What alternatives are there available to Nooelec SAWBird H1 with inbuilt resistor to use to generate reference samples for hydrogen line work?
Short answer: yes — but almost never as a single consumer LNA product.
Two common approaches are used in practice:
- radiometer / scientific front-ends that integrate an RF switch + calibration load(s) and an LNA (common in space/instrumentation receivers); and
- hobbyist/ham solutions where you add a low-loss coaxial RF switch or relay ahead of the LNA to switch the LNA input between the antenna and a 50-Ω (dummy) load.
Details, tradeoffs and concrete parts:
Why you don’t usually see “LNA with built-in antenna→dummy-load” as a consumer product
Manufacturers for astronomy/radiometry often build the whole front-end (switch, loads, LNA) as a system because the switch insertion loss and isolation matter for calibration. Most off-the-shelf hobby LNAs (Nooelec, Mini-Circuits wideband LNAs, etc.) do not include an internal antenna→dummy-load switching mechanism as standard — instead instrument-grade front-ends implement the switching separately. (arXiv)
Practical options you can use right now
A — Buy a radiometer/front-end module (integrated): several research projects and commercial radiometer front-ends include internal switches and matched loads for on-the-fly calibration (Dicke or multi-load schemes). These are the cleanest option for accurate temperature/reference samples, but they are usually aimed at instrumentation labs or space projects. Examples/refs showing this approach: REACH front-end, SMAP/Planck style radiometers. (arXiv)
B — Add an external 50-Ω coaxial RF switch / coaxial relay ahead of the LNA: this is the most common amateur approach. Use a high-quality coaxial SPDT or transfer switch (mechanical coaxial relay or a good solid-state/pin-diode switch) to select ANT ↔ 50 Ω load, then feed the LNA. Low-loss mechanical coaxial relays give the best calibration performance (very low insertion loss, high isolation) and are widely available. Examples you can buy off the shelf:
- Pasternack / PE71S63xx series SPDT coaxial relay (DC–18 GHz electromechanical SPDT). (pasternack.com)
- Mini-Circuits / MSP2T-18XL+ (mechanical coaxial SPDT) and many other Mini-Circuits RF switches. (Mouser Electronics UK)
- Tohtsu CX-600NC coaxial relay (popular in ham shops). (The DX Shop Limited)
- Pickering / Panasonic high-frequency RF relays and many small coaxial reed relays (useful if you want to mount on a PCB). (Pickering Electronics Ltd)
Tradeoff: any switch placed before the LNA adds insertion loss (and so increases system noise when the antenna is connected). That’s why instrument systems pick very low-loss mechanical switches (insertion loss ~0.01–0.1 dB) rather than lossy solid-state switches for calibration paths. The REACH radiometer front-end paper explicitly mentions using low-loss mechanical switches for switching between antenna and references. (arXiv)
C — Design option (if you like hardware design): use a dual-input pseudo-switch LNA topology or duplicate the LNA input stage so you can switch in a reference without routing through a lossy switch — there is academic work describing dual-input LNAs that avoid a conventional RF switch. This is a custom route but eliminates the switch loss penalty at the price of extra complexity. (ResearchGate)
Other calibration approaches (alternatives to antenna↔dummy-load switching)
- Noise-diode injection (calibrate by switching a calibrated noise diode in/out via a coupler or injection path). Common and simple for many amateurs.
- Directional coupler + known termination (tap a fraction of signal for reference).
- Dicke switching / continuous comparison receivers (used on sensitive radiometers). See SMAP/Planck references. (NASA Technical Reports Server)
Recommendation (what I’d do for hydrogen-line radio astronomy)
- If you want simple, robust reference samples: buy a low-loss coaxial SPDT mechanical relay (SMA/N connectors) and a quality 50-Ω dummy load, put the relay immediately at the antenna feed so the reference and antenna see the same cable/connector conditions, then feed the LNA. Use a mechanical/transfer relay with very low insertion loss (Pasternack / Mini-Circuits / Tohtsu examples above). (pasternack.com)
- If you need the best calibration accuracy and can afford a more complicated solution: look at integrated radiometer front-ends (or design a dual-input LNA) that provide internal loads and switching specifically engineered for low added noise and high stability. (arXiv)
Following is:
- list 3–4 specific coaxial relay parts (with short shopping links and UK suppliers) sized for 1.42 GHz and with typical insertion-loss/isolation specs; or
- sketch a wiring diagram showing where to place the relay, dummy load, LNA and SDR/receiver for minimum added noise; or
- search for a commercial radiometer front-end that already includes the antenna↔dummy-load switching for L-band (1.4 GHz).
