Basic Radiomm Astronomy

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Amplifier

amplifies a very weak radio frequency (RF) signal, is stable & low noise

Mixer

produces a stable lower, intermediate frequency (IF) signal by mixing the RF signal with a stable local oscillator (LO) signal, is tunable

Filter – selects a narrow signal band out of the IF

Backend – either total power detector or more typically today, a correlator

Basic Radio/mm Astronomy

Feed Horn

Slide 9

Origin of the Beam Pattern

Antenna response is a coherent phase summation of the E-field at the focus

First null occurs at the angle where one extra wavelength of path is added across the full aperture width,

i.e., q ~ l/D

Basic Radio/mm Astronomy

Slide 10

Antenna Power Pattern

Basic Radio/mm Astronomy

The voltage response pattern is the FT of the aperture distribution

The power response pattern, P(q ) µ V2(q ), is the FT of the autocorrelation function of the aperture

for a uniform circle, V(q ) is J1(x)/x and P(q ) is the Airy pattern, (J1(x)/x)2

Slide 11

The Beam

The antenna “beam” solid

angle on the sky is:

Basic Radio/mm Astronomy

Telescope beams @ 345 GHz

Slide 12

Sensitivity (Noise)

Unfortunately, the telescope system itself contributes noise to the the signal detected by the telescope, i.e.,

Pout = PA + Psys  Tout = TA + Tsys

The system temperature, Tsys, represents noise added by the system:

Tsys = Tbg + Tsky + Tspill + Tloss + Tcal + Trx

Tbg = microwave and galactic background (3K, except below 1GHz)

Tsky = atmospheric emission (increases with frequency--dominant in mm)

Tspill = ground radiation (via sidelobes) (telescope design)

Tloss = losses in the feed and signal transmission system (design)

Tcal = injected calibrator signal (usually small)

Trx = receiver system (often dominates at cm — a design challenge)

Note that Tbg, Tsky, and Tspill vary with sky position and Tsky is time variable

Basic Radio/mm Astronomy

Slide 13

Sensitivity (Noise)

In the mm/submm regime, Tsky is the challenge (especially at low elevations)

In general, Trx is essentially at the quantum limit, and Trx < Tsky

Basic Radio/mm Astronomy

“Dry” component: O2

“Wet” component: H2O

Slide 14

Sensitivity (Noise)

Q: How can you detect TA (signal) in the presence of Tsys (noise)?

A: The signal is correlated from one sample to the next but the noise is not