How about 6 x 7.5 inch can for hydrogen line? How does that compare with a can 160mm diameter x 165 mm height?

December 12, 2025 Admin

How about 6 x 7.5 inch can for hydrogen line?

Short answer: you can use a 6″ × 7.5″ can-sized aperture, but it’s very small for the 1420.40575 MHz hydrogen line — expect only a few dBi of gain and much lower sensitivity than a typical amateur 1 m+ dish. If you’re actually switching to the antenna you described (two cables joined in the middle, suspended between poles, feed dropping to the radio/computer), that’s basically a centre-fed half-wave dipole — which is a sensible, simple option. Below are the concrete numbers and practical suggestions.

Numbers (1420.40575 MHz)

wavelength λ ≈ 0.211 m ≈ 8.31 in.
A 6 in × 7.5 in aperture = 0.1524 m × 0.1905 m → area ≈ 0.0290 m².
Equivalent circular diameter ≈ 0.192 m (≈ 7.6 in).
With a reasonable aperture efficiency (~55%) that aperture would give a gain ≈ 4.5 (≈ 6.5 dBi).
Half-power beamwidth (approx.) ≈ 64° — very wide, poor angular resolution.
Collected area is only ≈ 3.7% of a 1-m diameter dish, so raw sensitivity is much lower.

If you build the two-cable centre-fed antenna (dipole)

This matches your “two cables joined in middle, held aloft on poles, single cable drops to floor and goes to computer” description:

Make it a half-wave dipole: total length ≈ λ = 8.31 in (211 mm).
- Each arm ≈ 4.155 in (105.5 mm) from feedpoint to tip.
Feedpoint: a plain dipole in free space has about 73 Ω feed impedance.
- Use a 1:1 balun (common-mode choke or ferrite sleeve) at the center to stop the feedline radiating and to present the dipole properly to your 50 Ω receiver.
- You can connect the dipole (via the balun) directly to 50 Ω — mismatch is small and usually acceptable for a simple setup; if needed add a small matching network.
Mounting: suspend horizontally (for linear horizontal polarization) or vertically for vertical pol. Keep it clear of nearby metal/structures for best performance.
Height: several λ above ground is ideal, but even a metre or two will work for a stable beam. Height changes the ground-gain and pattern.
Filtering & LNA: because the aperture is small, use a low-noise amplifier (LNA) at the feed and a narrowband filter centered on 1420.40575 MHz to improve SNR.

Practical tradeoffs & suggestions

The dipole is easy, cheap, and broadband — good for experiments and monitoring HI if you accept lower sensitivity.
If you need more sensitivity for faint galactic HI or narrow spectral work, consider:
- A larger aperture (bigger reflector or larger rectangular aperture) — even ~0.5–1 m diameter gives a huge improvement in collected power.
- Or a directional Yagi (several elements) designed for 1.42 GHz — a compact Yagi can get you higher gain than the tiny can/dipole while remaining portable.
Always use a band-pass filter and LNA near the antenna for hydrogen-line work when using small apertures — that helps recover weak signals.
If you keep the can physically (as a short cylindrical backing) you could still mount the dipole in front of it as a simple reflector/ground plane, but the tiny can won’t add much unless carefully designed as a tuned cavity/horn.

Is the above better or a can that is 160mm diameter x 165 mm height?

Short answer: the 6″ × 7.5″ (152.4 × 190.5 mm) aperture is the better performer — it collects ~44% more power and gives ~1.6 dB more gain and a noticeably narrower beam than the 160 mm-diameter can (160 × 165 mm).

