Radio Astronomy SA

An antenna is a specialized transducer that converts electromagnetic (EM) waves into an electric current or vice versa.  Antennas are used to receive (or transmit) non-ionizing EM fields, which include radio waves, microwaves, infrared radiation and visible light.  In our application, a receiving antenna intercepts EM (radio) waves transmitted through the Earth's ionosphere and atmosphere from space.  From these waves, the antenna generates a small amount of current, which varies depending on the strength of the original EM wave.  The current is coupled/passed to a low noise amplifier and receiver system for processing.

This antenna is a log periodic dipole array (LPDA), a design which provides a moderate gain with a wide bandwidth and reasonable front to back ratio.  The LPDA follows standard design procedures where the length of the rear element is set for about 7% lower than the lowest design frequency and the length of the upper element is set at about 1.3 times the highest design frequency.  For guidance, several texts were consulted to confirm the suitability of the LPDA for this task.  A complete design description and supporting calculations were taken from an example beginning on page 554 of the text “Modern Antenna Design 2nd Edition”.  Dimensional calculations for the LPDA were confirmed using the java script calculator(s) found here and here.  The key characteristics used were: lowest frequency;  92.5MHz,  highest frequency;  1000MHz,  diameter of shortest element;  12mm,  characteristic input impedance; 50Ω,  relative spacing;  0.150,  taper;  0.900,  number of elements;  28,  maximum boom length;  < 5000mm.
Basic 6060 grade aluminium materials were selected and purchased as standard lengths of 6.5m (12mm tube) and 7.0m (25.4mm hollow square) stock.  The heaviest wall material was selected due to the lengths required.  Aluminium is easy to work and cut in a garage or open area using basic hand tooling.  Due to the length and weight of both booms and elements, the structure needs to be constructed and fixed with materials suitable for outdoor conditions.  The 12mm tube has a wall thickness of 1.6mm, making the internal diameter of the tube approx. 8.8mm.  Suitable machine screw sizes to suit a drilled hole of this diameter and tapped were M10-1.25 or 3/8”UNF.  Funnily enough, it’s easier and cheaper to buy 3/8”UNF-24 pitch screws and taps in this country than M10-1.25 pitch, so that’s what was used to fix each of the 28 elements to both booms.

We needed to make insulators from a material with good ultraviolet (UV) radiation characteristics and sufficient strength to hold the LPDA structure together in all weather conditions.  To provide adequate strength, 25mm High-Density Polyethylene (HDPE) was selected and proved relatively easy to cut and mill.  A linear taper from 3.5mm (feedpoint) to 45mm (stub) is made by four insulators spaced approx. 1.2m apart, centred on the balance point of the antenna.  This also allowed us to create an electrically isolated and workable coupling to connect the antenna to the solar tracking mount.  We were fortunate to have ASSA member Alan willing and available to manufacture the four insulators for us.

This antenna is a Long Wavelength Array crossed-dipole antenna.  The antenna consists of two independent tied-fork dipoles with their vertical planes perpendicular to each other.  Each blade (or wing) is sloped 45° to improve response to radio waves whose plane of polarization is not aligned with the antenna axis.  The horizontal components are fiberglass rod braces.  The frequency range of the LWA antenna is approximately 10MHz to 90MHz.  The design details are freely available on the interblurb so we won't bother repeating them here.  The primary variation between the original (Burns Industries) and the ASSA version is the 600mm² concrete base we used, compared to the in-ground post (STD).

This Murchison Widefield Array antenna (MWA) is a dual, linear-polarization, wide-beamwidth, crossed-dipole over a ground screen.  The crossed-dipole dimensions are 74cm (L) x 74cm (W) x 50cm (H).  The frequency range of the MWA antenna is approximately 70MHz to 300MHz.  The design details are freely available on the interblurb so we won't bother repeating them here.

At the MWA site in Western Australia, these antennas are deployed in small phased arrays of 16 individual antennas in a planar,  4 x 4 grid per tile.  This makes a total of 4096 antennas (256 tiles) onsite.  Very impressive indeed!

At Sunnydale, we have one (1) individual antenna deployed onsite.  A more modest installation in our case.

This antenna is based on the Moxon antenna, which is one of the favorite antennas among amateur radio hobbyists.  The Moxon antenna consists of a bent dipole over the ground reflector which produces outstanding front to back ratio of radiated power, good match over a relatively wide band and lower elevation height.  In essence, it is a two element Yagi-Uda antenna.  Cross-Moxon antennas fed through a suitable SATCOM (225MHz to 450MHz) hybrid quadrature coupler may be used to obtain circular polarization when used with a pair of Callisto Spectrometers.  Extending the width of the strip of the dipole elements leads to wider bandwidth and improved cross-polarization ratio.  While we don't have to capability to model and test this antenna properly, we should be able to tune it to an acceptable level of performance for the task with a bit of tweaking.

Unlike the Phase 3 Spectrometer antenna system, the Phase 4 Spectrometer antenna system is kept in 50Ω impedance up to the Callisto Receiver(s) RF Input.  This is because the LNA output impedance is 50Ω, the LMR195 coaxial cable is 50Ω and the Merrimac QHM-2-333.3 coupler impedance is 50Ω on all ports.  The isolation provided by the coupler minimises the VSWR effects of the receiver's RF Input impedance of 75Ω on the Phase 4 antenna system.  This link to Impedance Matching Issue with the CALLISTO Solar Radio Spectrometer is a discussion on the subject of the receiver's mismatch .

This quarter wave ground plane was purchased from eBay store Ancient Electrons. The seller was happy to cut the elements to suit the frequency change from 440MHz down to 401MHz. Apart from the clunky UHF connector used, which could easily be replaced by a slightly less clunky Type N connector if need be, it has turned out to be very good.
From Middleton, SA it will pick up radiosonde transmissions from around Mt Gambier, 320km to the southeast without any trouble at all.  Occasionally, the radiosonde tracking software will lock to and decode balloon flights from Mt Gambier over concurrent flights from Adelaide only 80km to the north.
This antenna is constructed from two 3D printed components, stainless steel elements and some cable ties.  Once you have one, you could easily have a couple of spares without too much effort.

While the above quarter wave ground plane antenna above has proved to be a good choice, it's omnidirectional characteristics can be annoying when concurrent weather balloon flights are nearby.  This LPDA was made up from a Fracarro LP345HV  TV antenna with the 16 elements arranged and cut for 403MHz.  This should provide some directivity and additional gain for tracking flights heading eastward toward the South Australia/Victoria border.  The primary disadvantage of using this LPDA is that it's characteristic impedance of 75Ω will require some impedance matching to the receiver.