Configuration #1 - HI Line


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.

Bandpass Filter

The bandpass filter placed after the first LNA is a unit bought from M. Leech (SBRAC)...

...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.

Line Drivers

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.