Radio Astronomy Antennas

 

For HF reception of Jupiter Noise storms, the well documented and popular choice is the phased dual dipole. For those of us with a little less space to play with, a single dipole with a low noise amplifier (LNA) before the receiver has to suffice. In addition to the Radio Jove designs in the book “Listening to Jupiter 2nd Edition”, there are many designs for Yagi-Uda's in this wavelength which are freely available on the net but they are quite large. Hydrogen and other spectral line monitoring systems usually rely on parabolic dish collectors for the high gain required. The signal is reflected and focussed to a feedhorn or helix to couple signals to an LNA(s) and receiver(s). A 3.0m diameter dish can provide about 30dB of gain at 1420MHz with a beamwidth (resolution) of about 4° of the sky. The same size dish will increase in gain and its beamwidth will decrease (higher resolution) at higher frequencies, assuming the surface accuracy is adequate.

Antenna Aperture

A receiver antenna aperture or effective area is measured as the area of a circle to incoming signal as the power density (watts per square metre) x aperture (square metres) = available power from antenna (watts).

the aperture of different antenna

Antenna gain is directly proportional to aperture and generally antenna gain is increased by focusing radiation in a single direction, while reducing all other directions. Since power cannot be created by the antenna the larger the aperture, the higher gain and narrower the beam-width.

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.

Simply increasing the size of antenna does not guarantee an increase in effective area; however, other factors being equal, antennas with higher maximum effective area are generally larger.

It seems obvious to optical astronomers that a parabolic dish antenna that is many wavelengths across, will have an aperture nearly equal to their physical area. However other antenna such as a Yagi and Col-linear arrays may not look to be the same at first glance but they do achieve the same result using other means at radio frequencies.

the aperture of different antenna


Antenna Polarisation

Most natural signals (i.e cosmic sources) are almost always non-polarized (which is the same as "random polarized"), so the use of any single polarisation method either linear or circular will achieve the same result. The slight polarisation present in such signals do not bring any significant "power advantage" so in practice linear polarised antennas are preferred more in Radio Astronomy as they are more practical to construct for a specific gain over a circular polarised antenna.

Polarisation can however carry interesting information about the source, so radio astronomers sometimes want to measure this. However it is quite difficult to do, because the signal characteristics are so weak, and below a few 100 MHz, the polarisation information is usually too mixed up by the ionosphere to be of any practical use.

Log periodic Antennae

Broad band 'Yagi' antennae are some times used if there is a need to receive a large range of frequencies with the same antenna, as in the e-Callisto Solar Radio Spectrometer 45-870MHz.

Peter has a write up on the construction of a 5 metre long log periodic antenna for this receiver here.

 Radio Telescopes

There is so much written about Radio Telescopes by the professionals, it seems silly to try and write another.

The link Radio Telescopes takes you to an excellent article by the National Radio Astronomy Observatory (NRAO)

Computers and Software

You don't need these unless you are going to take the Software Defined Radio (SDR) option but they do come in handy for just about everything you'll ever want to do. The main thing to remember is that whatever hardware platform you choose, if you are going to do digital signal processing (DSP) you're going to want a fast processor because lots of DSP is quite heavy going for the computer.

Software is available from many sources and you may even have to buy some, god forbid.

Most DSP software contains a fast fourier transform (FFT) spectrum analyser, waterfall display spectrograph (frequency & amplitude/time) and audio record/playback function from your radio via the soundcard or from files on your selected storage disk. The more exotic packages offer additional capabilities such as auto correlation and other advanced noise reduction techniques.

If you are planning to take the SDR option you'll probably get a DSP package with the receiver, then again, maybe not. The USRP is made to work with the GNU Radio suite on a Debian Linux operating system (OS). WinRadio G3xx receivers are primarily made for the various Micro$oft Windows OS and come with standard or optional DSP packages, with limited resources and support for operation under Linux. For other SDR models, check the manufacturers sales information regarding hardware/OS/software requirements.

Some links you might find helpful are:

Linux

Windows

MacOSX


Receivers

Lots of choices here but they generally fall into the two categories of communications receivers/ scanners and software defined radio (SDR). In the first category, these receivers tend to operate up to several hundred megahertz and are usually reasonably sensitive. If you have an old shortwave receiver, first dust off the spiders, connect it to your new 15 metre band dipole array and you should receive the Sun or even Jupiter if you're lucky. The quality HF communications receivers used by ham radio enthusiasts are a good option, there is quite a lot of software support for the Icom IC-7000 Series and if you look hard enough, quite a few others as well. There are also many ARRL members and enthusiasts developing their own radio hardware which are often better than many commercial models.

Some links you might find helpful are: Rick Campbel KK7B & Bill Kelsey N8ET R1/R2 & Mini R2 Pro Direct Conversion Receiver QPL2000 Project

SDR is the new toy of choice in the radio world. There are many models appearing not only in the HF and amateur radio bands but wideband models as well operating well into the gigahertz ranges.

There are also many ARRL members and enthusiasts developing their own SDR hardware.

Some of the more popular models are listed below along with links to their respective websites: