Thoughts on Building an Amateur 6.668 GHz Methanol Maser Receiver

I am exploring the possibility of building a receiver to observe the 6.668519 GHz Class II methanol maser line using amateur radio astronomy equipment.

The 6.7 GHz methanol line is an attractive target because it is one of the strongest maser transitions in the Galaxy, but it is more challenging than the 1.420 GHz hydrogen line or the 22.235 GHz water maser line because there are no simple off-the-shelf satellite LNB solutions.

Why a normal satellite LNB is not suitable

The main satellite frequency bands are:

• C-band: approximately 3.4–4.2 GHz (extended versions sometimes higher)
• X-band: approximately 7.25–7.75 GHz
• Ku-band: 10.7–12.75 GHz
• Ka-band: 17 GHz and above

The 6.668 GHz methanol line sits between conventional C-band and X-band, so standard satellite LNBs do not cover it.

Antenna/feed

A cantenna or waveguide feed should be practical at 6.7 GHz.

At 6.668 GHz:

• Wavelength ≈ 45 mm
• A suitable waveguide/feed diameter is roughly 30–35 mm
• A probe feed could be positioned around 11 mm from the back wall (approximately quarter wavelength)

A 1.5 m dish should perform well:

• Beamwidth approximately 2°
• Gain approximately 35 dBi

This makes a 6.7 GHz feed a realistic project using a dish similar to that already used for hydrogen-line work.

Receiver concept

The proposed receiver chain is:

6.668 GHz feed/cantenna
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        ▼
6–18 GHz LNA
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        ▼
6.668 GHz cavity filter
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        ▼
Mixer
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        ▼
968 MHz IF
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        ▼
SDRplay RSPduo / USRP B210

LNA choice

A possible LNA found was:

5–18 GHz RF LNA

• Gain: 26 dB
• Noise figure: 1.5 dB

This is a suitable specification for 6.668 GHz:

• Frequency coverage includes the methanol line
• Noise figure is reasonable for amateur microwave astronomy
• Gain is sufficient without excessive risk of mixer overload

Filtering

A custom cavity filter from WTMicrowave could be very useful.

A filter centred at:

6668.5 MHz

with the narrowest available bandwidth:

100 MHz

would cover:

approximately 6618.5–6718.5 MHz

This corresponds to thousands of km/s in velocity coverage, far wider than required for Galactic methanol masers.

Although it is much wider than the actual maser lines, it would still help by:

• reducing unwanted signals
• protecting the mixer
• improving receiver dynamic range

The final spectral resolution would come from the SDR/software, not the cavity filter.

Mixer/downconverter

A suitable mixer is required to convert 6.668 GHz down to an SDR-friendly frequency.

The AliExpress mixer found:

5–21 GHz RF mixer

should be usable.

Example conversion:

RF: 6668.519 MHz

LO: 5700 MHz

Output:

6668.519 − 5700 = 968.519 MHz

This IF is suitable for both:

• SDRplay RSPduo
• USRP B210

Higher quality alternatives would include microwave mixers from companies such as Mini-Circuits or Analog Devices, but the inexpensive mixer is likely adequate for initial experiments.

Local oscillator (5.7 GHz)

The 5.7 GHz LO is one of the most important parts of the system.

A NanoVNA is not ideal as a permanent LO:

• frequency stability is insufficient
• phase noise may be poor
• it is not designed as a signal source

An SDR transmitter could possibly generate 5.7 GHz for testing, but it is not the best long-term solution.

Better options:

1. ADF4351 synthesiser
   • inexpensive
   • capable of producing 5.7 GHz
   • much better if locked to a stable reference
2. ADF5355 synthesiser
   • higher performance
   • covers a much wider frequency range
3. Best amateur solution:

GPS-disciplined 10 MHz reference
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          ▼
PLL microwave synthesiser
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5.700 GHz LO
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Mixer

LO stability is essential because frequency drift can mimic Doppler shifts and broaden the narrow maser lines.

Required bandwidth

At 6.668 GHz:

• 1 km/s corresponds to approximately 22 kHz

Typical useful observing bandwidths:

• 2 MHz ≈ ±45 km/s
• 5 MHz ≈ ±110 km/s
• 10 MHz ≈ ±225 km/s

A 10 MHz observation bandwidth would cover almost all Galactic methanol maser velocity ranges.

Overall assessment

A realistic amateur 6.7 GHz methanol maser system would consist of:

• 1.5 m dish
• home-built 6.668 GHz cantenna/feed
• 6–18 GHz low-noise amplifier
• 100 MHz cavity filter centred at 6668.5 MHz
• microwave mixer
• stable 5.7 GHz synthesised LO
• SDRplay RSPduo or USRP B210
• FFT spectroscopy software

The antenna is probably the easiest part. The main engineering challenges are the low-noise front end and a very stable microwave local oscillator.