3 m TVRO Dish Antenna
The antenna used is a re-purposed 3 metre diameter TVRO mesh dish.
The gain @ 1420 MHz is calculated to be about 30 dBi with a 3 dB beamwidth of about 60. The dish is attached to an azimuth-elevation style mount - but is motorised in elevation only. The azimuth direction is set manually by hand. The elevation movement is adjusted semi-manually via a TVRO actuator and is set by applying power to the actuator while monitoring elevation via a digital inclinometer.
The dish, therefore, has no real-time tracking ability and is operated in drift scan mode only.
The azimuth-elevation mount is the standard polar-type mount supplied with the dish, but adjusted such that the polar rotation axis is exactly horizontal - and so becomes the elevation axis...
The azimuth axis is the mounting pole and azimuth settings are done by loosening the bolts securing the mount to the pole and rotating the whole mount.
To get the elevation axis exactly horizontal required a slight modification to the mount. The travel of the polar axis setting is limited by the minimum length possible of the adjustment arm and so the lower mounting bolt was removed to allow greater movement. The adjustment to exactly horizontal can now be done...
...with the weight of the dish keeping the adjustment arm en-trapped in its normal u-bracket cup mount location
The feed at the focus of the dish is a DL4MEA-type loop tuned for 1420 MHz...
Adjustment can be made to position the feed at the focus position.
First Low Noise Amplifier (LNA)
The first LNA is mounted right behind the feed groundplane...
...and is a 0.8 dB NF 20dB gain unit bought on eBay...
This LNA model has been tested (by another party) against a Mini-Circuits ZX60-33LN and found to have similar characteristics with slightly better NF.
A 5 metre length of LMR-195 brings the signal down to the base of the dish mount where filtering and further amplification takes place.
The bandpass filter placed after the first LNA is a unit bought from M. Leech (CCERA : https://www.ccera.ca/)...
...which gives a passband from about 1395 MHz to 1430 MHz...
This filter reduces spurii from intermodulation products caused by strong signals at frequencies outside the range of interest.
After the bandpass filter two amplifiers are run in series to raise the level of the signal to overcome the loss in the 25 metre run of RG-213/U coaxial cable from the antenna base into the observatory station...
The first one is a LNA-1800 from RF Bay (NF=2.2 dB, gain=30 dB).
The second one is a ZKL-2R7 from Mini-Circuits (NF=5 dB, gain=25 dB).
The two amplifiers and the bandpass filter are housed in an enclosure made from 90 mm diameter plastic plumbing tube stock and appropriate end caps...
Second Low Noise Amplifier
The second LNA is located inside the observatory and is the same model 0.8 dB NF 20dB gain unit bought on eBay. This LNA is primarily used to amplify the signal to a level where gain settings of around 30 dB can be used on the following RTLSDR dongle to ensure enough bits of digitisation occur.
At this point in the RF chain an LNA is not needed (as a higher noise figure general purpose amplifier will not degrade the noise figure significantly), but it was available and so was used. The LNA could be replaced by an MMIC amplifier (ERA-5) without significant NF penalty.
NOTE: this 2nd LNA was removed in later activities as it was found that the dongle seemed more stable running at maximum gain (49.6 dB) with Digital AGC enabled and input levels which exercised only about 5 bits of the 8-bit ADC range. This may be entirely a local effect and no statement is made other than what was observed.
This configuration was a development of the configuration used for HI observations - but was never used operationally as the benefit from sidereal motion tracking was considered not sufficient enough to warrant the extra complexity. This is brought about by the limited view of the sky here at HawkRAO due to extensive tree coverage at the observatory site.
True Equatorial Mount
The following are details of converting the 'polar mount' of the satellite dish into a true equatorial mount.
TVRO Polar Mount
The mount supplied with the 3 m TVRO dish is called a 'polar mount' - a name, presumably, derived from the need to align the rotation axis of the mount to either the north or south celestial poles (depending on the location of the dish north or south of the equator respectively). The mount is designed to allow the dish to be moved (by the actuator) around the axis to point the antenna at various satellites placed in a circle above the equator in the Clarke Belt. The rough diagram below shows the original intent of the 'polar mount'.
NOTE: there is a typo in both the following diagrams: 34.5° should be 33.5°.
The normal TVRO satellite configuration involves aligning the rotation axis with the north or south celestial pole. If you live on the equator the rotation axis is simply horizontal (elevation = 0°), azimuth due north/south and that would be the end of it as the satellites are overhead in an arc and the dish pointing direction is at right angles to the rotation axis. However, if you live north or south of the equator two adjustments to that simple case are needed.
First, the rotation axis would need to be elevated by the same angle as the local latitude and the azimuth, once again, north for the northern hemisphere, and south for the southern hemisphere.
Second, an offset adjustment needs to be made because of the 'parallax error' at different receiving latitudes due to the satellites' distance (35,790km) being a relatively small multiple of the Earth's radius. If the satellites were millions of kilometres away then no offset would be required. For receiving locations south of the equator the offset kicks the dish north to point at the satellites, while for south locations the kick is to the south. See the green detail in the above diagram. This offset is provided by the threaded rod mounting point - 0° for equatorial latitudes and a maximum of 10° at polar latitudes.
Equatorial Mount Pointing to Vela
At the latitude of HawkRAO (33.5°S), the required offset for the Vela Pulsar is in the opposite direction to the offset for TVRO satellites. The required offset magnitude is 45.18° as shown in red in the " 'Polar Mount' Configurations" diagram above. The basic arrangement for a true equatorial mount is shown below…
The dish would need to be jacked up around pivot point 'A' by the grey bar ('B-C') by 45°, the declination of Vela. The offset adjustment threaded rod at point 'A' is normally used when pointing at satellites to provide an offset to the dish needed compensate for the relative closeness of the satellites - i.e., they are not exactly at declination 0°. This adjustment can conveniently be used to allow trimming of the declination pointing of the dish to account for small deviations of the antenna beam from the measured physical orientation of the dish.
Taking some measurements and drawing a rather busy diagram below allows the calculation of the length of the 'jack up' extension bar.
The threaded 'TVRO offset' rod can now be used to trim the declination around 45°S with a range of about ±5° to compensate for any smallish skew of the beamwidth which may occur. In the diagram the threaded rod has been set to the middle of its range. The calculated distance between the mounting holes for the 'jack up' extension bar (attached to points 'B' and 'C') is 291 mm.
NOTE: a typo was made in the drawings. An angle of 34.5° was marked - whereas it should have been 33.5°.
This 'jacked-up' mounting arrangement was, to use a technical term, a bit 'wobbly'. To stabilise the mounting on the extension bar two turnbuckles plus D-buckles were used to triangulate the mounting ring and the base of the extension bar as shown below.
Equatorial, or polar mounts, are not common amongst radio telescopes - especially large ones (the Green Bank 42m is one of the few). Probably the largest number of examples would be found in small amateur radio telescopes which are converted TVRO polar mounts (like the one being described here) or, for very small antennas, converted optical astronomy telescope mounts.
The next task is attaching a linear actuator (TV satellite actuators are fine as they have a pulse output to keep track of the movement of the actuator arm. The actuator is attached to the standard mounting point.
The upper mounting point attachment also had to be modified due to the dish being jacked up at 45° from its usual TVRO position...
Further development was terminated as the extra complexity required to add declination pointing to sidereal tracking was considered too complex for the benefit received.