Yes—central obstruction from your feed does matter for a hydrogen line (1420 MHz) dish, but how much it matters depends on your goals and geometry.
What central obstruction does
When your feed (and its supports) sits in front of the dish, it blocks part of the aperture:
- Reduces effective collecting area → slightly lowers sensitivity
- Introduces diffraction → raises sidelobes and can distort the beam
- Adds spillover and scattering → increases system temperature (noise)
How significant is it?
For most amateur hydrogen line setups:
- If obstruction is <10% of dish diameter → impact is usually small
- Around 10–20% → noticeable but often acceptable
- >20% → begins to significantly degrade performance
The loss in gain is roughly proportional to the blocked area, not diameter.
So a feed that “looks big” might still be okay if its area is modest.
Why it matters more for hydrogen line work
Hydrogen line detection is signal-to-noise limited, not resolution limited.
So:
- A bit of beam distortion is tolerable
- But extra noise from spillover and blockage hurts detection sensitivity
Practical guidance for your setup
Since you’re working on feeds and antennas:
- Keep the feed physically as small as practical
- Use thin supports (or low-scattering materials)
- Ensure good illumination taper (don’t over-illuminate the dish edges)
- Minimize spillover onto the ground
When it becomes critical
Central obstruction becomes more important if you are:
- Doing interferometry (phase errors, sidelobes matter more)
- Trying to detect very weak galactic hydrogen structure
- Using a small dish (obstruction fraction becomes larger)
A useful alternative
If you want to push performance further:
- Offset dishes (no central blockage) are ideal
- Or creative feeds (like your cable-based antenna idea) can reduce obstruction significantly
Bottom line
Central obstruction does matter, but for most amateur hydrogen line systems it’s a second-order effect—important to manage, but not something that will make or break your observations unless it’s large.