In this band of frequencies (SHF: Super High Frequency, from 3 GHz to 30 GHz) it is important to the mechanism of thermal emission of radio sources, connected to their surface temperature: according to Planck's law all material bodies radiate, for thermal effect, more or less efficiently depending on the physical temperature and the absorption capacity of the body.
The following figure shows the so called "microwave radio window": you see as the frequencies region between approximately 1 GHz and 10 GHz that is characterized by the lower background noise, therefore particularly suitable for the production of small amateur radio telescopes.
Small microwave radio telescopes are easily achievable at an amateur level using commercial components and modules from the market of TV SAT (including the antennas).
The 10-12 GHz band receivers are relatively immune to artificial noise radio so you can successfully install a small radio telescope in the urban area.
Unfortunately, the radio sources in this frequency band are not numerous or powerful: to observe it are essential not negligible dimensions antennas.
With small antennas (from 0.80 to 1.5 meters) are possible good observations of the Sun (thermal component of the radiation, monitoring of solar flares microwave) and the Moon.
Feasibility analysis of a simple amateur radio telescope microwave band (10-12 GHz)
The simplest structure of radio telescope is the Total-Power radiometer:
The instrument response is directly proportional to the power associated with the incident radiation mediated by the bandwidth of the instrument, then the brightness temperature in the region of "seen" sky by the antenna beam. The radiometer behaves like a thermometer that measures the equivalent temperature noise of the observed celestial scenery.
If the antenna is pointed at a clear and dry sky region where radio sources are absent (the so called clear atmosphere, with negligible atmospheric absorption), we measure a very low equivalent temperature noise (due to fossil radiation at 3 K), generally the order of 6-10 K (cold sky), which corresponds to the minimum temperature detectable by the instrument and takes into account the losses instrumental.
Even allowing for minimum contributions due to small radio sources and the noise of the atmosphere, if your antenna is kept at least 15° above the horizon and away from the Sun, we can assume an equivalent noise temperature of a few degrees between antenna and a few tens of degrees (primarily due to the secondary lobes).
Pointing the antenna on the ground, the temperature rises to values of the order of 300 K (about 273 K + temperature), if it is interested the entire beam of the antenna.
When a typical antenna for TV-SAT (beam width of the order of 2° -3°) is focused on the Sun (apparent size of about 0.5° and characterized, at a frequency of 11 GHz, a temperature noise approximately equal to the surface of 6000 K), the system "sees" a source with a temperature of about 396 K. The background cosmic radiation "dilutes" the powerful radiation of the Sun (6000 K value is passed to much lower than 396 K value) if the antenna beam is broad to the point of collecting a significant contribution of external radiation (with much lower brightness). The background radiation, captured in good percentage from the crown of the outer lobe of the antenna, decreases the amplitude of the received signal as if it came from a source with a temperature considerably lower than the real one.
A similar situation occurs if between the "cold sky" and the observer are localized variable cloud formations (hydrometeors in general) in composition, thickness and height from the ground, characterized by a equivalent noise temperature widely variable between 200 K and 260 K. These operating frequencies can become a major contribution to disturbing the troposphere with its fluctuations and irregularities: it is interesting to note that any "seen" hydrometeors by the antenna are able to mask the cosmic radiation of the cold sky acting as point sources and characterized by irregular temperature higher than the bottom.
This characteristic is widely used in applications of meteorology (operating frequencies around 22 GHz), where sensitive microwave radiometers "watch" the earth from satellites locating and measuring the equivalent noise temperature of the hydrometeors and clouds of gas and/or pollutants vapor.
The following figures show some simulations (where possible supported by actual measurements) for receiving some tests radio sources carried out with typical amateur radio telescopes operating in the frequency band 10-12 GHz (components from the market of TV-SAT): This information is useful to understand what are the real possibilities of observation accessible to amateur radio astronomers with simple equipment.
Then is illustrated the importance of a correct choice for the constant of integration of the amplifier chain of post-integration: if the emission of the observed radio emission exhibits characteristics of stationarity, you can increase the value of the time constant of the integrator to all advantage of the sensitivity of the system (equation of the radiometer).
At the limits of instrument sensitivity...
The previously simulations performed are theoretical, although quite close to the real behavior of the instrument: possible limitations are mainly due to the problem of stability instrumental.
The stability of a total-power receiver, which is one of the most important parameters for this type of receiving systems is greatly influenced by the temperature of the environment where the system is installed and the achievement of the operating temperature of the system of internal electronic device: connect all components of the receiving chain, it is advisable to finalization of the system only after one hour after power of the system.
From experimental measurements it is seen as the main factor that limits the stability of the base line radiometric (the response of the receiver when total-power radio sources are absent on the antenna beam) is the daily temperature range to which the unit external LNAC (illuminator and any front-end RF preamplifier placed outside, typically on the antenna focus parabolic reflector) is subjected: temperature variations cause very small variations in the gain of the front-end sufficient, however, to cause significant fluctuations in the reference level due to the considerable total amplification of the receiver.
The entity of changes depends upon the temperature and the quality of LNAC used.
For best performance it is recommended to stabilize thermally the outdoor unit and the receiver. This intervention is decisive for the quality of the observations.
In the following pages you will see how the Dicke-switch for the radiometer (Dicke - 1946) reduces considerably (at the expense of sensitivity) problems related to the stability of the receiver gain fluctuations of the system to changes in temperature.
The end chain of the measurement process of the radio signal is formed by the interface of analog-digital conversion which allows to record automatically and continuously the data on the PC from station, without the presence of the operator.
The following diagram illustrates the concept and highlights the importance of the correct choice of the dynamic range (resolution) of the ADC.
Is shown as a poor choice for the ADC dynamic range involves sensitive material error and distortions in the representation of the received signal.