Numbers (carefully worked out)

wavelength at 1420.40575 MHz: λ ≈ 0.211 m (8.31 in) (same as before).
6.0″ × 7.5″ aperture: physical area = 0.0290322 m².
160 mm diameter can: physical area = 0.0201062 m².
Assuming an aperture efficiency η = 0.55 (typical for small, imperfect apertures):
- effective area (6×7.5): A_eff = 0.0159677 m².
- effective area (160 mm dia): A_eff = 0.0110584 m².
Predicted linear gain (4πA_eff/λ²):
- 6×7.5: 4.507 → ≈ 6.54 dBi.
- 160 mm dia: 3.121 → ≈ 4.94 dBi.
Gain ratio: 4.507 / 3.121 ≈ 1.444 → the rectangular aperture collects about 44% more power.
dB difference: ≈ 1.59 dB (6×7.5 better).
Approximate half-power beamwidth (rule of thumb HPBW ≈ 70·λ/D):
- 6×7.5 (equivalent circular diameter ≈ 0.1923 m): ≈ 77°.
- 160 mm dia: ≈ 92°.
  So the 6×7.5 aperture is significantly more directional (narrower).

Practical points & caveats

Aperture area dominates for simple open apertures: the 6×7.5 rectangle has substantially larger area so it’s the better choice for raw sensitivity and gain.
The height of the can (165 mm) only matters if you plan to use the can as a tuned cavity, short horn, or ground-plane — otherwise it has little direct effect on aperture-gain which is governed by the opening area.
A circular can can be convenient mechanically (fits coax/waveguide easily), and a deeper can might be formed into a small horn or used as a partial cavity to improve pattern or reduce backlobes — but designing a useful horn/cavity at 1.42 GHz requires care; a 160 mm dia × 165 mm height is quite small and won’t magically beat a larger open aperture.
If you’re switching to the two-cable centre-fed dipole you mentioned previously, remember the dipole’s effective area is much smaller than either of these cans — the cans (acting as simple apertures or backing reflectors) will generally outperform a lone short dipole unless you add an LNA and very good filtering.

Recommendation

If your goal is maximum sensitivity from small, easily fabricated hardware: use the 6″ × 7.5″ aperture (the rectangular can/open aperture).
If you need a physically neat, round enclosure for mechanical reasons or to try making a small horn/cavity, the 160 mm dia can is fine — but expect ~1.6 dB less gain than the 6×7.5 option.
In either case, add a bandpass filter and a low-noise amplifier at the feed to recover as much SNR as you can from these small apertures.

The diameter of the 6 inch diameter can might be the outside diameter so the actual internal diameter may only be 145mm. Does that change the analysis?

Yes — if the actual internal diameter is 145 mm that does change the numbers, and it makes the small can noticeably worse than the 160 mm can. I recalculated with λ ≈ 0.211 m (1420.40575 MHz) and a representative aperture efficiency η = 0.55.

Quick numbers (internal aperture area → gain)

6.0″ × 7.5″ rectangular opening (152.4 × 190.5 mm)
- Area = 0.02903 m² → A_eff = 0.01597 m² → Gain ≈ 4.51 (6.54 dBi).
160 mm diameter can (internal diameter = 160 mm)
- Area = 0.02011 m² → A_eff = 0.01106 m² → Gain ≈ 3.12 (4.94 dBi).
145 mm internal diameter can (your updated number)
- Area = 0.01651 m² → A_eff = 0.00908 m² → Gain ≈ 2.56 (4.09 dBi).

What that means

The 145 mm internal can collects ~21.8% less power than the 160 mm can (and is ~75.8% less than the 6×7.5 opening). In dB: the 145 mm can is about 0.85 dB down on the 160 mm can and ~2.45 dB down on the 6×7.5 aperture.
So: if the inside diameter is 145 mm, the 160 mm can is the better circular choice. The rectangular 6″×7.5″ opening still wins by a clear margin for raw sensitivity and gain.

Practical reminder

These numbers assume η ≈ 0.55 — if your real aperture is better or worse (due to edges, depth, feed losses, or using it as a short horn/cavity), the relative ranking can shift slightly, but area dominates at this scale.
If you plan to use the can as a small horn/cavity, depth and precise shaping can improve pattern and front-to-back ratio; that may partially compensate for smaller area but requires careful design.

The can will be used as a waveguide/feed for my 160mm dish. Does this change things?

Short answer: yes — using the can as a short circular waveguide / horn feed for your 160 mm dish is a good approach and will usually give better illumination control and pattern (less spillover, better forward gain) than simply sticking a bare dipole in front of the dish. Below I give the concrete EM checks and practical, buildable starting dimensions so you can try it and tune quickly.

1) Is the can a usable waveguide at 1420.40575 MHz?

Yes. the dominant circular-waveguide mode (TE₁₁) cutoff frequency for a circular bore of the sizes you mentioned is well below 1420 MHz, so the can will propagate TE₁₁ and can be used as a feed/waveguide section.

Calculated numbers (rounded):

For 160 mm internal diameter:
- TE₁₁ cutoff ≈ 1,098 MHz → well below 1420 MHz.
- Guide wavelength λg ≈ 0.333 m (333 mm).
For 145 mm internal diameter:
- TE₁₁ cutoff ≈ 1,212 MHz → still below 1420 MHz.
- Guide wavelength λg ≈ 0.404 m (404 mm).

So both diameters support propagation at 1420 MHz; the 160 mm bore is further above cutoff (a bit “less dispersive”), which is generally helpful for simpler matching and slightly smaller guide wavelength.

2) Practical feed design — simple, reliable starter recipe

These are practical starting dimensions you can build quickly and then tune with an SWR meter / VNA or by measuring SNR on the receiver.

Waveguide-to-coax probe + backshort (easy, commonly used):
- Make a circular backshort (metal end plate) at the back of the can.
- Backshort distance (from probe to short) — start with ≈ λg / 4:
  - 160 mm can → ~83 mm from probe to backshort.
  - 145 mm can → ~101 mm from probe to backshort.
- Feed probe: bring the centre conductor of a short length of coax through the side wall so the probe tip extends into the bore pointing toward the axis (a “radial/side” probe couples nicely to TE₁₁).
  - Start with probe axial insertion of 20–30 mm inside the bore (this is a starting point — you will shorten/lengthen slightly while tuning).
- Use the coax shield bonded to the can (ensure a solid RF seal).
- Put a small dielectric (PTFE or foam) support for the coax if needed.
Open-ended can (short horn) — simpler but a little less matchable):
- Use the can as a short cylindrical horn (open at the front, backshort at some distance). The open end illuminates the dish.
- Try a backshort a little deeper than the λg/4 starting point and experiment ±10–20 mm to optimise return loss and dish illumination.
Choke / rim treatment:
- To reduce edge currents and control spillover, consider adding a simple choke ring (one or two circumferential grooves or a flange at the can rim) or a small lip (10–20 mm) around the open end. This improves pattern and front-to-back ratio.
Illumination & taper:
- Aim for an edge taper around −10 to −12 dB at the dish rim for a good balance of aperture efficiency vs spillover. That comes from the geometry of the open can vs dish focal length — you’ll tune the probe/backshort to get the desired illumination pattern. For a very small dish (160 mm) over-illumination is easy — so reduce the feed power radiated at wide angles.
Match to 50 Ω:
- Expect to need some tuning. The probe + backshort is a tried method — adjust probe length and backshort position to get the best S11. If you have an SWR meter or VNA you can tune to S11 < −10 dB across a narrow band around 1420 MHz.
LNA & filtering:
- Place a low-noise amplifier (LNA) as close to the feed as practicable (inside/at the back of the can) and a narrow bandpass filter centered at 1420.40575 MHz ahead of the receiver. This significantly improves system sensitivity, especially with a small dish.

3) Which bore (145 mm vs 160 mm) is preferable as the feed?

