTFE dielectric means it’s stable over temperature and resistant to UV and weather, but the silver plating can tarnish, so seal connectors well.
• Connector choice — Use precision N-type or SMA connectors rated well above 1.5 GHz to avoid mismatch losses.

Below are losses for RG142 at 1420 MHz, using the typical attenuation range 14.0–15.0 dB/100 m (I also show the midpoint 14.5 dB/100 m as a handy single-number estimate).

Loss table (dB and linear power fraction)

Using 14.5 dB / 100 m (0.145 dB/m) as the nominal value:

Length (m)	Loss (dB)	Power received fraction
1 m	0.145 dB	0.967 (96.7%)
2 m	0.290 dB	0.935 (93.5%)
3 m	0.435 dB	0.905 (90.5%)
4 m	0.580 dB	0.875 (87.5%)
5 m	0.725 dB	0.846 (84.6%)
10 m	1.450 dB	0.716 (71.6%)

Range across the stated spec (14.0 → 15.0 dB/100 m):

• 1 m: 0.14 → 0.15 dB (power ≈ 96.6–96.8%)
• 5 m: 0.70 → 0.75 dB (power ≈ 84.1–85.1%)
• 10 m: 1.40 → 1.50 dB (power ≈ 70.8–72.4%)

Quick practical takeaways

• For runs ≤ ~5 m, cable loss is very small (<~0.75 dB). You’ll lose <16% of received power — fine for most setups.
• At 10 m, loss approaches ~1.45 dB (≈28% power lost). Still modest, but not negligible for very weak HI signals.
• Best practice: if you can put a low-noise amplifier (LNA) at the feedpoint, you recover the most SNR. For these short runs it’s not strictly necessary, but it’s still recommended for radio astronomy.
• Use good N-type connectors, weatherproof them, and keep connectors few and tight to avoid extra mismatch/ohmic loss.
• If you want I can compute the expected noise figure / system temperature impact for a given LNA NF and receiver chain using these losses.