Hydrogen-Line Detection vs Antenna Gain
A measurement of 0.73 dB above background does not directly convert to dBi because they describe different things:
- dB above background = how much stronger the received hydrogen-line signal is than the background level.
- dBi = antenna gain relative to an isotropic radiator.
For your measurement:
0.73 dB corresponds to a power ratio of
10^(0.73/10) = 1.183
so the hydrogen-line peak is about 18.3% stronger than the background power.
This does not mean your horn gain is 0.73 dBi.
For a typical Horn of Plenty with an aperture approximately 4 ft x 3 ft (1.22 m x 0.91 m) operating at the hydrogen line frequency of 1420.405 MHz:
- Aperture area A = 1.22 x 0.91 = 1.11 m^2
- Wavelength lambda = 0.211 m
The horn gain is given by:
G = eta * (4 * pi * A / lambda^2)
where:
- G = gain (linear)
- eta = aperture efficiency
- A = aperture area
- lambda = wavelength
Using a realistic efficiency of 55% to 65%:
- Ideal (100% efficient): about 25.0 dBi
- 65% efficient: about 23.1 dBi
- 55% efficient: about 22.4 dBi
Therefore a practical estimate for your Horn of Plenty is:
Gain approximately 22.5 to 23 dBi
Your observed hydrogen-line enhancement of 0.73 dB above background is quite reasonable for a horn of this size and indicates a successful detection of the hydrogen line, not a gain of 0.73 dBi.
Equation sheet (PDF):
https://www.astronomy.me.uk/wp-content/uploads/2026/05/Hydrogen_Line_Horn_Equations-290526.pdf