160 mm internal bore is the easier, more forgiving choice: it is further above cutoff, has a shorter λg (so a shorter backshort), and typically yields a better match and slightly broader usable bandwidth for a simple probe/backshort feed.
145 mm bore will work, but it’s closer to cutoff, giving a longer λg and a longer backshort distance (≈ 101 mm) — matching may be slightly more sensitive to probe position and physical tolerances.

In short: if you can use the 160 mm internal bore, do so. It’s more convenient and slightly more robust.

4) Expected performance / why this is better than a bare dipole

A short waveguide or short-horn feed controls the radiation pattern so the dish is more uniformly illuminated and spillover is reduced → higher aperture efficiency and better system temperature for dish-based reception.
The can-waveguide feed will therefore likely increase system gain and SNR versus a bare dipole placed in front of the dish, provided you match it reasonably and use an LNA/filters.

5) Quick checklist for building and tuning

Use the 160 mm internal can if available. If only the 145 mm version is at hand, you can work with it — expect more tuning.
Start with backshort ≈ λg/4 (83 mm for 160 mm can; 101 mm for 145 mm can).
Insert a coax probe through the sidewall, tip projecting ≈ 20–30 mm into the bore; shield bonded to can.
Add an LNA right at the feed and a 1420 MHz bandpass filter after the LNA.
Tune probe length / backshort position while monitoring S11 or receiver SNR; add a small rim flange/choke if spillover is high.
If you have a VNA, aim for S11 < −12 dB across a few MHz centered on 1420.40575 MHz.

If you want, I can do one of the following next (pick one) and I’ll give the full step-by-step drawing and parts list ready for you to build:

A labelled construction drawing (dimensions) for a 160 mm internal can feed with probe & backshort positions.
The same drawing tuned for a 145 mm internal can.
A short comparison showing expected aperture efficiency / edge taper if you mount that feed at the focal point of your 160 mm dish (I’ll need the dish focal length or F/D ratio — if you don’t have it I’ll assume F/D = 0.4 and show both cases).

The edge of the base where it joins sides of 160cm can are slightly curved. Does that matter?

Ah, yes — a small curvature at the base where the sides meet the bottom does matter slightly, but in practice it’s not a showstopper. Here’s a careful breakdown:

1) What the curvature actually does

A perfect circular waveguide assumes a sharp 90° junction at the base (or a smooth backshort wall) so the TE₁₁ mode reflects predictably.
If the edge is slightly curved or rounded, a few things happen:
1. The effective internal diameter at that point is slightly smaller.
2. The field distribution near the backshort is slightly perturbed.
3. For a short waveguide (~a few cm long for your 1420 MHz feed), this can slightly shift the backshort position needed for best matching.

2) Magnitude of effect

For small curvature (a few mm radius on a 160 mm bore), the effect on impedance and mode is minor.
The TE₁₁ cutoff and guide wavelength barely change.
You may need to move the probe back a few mm or adjust the backshort slightly to get the best S11.
This is exactly why short-cylindrical waveguide feeds are usually tuned experimentally rather than relying purely on theoretical formulas — the final backshort/probe combination is found by trial for minimum reflection.

3) Practical takeaway

The curvature will not prevent the feed from working.
Expect to adjust the probe length and/or backshort position by a few mm compared with the “ideal” flat-bottom formulas.
If the curvature is extreme (say, a 10–15 mm radius), it may reduce the effective aperture slightly or create small higher-order mode effects, but for your described cans (slight curve), this is negligible.

✅ How to compensate

Build the probe and backshort with a small adjustable range: e.g., make the probe mount adjustable by ±5–10 mm and backshort movable or with shims.
Measure S11 or receiver signal while adjusting.
Fine-tune until you get best reflection (< −10 dB) and best dish illumination.

Bottom line: Slight curvature at the base is normal for commercial cans; just plan a bit of tuning. Your 160 mm can is still the better choice for a dish feed over the 145 mm can.

astronomy.me.uk The Website of Lichfield Radio Astronomy Observatory (LRO)