A surprisingly effective improvement for hydrogen-line reception is optimising the feed illumination of the dish. Many amateur systems lose several dB simply because the feed does not illuminate the reflector correctly.
This matters when observing the Hydrogen line at 1420.405 MHz because the signal is extremely weak and every dB of antenna efficiency helps.
The key idea: proper edge illumination
The feed should illuminate the dish so that the edge of the reflector receives about –10 dB to –12 dB relative to the centre.
If the feed pattern is wrong:
| Problem | Result |
|---|---|
| Feed too wide | Energy spills past dish → lost signal |
| Feed too narrow | Dish edges under-illuminated → reduced effective aperture |
Both cases can reduce effective gain by 3–6 dB, which is huge in radio astronomy.
Simple improvements that help
1. Add a choke or scalar ring
Many hydrogen-line observers add rings around the feed horn to control the radiation pattern.
These are common in satellite feeds like:
- Bullseye LNB
- Octagon PLL LNB
The rings suppress edge spillover and improve dish illumination.
2. Adjust feed position
Moving the feed a few centimetres in or out from the focal point can change illumination dramatically.
Typical method:
- Observe the hydrogen line while integrating.
- Move the feed slightly along the focal axis.
- Stop when the spectral peak is strongest.
3. Reduce ground noise pickup
If the feed pattern is wide, it “sees” warm ground outside the dish.
Ground ≈ 300 K noise source
Sky ≈ 5–20 K
Reducing spillover lowers the system temperature, improving SNR.
Example improvement
Typical amateur dish results:
| Setup | Effective gain |
|---|---|
| Poor illumination | baseline |
| Optimised feed | +3 to +6 dB |
That is equivalent to increasing dish diameter by roughly 40–70 %.