Natural radio emissions from space cover the total range of the electromagnetic spectrum. However, on the earth's surface the majority of this spectrum is blocked by the earth's magnetic field and atmosphere only allowing few regions to pass. In the radio spectrum the earth’s atmosphere becomes increasingly transparent above 18Mhz and then increasingly opaque at around 40Ghz.
Atmospheric Transparency to different wavelengths of the Electromagnetic Spectrum
Any frequency above 18Mhz free from terrestrial and satellite interference can be used for radio astronomy. The lower segments of the spectra are used for solar and Jupiter observations; the 73, 150 and 406 MHz segments are quite popular for pulsar, and the 1.4 Ghz band and above is used for spectral line or energy measurements.
The following frequencies below are generally accepted spectral regions for radio astronomical observations and so have just chosen list the official regions as a reference, most accessible for an amateur radio astronomer.
- 25.550 – 25.670 Mhz
- 37.5 – 38.25 Mhz
- 73 – 74.6 Mhz
- 150.05 – 153 Mhz
- 322 – 328.6 Mhz
- 406.1 – 410 Mhz
- 608 – 614 Mhz
- 1.4 – 1.427 Ghz
- 1.6106 – 1.6138 Ghz
- 1.66 – 1.67 Ghz
- 2.655 – 2.700 Ghz
- 4.8 – 5 Ghz
- 10.6 – 10.7 Ghz
- 18.28 - 18.36 GHz
Amateur Radio Astronomy Frequency Choices
Most natural cosmic sources have spectra that fall off with frequency, so even if you keep the same antenna aperture (effective area) the signals will decrease with frequency.
Consequently the lower the frequency that is still transparent to the ionosphere (e.g. above 18Mhz) the greater the energy (signal strength) that can be collected by a specific gain of antenna. Said another way the better chance you have in detecting it.
What you gain by going up in frequency is:
- a narrower antenna beam (if you keep the same antenna area),
- less man made interference,
- more transparency to the ionosphere,
- a bigger possible bandwidth (if your hardware can eat it)
The relation between gain and effective area is
G = 4 * PI * A / L2 or A = G * L2 / 4 / PI
Where G is gain (linear, not dB), A is the effective area, PI is 3.14... and L2 is wavelength squared. Units for A and L2 are not important, but both must be given in the same units. The same area means more gain at a higher frequency, and the same gain means less area at a higher frequency.
Consequently from this reasoning the best choice of frequency would then be the lowest frequency that is free of interference that can be installed on the land area available to the Amateur Radio Astronomer. Land area becomes even a greater concern with interferometry as the antenna must be space apart East to West by 15 or more wavelengths to achieve a suitable fringe pattern.