WHAT IS A DICKE SWITCH?
A Dicke switch is a rapid RF switch used in radio astronomy to reduce receiver gain drift.
It switches quickly between:
- Antenna (sky signal)
- Reference load (known temperature resistor)
This technique was invented by Robert H. Dicke.
THE PROBLEM
Radio receivers measure noise power:
P = G * (T_ant + T_rx)
Where: G = receiver gain T_ant = antenna (sky) temperature T_rx = receiver noise temperature
If G drifts slightly, it looks like a real sky signal.
When observing weak signals (like hydrogen at 1420 MHz), gain drift can dominate the measurement.
THE DICKE SOLUTION
Switch rapidly between:
ANTENNA <—-
>—- RF SWITCH —-> LNA —-> DETECTOR REFERENCE <—-/
Switching rate: typically 10 Hz to several kHz.
Now the receiver measures:
(Antenna signal) minus (Reference signal)
Because the switching is fast:
- Gain changes are slow compared to switch rate
- Both signals see the same gain
- Subtracting them cancels gain drift
WHAT THE OUTPUT REPRESENTS
Output proportional to:
T_ant – T_ref
If T_ref is known, you can calibrate absolute temperature.
NOISE PENALTY
There is a cost:
Sensitivity is reduced by about:
sqrt(2)
This is called the Dicke radiometer penalty.
WHY THIS HELPS
Instead of measuring a slowly drifting DC level, you are effectively measuring an AC-modulated signal.
That makes it much easier to detect small differences using synchronous detection.
PRACTICAL 1420 MHz USE
Without switching: Slow LNA or SDR drift looks like hydrogen signal.
With Dicke switching: Long integrations become much more stable.
Some amateurs implement it by:
- Switching between antenna and 50 ohm load
- Subtracting averaged spectra in software