For testing a 22 GHz LNB, you don’t necessarily need a laboratory-grade signal generator. The cheapest options are:
- 22 GHz Gunn diode oscillator (typically £20–£60 used, £50–£150 new). These were widely used in police radar trainers and microwave experiments. They produce a stable CW signal close to 22 GHz and are ideal if you can find one second-hand.
- 24 GHz automotive radar module (£5–£20). These modules operate around 24.125 GHz rather than 22.235 GHz. While not on the water maser frequency, they are still useful to verify that the LNB is working across the K-band if its input covers that frequency.
- Frequency multiplier chain. If you already have a 10–11 GHz signal source (such as an SDR with an upconverter or a microwave PLL), a ×2 active multiplier can produce around 20–22 GHz. This is usually more expensive than buying a Gunn oscillator unless you already own the equipment.
- Second 22 GHz LNB with a beacon. Some people test by transmitting a low-level IF signal into one LNB (if designed for transmit/receive) or by using another microwave source, but this is generally more complex.
If your goal is simply to confirm that your LNB and receiver chain work before looking for water masers, I would recommend searching for a 22 GHz Gunn oscillator. It is usually the lowest-cost solution that provides a signal very close to the 22.23508 GHz water maser line.
Could you tell me:
- Is your LNB a commercial satellite-style 22 GHz LNB, or one you’ve built yourself?
- Do you need to test just the LNB, or the entire receiver chain including your SDR?
Norsat 9000 commercial LNB. Test both.
Your Norsat 9000 Ka-band LNB receives 21.2–22.2 GHz (depending on the exact model), so a 22.235 GHz water maser is just inside its input band. It mixes this with a 20.25 GHz LO to produce an IF around 1.985 GHz.
For testing the entire chain (LNB + bias tee + coax + SDR), I would rank the options as:
- Used 22 GHz Gunn oscillator (£20–£60 if you can find one). This is the closest thing to a real water maser signal and is the best low-cost solution.
- 24 GHz radar module (£5–£15). These are extremely cheap and can confirm that the LNB is functioning, although 24 GHz is outside the specified 21.2–22.2 GHz input range, so there is no guarantee your particular Norsat will respond.
- Noise source. A broadband microwave noise source placed close to the feed can verify receiver gain even without a narrow spectral line.
An alternative that costs nothing if your dish is already assembled is to point at a strong geostationary Ka-band satellite beacon in the correct frequency range. You should see a clear peak in your SDR if the LNB is working. This is often the easiest end-to-end test before attempting water maser observations.
If your goal is specifically detecting water masers, I can also suggest an even better astronomical test than a signal generator: observing the 22.235 GHz atmospheric water vapour line. With a low-noise Norsat 9000 and a suitably sized dish, many amateur setups can detect the increase in sky temperature near the zenith compared with a cold sky reference. This tests the entire system at exactly the correct frequency without needing any transmitter.
When you say cold sky reference here qhat would you recommend I use if I am already using Zenith for the atmospheric water signal? In other words which part of sky gives me cold sky reference with less water signal?
For the 22.235 GHz atmospheric water vapour line, the “cold sky” reference is not a different frequency—it’s a different line of sight through the atmosphere.
If you use the zenith (straight up) as your measurement, there is actually less atmosphere above you than in any other direction. That means the water vapour signal is weaker at the zenith, not stronger.
To demonstrate the atmospheric water line, you normally compare:
- Zenith (90° elevation) – shortest atmospheric path, giving the lowest water vapour emission (your “cold sky”).
- Low elevation (e.g. 10–20° above the horizon) – much longer atmospheric path, giving stronger water vapour emission (your “warm sky”).
So, for your experiment:
- Cold reference: Point the dish at the zenith, well away from the Sun and Moon.
- Warm measurement: Point at the same azimuth but only about 10–20° above the horizon. Avoid buildings, trees, and the ground entering the beam, as ground spillover is much warmer than the sky and can dominate the measurement.
If your dish has sidelobes or a wide beam, I would suggest using 20° elevation rather than 10° to reduce ground contamination while still increasing the atmospheric path length by about a factor of three compared with the zenith.
With your Norsat 9000 and a dish suitable for 22 GHz, you should be able to see a measurable increase in received power as you move from the zenith to a low elevation on a reasonably humid day.
The brightest astronomical maser transitions that are practical for radio astronomy are:
| Maser species | Rest frequency | Amateur practicality |
|---|---|---|
| Water (H₂O) | 22.23508 GHz | Very popular with advanced amateurs |
| Methanol (CH₃OH) | 6.66852 GHz | Requires a 6.7 GHz receiver |
| Hydroxyl (OH) | 1612.231 MHz | Accessible with L-band equipment |
| Hydroxyl (OH) | 1665.402 MHz | Common star-forming region masers |
| Hydroxyl (OH) | 1667.359 MHz | Common star-forming region masers |
| Hydroxyl (OH) | 1720.530 MHz | Supernova remnant masers |
| Silicon monoxide (SiO) | 43.122 GHz | Challenging for amateurs |
| Silicon monoxide (SiO) | 86.243 GHz | Very challenging |
For your Norsat 9000, only the 22.23508 GHz water maser line is relevant.
Gunn oscillators
Most Gunn oscillators are sold by frequency band rather than an exact frequency. The ones of interest are:
- 22.2 GHz Gunn oscillator – Ideal. A unit specified at 22.2 GHz is close enough to the water maser frequency for testing.
- 22.235 GHz Gunn oscillator – Excellent if you can find one (rare).
- 22.0 GHz Gunn oscillator – Fine for checking LNB operation.
- 22–22.5 GHz tunable Gunn oscillator – Best option, as it can usually be mechanically tuned to 22.235 GHz.
- K-band (18–26.5 GHz) Gunn source – Many laboratory modules can be tuned over this range.
You do not need the test source to be exactly at 22.23508 GHz to verify that the LNB, bias tee, coax, and SDR are functioning. Anything within the Norsat’s input band (around 21.2–22.2 GHz, depending on the model) should produce a detectable IF signal.
If you’re looking on the second-hand market (e.g. eBay or surplus suppliers), search for terms such as:
- “22 GHz Gunn oscillator”
- “22.2 GHz Gunn diode”
- “K-band Gunn oscillator”
- “22 GHz microwave source”
- “22 GHz waveguide Gunn”
These surplus modules are often the most economical way to obtain a suitable microwave test source.