In order: (1) four good coaxial-relay/switch parts you can buy for 1.4 GHz use (with key specs + UK/retailer sources), (2) an explicit wiring diagram and wiring/placement notes so your calibration path adds minimal noise, and (3) commercial/academic radiometer front-ends that already include antenna↔dummy-load switching (L-band / 1.42 GHz). I’ve cited the seller/datasheet pages for the most important claims so you can click through and order or read datasheets.
1) Recommended coaxial relays / RF switches (good for ANT ↔ 50 Ω load switching)
Below are reliable, low-loss mechanical coaxial relays and one solid-state option (mechanical relays are preferred because of much lower insertion loss and better isolation for calibration). I list connector type, typical insertion-loss/isolation, drive voltage notes and a UK supplier or global datasheet.
- Mini-Circuits MSP2T-18XL (SPDT coaxial electromechanical switch)
- Frequency: DC – 18 GHz.
- Insertion loss: ~0.25 dB (very low). Isolation: ~80–85 dB depending on model.
- Connector / mounting: screw-mount / flange versions available. Lifetime: millions of cycles.
- Notes: widely used, rugged, low-loss — excellent choice for front-end calibration switching.
- Source / datasheet: Mini-Circuits product page/datasheet. (minicircuits.com)
- Pasternack PE71S (series) / PE71S6391 (example SPDT coaxial relay)
- Frequency: DC – 18 GHz (varies by sub-model).
- Insertion loss: typical 0.3–0.4 dB max. Isolation: 60–85 dB depending on part.
- Connector types: SMA / N / others depending on variant. Failsafe and latching options exist.
- Notes: many variants; Pasternack is easy to source worldwide and stocks multiple low-loss SPDT coaxial relays.
- Source / product listing / datasheet (Pasternack / Mouser). (pasternack.com)
- Tohtsu CX-600NC (N-type coaxial SPDT relay) — popular in ham shops / UK retailers
- Frequency: suitable for VHF/UHF and L-band use (N-type connections).
- Insertion loss & isolation: spec sheets show good performance for 50 Ω switching; commonly sold to amateurs for TX/RX switching.
- Notes: inexpensive, commonly sold in ham shops (The DX Shop, Henry Radio / eBay UK). Good value if you want N-type connectors and simple mechanical operation.
- UK retailer example: The DX Shop; also available on eBay/ham suppliers. (The DX Shop Limited)
- Mini-Circuits / Mouser absorptive solid-state switch (if you prefer a compact SS option)
- Example family: Mini-Circuits absorptive SPDT RF switches (various ZSWA / MSWA families) — frequency ranges and IL vary (solid-state IL is higher than mechanical for many parts).
- Insertion loss: can be higher (0.5–1.5 dB typical depending on device) and some are absorptive (no reflections when off). Good for fast switching or PCB integration but watch IL impact on system noise.
- Source / distributor (Mouser listings). (Mouser Electronics UK)
Where to buy in the UK / Europe (examples): Farnell / RS / Mouser UK / The DX Shop / Henry Radio (eBay) — these sellers stock coaxial relays and can ship to the UK. For high-performance low-loss switching choose a mechanical coaxial relay from Mini-Circuits or Pasternack. (Farnell UK)
2) Wiring diagram + placement + control notes
Put the switch as close to the antenna feed as possible so the antenna and dummy load see the same feedline and connector environment (this is crucial for clean calibration).
ASCII wiring diagram (best practice):
Antenna | [Coax feed] | +----+----+ | SPDT | <- Coaxial relay/transfer switch at the antenna/feed | Relay | +----+----+ | | | +-----> 50 Ω dummy load (at the same physical location) | +-----> LNA input (SMA/N) ---> LNA ---> Receiver/SDRKey notes and practical tips:
- Place the SPDT relay at the antenna feed (or at the mast/head): this ensures the load and antenna share the same feedline losses and temperature/environment, reducing calibration errors. Use an N or SMA relay to match your feed/feedline connectors. (The DX Shop Limited)
- Use a high-quality 50-Ω dummy load rated for the expected power (for calibration you usually use tiny power so standard 1–5 W loads are fine). The load must be right at the relay output so stray cable between relay and load is minimal.
