HI feedhorn prototyping – early work in progress, looking for guidance
Tiago Baroni
Dear all,
I would like to briefly share some very early, work-in-progress experimentation and ask for guidance from those with more experience. I am at the beginning of my HI radio astronomy learning curve and am currently focusing more on understanding the instrumentation than on obtaining results.
Over a short vacation period, I started prototyping a simple cylindrical feedhorn for 1420 MHz, intended eventually to be used with a parabolic dish. To keep the initial build manageable, I deliberately omitted any flare/choke at the aperture, accepting the known trade-off (higher spillover, lower gain) in exchange for simpler geometry and fabrication. The goal at this stage was only to validate basic dimensions, probe placement, and the initial RF chain before committing to a full metal build.
Before moving to aluminum, I built a PLA prototype, internally lined with aluminum tape to ensure electrical continuity, and implemented an adjustable brass probe to allow small changes in exposed length. The current RF chain is very simple (feed → LNA → external bias tee → 1420 MHz band-pass filter → RTL-SDR), and all testing so far has been done in a challenging urban environment (ground floor of a high-rise building), where RFI and nearby structures clearly dominate.
I am not claiming any HI detection at this point; this is purely a learning exercise to identify practical limitations and plan the next steps, which likely include testing the feed with a small offset dish on a portable mount to reduce spillover and environmental pickup.
I would really appreciate comments on a few specific questions:
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Are there common geometric or electromagnetic pitfalls in cylindrical feedhorn prototypes that are worth double-checking before moving to an all-metal version?
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When transitioning from a lined plastic mockup to an aluminum horn, are there typical “gotchas” (surface finish, seams, probe repeatability) that tend to cause surprises?
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For those who have worked in urban or semi-urban locations, how effective have small offset dishes been in reducing rear/side pickup, and are there positioning strategies that helped in constrained spaces?
Many thanks in advance for any feedback. I am very open to criticism and corrections, and I greatly value the collective experience of this group.
Best regards,
Tiago
Stephen Arbogast
fasleitung3
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Tiago Baroni
Dear Stephen and Wolfgang,
Thank you both for your helpful comments.
Stephen, regarding the proximity of the RTL-SDR, I have ordered a 1-meter extension cable so that I can physically separate the SDR from the feed and test whether any locally generated RFI is coupling back into the system. Once the cable arrives, I will repeat the measurements and compare the spectra.
Wolfgang, I checked the electrical continuity between the interior foil lining and the ground of the SO-239 connector, and you were absolutely right: the continuity was not as good as it should be. This may very well be affecting the behavior of the probe and the waveguide section.
For this PLA prototype, I will redesign and reprint the horn to ensure a reliable mechanical and electrical connection between the connector ground and the internal conductive surface, rather than relying on incidental contact. After rebuilding it, I will run a new set of tests.
I appreciate both of you taking the time to look carefully at the details!
Best regards,
Tiago Baroni
Jimmy Fransson
This is not critique, I am curious as I am building a similar waveguide myself and there's a lot I don't know. :)
tba...@gmail.com
Hi Jimmy,
Thank you for your reflections, I appreciate the curiosity.
Yes, the internal aluminum foil is visibly wrinkled. I agree that this is not ideal from an electromagnetic perspective. At 1420 MHz, surface currents are confined to a very thin layer, and mechanical irregularities may introduce additional losses or small impedance perturbations. For this initial prototype, however, my goal is not to characterize performance precisely, but simply to verify basic geometry, probe positioning, and overall system integration. I do not intend to analyze or trust any scientific data obtained with this mockup.
Once I am confident in the mechanical dimensions and RF chain, I plan to build a proper aluminum horn with continuous conductive surfaces.
Regarding the short coaxial cables between the probe output and the LNA/filter chain: you are correct that a direct mechanical connection could reduce one interface and possibly minimize losses or mismatch. At this stage, however, I am frequently assembling and disassembling the setup, and a rigid direct connection would place mechanical stress on the LNA board and solder joints.
My current assumption (based on what I have read, but very much open to correction) is that using very short, good-quality coaxial jumpers (5–10 cm) should not introduce a dominant degradation compared to the other uncertainties in this prototype. That said, I may be completely wrong, and I am still learning.
If you or others have measured differences between direct coupling and short coax connections in similar setups, I would be very interested to hear about it.
Best regards,
--
At.te
Tiago H. França Baroni
Mestre em Transportes
Engenheiro Civil
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edhar...@gmail.com
The “rule of thumb” (or all thumbs in my case) that I know are:
Foil wrinkled – Is it more than 1/20th of a wavelength? 21 cm/20 = 1.05 cm. For amateur work definitely not a concern. For professional… Not anywhere near knowledgeable enough…
Why do cables between the probe … to the LNA? Preferred none. Cables result in lose and we have an extremely small signal to begin with. That said, practical matters can (somewhat) overrule. Preferred as short of a run of cable as possible to the first LNA. After the first LNA, longer runs (with good cable) are acceptable as most people do not have their receiver (SDR) out in the weather.
(Unwritten but…) Why use white for the dish, other containers? Temperature and temperature swing.
- For the probe, reflected sunlight (for those times when the Sun passes through the beam) can melt the probe (or at least any dielectrics supporting the probe. A black or metal dish can reflect enough IR to melt a copper probe.
- For the filter, the tuning will shift with temperature as it expands and contracts. Usually the tuning is wide enough to not be a concern.
- For cable, the length will change as it expands and contracts. (See Wolfgang for some of their experiences with cables and an interferometer. The things radio astronomers have to think of…)
- For the LNA, the gain can change enough to think you have a signal… even with a dummy load instead of the probe. (e.g. a 0.01 dB change in gain is a 1K change in temperature.) Temperature stability (usually heating) is well advised. Why heating? Because generally our systems are not calibrated and we are looking for a change in signal, not an absolute value. (An absolute value is a next level game and is seriously hard work to get stable. There is a reason, beyond the big cost of the dish, that professionals have to pay a lot of money for their equipment.) If you have a stable gain, you can detect a change more easily (within bounds). Think of it this way: If you have a stable noise floor of 100K +/- 1k, you can reliably detect a smaller change in signal than a 50K noise floor +/- 50K. Cooling systems are expensive and (generally) generate a lot of RFI.
These are the rules of thumb that I remember from others (with my first cup of coffee).
Best wishes,
Ed
From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Jimmy Fransson
Sent: Friday, February 13, 2026 5:19 AM
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: [SARA] Re: HI feedhorn prototyping – early work in progress, looking for guidance
Hi, I will NOT give you professional tips just some of my own reflections here.
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tba...@gmail.com
Hi Ed,
Thank you for taking the time to write such a detailed set of reflections, that was extremely helpful.
Regarding the wrinkled foil, your λ/20 rule of thumb makes sense. The irregularities are well below that scale, so for this early prototype I am assuming they are not the dominant limitation. Still, the final aluminum version will of course have continuous conductive surfaces.
On the short cable between the probe and the first LNA, I fully agree that minimizing loss before the first gain stage is important. In this mockup, I am using very short jumpers mainly for mechanical flexibility during frequent assembly and disassembly. As the system stabilizes, I will revisit whether a more direct connection is practical without compromising mechanical robustness.
Your comments on thermal effects are particularly valuable. I had not yet considered gain drift at the level of hundredths of a dB translating into kelvin-scale variations. Since I am currently working with an uncalibrated total-power setup and looking only for relative changes, thermal stability (especially of the LNA) is clearly something I need to plan for as the project progresses. For now, I will at least ensure passive stability and avoid direct solar exposure when testing with a dish.
I am very much in the “learning by building” phase, so I appreciate these reminders about second-order effects that can easily become first-order problems.
Best regards,
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At.te
Tiago H. França Baroni
Mestre em Transportes
Engenheiro Civil
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Robert Meade
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