Short answer up front: a 2.4 GHz LHCP helical dish feed is a perfectly sensible and widely-used solution for uplinking to Esʼhail-2 / QO-100 — but don’t confuse that with the hydrogen-line (1420 MHz) — the HI line is a different frequency and needs a different feed. Below I explain why, what to watch for, and rough numbers you can expect.

Key facts you must know

• QO-100 (Esʼhail-2) uses 2.4 GHz for the amateur uplink (the NB/WB transponders). Uplink polarisation is specified as RHCP (so the feed arrangement at the dish must produce the correct sense at the satellite).
• Many dish-feed designs for QO-100 use a helical feed at 2.4 GHz (axial-mode helix). DIY guides and published builds exist and work well when installed correctly.
• Hydrogen line = 1,420.4058 MHz (L-band). That is a completely different frequency band from 2.4 GHz (S-band). A helix tuned for 2.4 GHz will not perform acceptably at 1.42 GHz. If you want to do HI work you need a dedicated 1.42 GHz feed (or a proper dual-band feed). (I mention this because your phrase included “hydrogen line”.)

Polarisation / helix handedness — the tricky bit

• The satellite expects RHCP on the uplink. However a prime-focus dish inverts the circular sense once the feed is reflected from the paraboloid, so builders commonly use a LHCP helix as the feed to produce the correct RHCP wave toward the satellite. AMSAT documentation and several build guides explicitly state this. Make sure your helix is built and mounted to produce the required handedness in the far field.

Dish size and expected gain (practical numbers)

Using a typical dish efficiency ~55% you can expect approximate on-axis gains at 2.4 GHz:

• 0.6 m dish → ≈ 21 dBi
• 0.9 m dish → ≈ 24.5 dBi
• 1.2 m dish → ≈ 27.0 dBi (a common grassroots uplink size)
• 1.8 m dish → ≈ 30.5 dBi
• 2.4 m dish → ≈ 33.0 dBi

(These are computed from standard dish gain formula G = (πD/λ)²·η and are meant for order-of-magnitude planning.) With these gains people typically aim for moderate uplink EIRP (and many recommend aiming so your downlink is below the beacon and watch for LEILA warnings).

Practical performance notes / gotchas

• Helix design: axial-mode helix parameters (diameter ≈0.3–0.4λ, spacing ≈0.2–0.25λ, 2–4 turns depending on bandwidth/axial ratio) are commonly used for 2.4 GHz QO-100 feeds. There are many simple successful DIY designs.
• Feed placement/phase center: axial helix must be positioned properly in front of the dish (focus) for good illumination and to avoid spillover. Mechanical mounting and exact focus position matter.
• Losses / efficiency: cheap Wi-Fi antennas or poorly matched feeds can cost you ~3 dB or more. Use low-loss coax and a good transverter / linear amplifier if required.
• Power & LEILA: QO-100 has a power-monitoring system (LEILA). Keep your transmitted downlink (what the satellite sends back) below the beacon — reduce uplink power if you see LEILA. Full-duplex monitoring (hear your own downlink while transmitting) is recommended/mandatory.
• Regulatory / station safety: 2.4 GHz is an ISM band in many countries — but QO-100 uplink operation must also follow local amateur regulations and Esʼhail-2 operating guidelines.

If your goal is both hydrogen-line (1.42 GHz) and QO-100 (2.4 GHz)

• A single 2.4 GHz helix will not serve for 1.42 GHz HI observations. Options:
  • Install a separate 1.42 GHz feed (e.g., small probe, patch or dual-linear feed) for HI work.
  • Design/create a dual-band feed (possible but more complex) that covers both 1.42 GHz and 2.4 GHz with acceptable performance.
  • Use two feeds and a flip-mount / feed switch so you use the appropriate feed for each purpose.

Quick practical checklist if you plan to build/use a 2.4 GHz LHCP helix dish feed for QO-100 uplink

1. Build helix to axial-mode dimensions for 2.4 GHz (turns, diameter, spacing).
2. Make it left-hand helix so dish + feed produce RHCP toward the satellite. Verify the handedness.
3. Mount at correct dish focus and measure SWR/matching with a VNA or good analyser.
4. Use low-loss coax or waveguide to your transverter; minimise connector and cable loss.
5. Use a transverter/PA that’s clean (low IMD) and monitor downlink (full-duplex) to avoid LEILA.

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