- Switching before the LNA means the switch insertion loss only affects the antenna path: mechanical relays have very low IL (~0.01–0.4 dB for high quality coaxial relays) so system noise penalty is small; avoid lossy solid-state switches if lowest system noise is required. (pasternack.cn)
- Power/drive: mechanical coaxial relays typically use +12 V or +24 V coils (check datasheet). Mini-Circuits and Pasternack parts list coil specifications on datasheets — you’ll need a small driver (GPIO + driver transistor/relay driver or a 12 V supply + MOSFET/transistor) and a flyback diode on coil drive. (pasternack.cn)
- Control & timing: for calibration sequences you can drive the coil from a microcontroller (Arduino/RPi + transistor) or an external relay driver. If you need fast, repeated switching for Dicke-style measurements consider relay life ratings; Mini-Circuits parts often specify very high cycle life (10M cycles). (minicircuits.com)
- Mechanical mounting & connectors: pick an N-type relay if the mast/feed uses N connectors (better weatherproofing options), or SMA if small. Ensure the relay housing can be weatherproofed at the mast (or place inside a small weatherproof box). (The DX Shop Limited)
3) Commercial radiometer front-ends / integrated solutions (L-band / switch+load included)
If you want a single, instrument-grade unit that already implements reference switching and loads internally (instead of adding an external relay), consider the following:
- Spacek Labs — Radiometer front-ends (custom/standard)
- Spacek Labs advertises radiometer front-ends and assemblies across wide frequency ranges and mentions integrated noise sources, couplers and front-end assemblies. They work with customers to produce front-ends spanning <1 GHz up to tens of GHz. Contact them for a custom L-band front-end with internal REF load/switching. (spaceklabs.com)
- Research / project front-ends (examples / references)
- The REACH radiometer project (academic) describes a front-end design used for 21-cm (1.42 GHz) cosmology that houses the receiver and a calibration subsystem (switching and reference loads) in the front-end; see the REACH papers and front-end design descriptions for architecture and practical implementation details. These are typically custom or research devices, but their published designs are useful references. (CORE)
- Custom OEM builders / university labs
- Many instrumentation vendors or university labs build custom L-band radiometer front-ends including calibrated loads and switching (SMAP/Planck style designs). For a turnkey purchase, contact instrumentation vendors (Spacek Labs is one example) and request a “radiometer front-end for 1.42 GHz with integrated switch & matched loads”. Expect quotes (these are more expensive than simple relays) but you’ll get engineering documentation and better stability for precision radiometry. (spaceklabs.com)
4) Quick shopping + where I’d order for each option (UK-friendly)
- Best low-loss mechanical option (professional): Mini-Circuits MSP2T-18XL — order from Mini-Circuits or Mouser/Mouser UK / Farnell (datasheet on Mini-Circuits). Good for high isolation and long life. (minicircuits.com)
- Good cost/availability (ham shops / UK): Tohtsu CX-600NC — sold by The DX Shop (UK) and via eBay/Henry Radio — cheap and easy to get in the UK. (The DX Shop Limited)
- Wide variant choice / quick shipping: Pasternack PE71S series — many SMA/N variants and Mouser/Pasternack list multiple in-stock options. Great if you want a particular connector or latching/failsafe style. (Mouser Electronics)
- Integrated front-end / turnkey radiometer: Contact Spacek Labs with your specs (1.42 GHz center, bandwidth, calibration scheme) — they can provide an integrated radiometer front-end or point you to similar OEM offerings. (spaceklabs.com)
5) Short calibration checklist and pitfalls to avoid
- Put the switch at the antenna feed so antenna and load share the same cable. (The DX Shop Limited)
- Use mechanical coaxial relays for lowest insertion loss (minimise added noise). (minicircuits.com)
- Weatherproof the relay or place it in a small radome/box — moisture causes big impedance changes.
- If you want frequent, high-speed switching, confirm relay life (mechanical relays — check cycle rating) or consider solid-state (accepting higher IL). (minicircuits.com)