Parabolic reflector for HRPT weather satellite reception and Moonbounce EME communication
The setup consists of the following :
1) Initially the dish was a 2m (4 pieces) reflector with f/D=0.5 from Gert Liebal DG0CN. Recently extended the dish to 2.5m and f/D became 0.4
2) Big RAS SPID rotator, 0.5 deg pointing accuracy. Controller used : SPID or homemade “EME FUN antenna controller” (see related article)
3) The “EME FUN software” is used to track satellites/Moon/Sun and controls the radio in the shack (see related article)
4) For Moonbounce a home-made septum feed is used for 23cm based on the W2IMU / N2UO design
To improve Tx/Rx isolation up to 60 dB a metal disk has been placed additionally in the dual mode section of the feed (see details below).
For HRPT reception at 1.7 GHz a helical feed is used,
5) Home-made 45 W PA based on XRF286
6) G4DDK LNA (MGF4919G) (measured NF in the lab 0.3 dB)
7) Home-made 144/1296 MHz up/down converter and PLL LO (1152 MHz) . Currently adding an option to use 10 MHz input as reference.
8) 28V, 13.8V power supplies
9) Home made PIC-based VSWR and over Temperature protection
The 2m dish initially installed resulted in 10.2 dB sun noise at SFU=94 (10.7cm )using an Agilent E4404 spectrum analyzer. Sun noise without the metal disk 9 dB
Measured 0.32 dB system noise figure from the input of the LNA to the IF (144 MHz) using an Agilent 346A Noise Source
Metal disk placed to improve Tx/Rx Isolation
HRPT feed can also be seen on picture above
Extended dish to 2.5m
Measured Isolation between Tx and Rx ports with a E5071 network analyzer
The feed alone looking at cold sky results in better than 45 dB Tx/Rx isolation , but when the feed is at the focus of the dish (f=1m) the isolation becomes only 13 dB. This means that from the ~300K at the Tx port, 15 K would reach the LNA port (Rx) ! For a W2IMU feed and a 0.4 f/D dish minimum antenna noise temparature is close to 10K, a 0.2 dB LNA adds 14K,thus totally 10 + 14 =24K (LNA protection switch not taken into account, typically 0.05dB loss or 3.5 Kelvin)
Now one can imagine that adding extra 15K due to poor Tx/Rx isolation significantly reduces sensitivity !
If we do the math, the improvement in sensitivity would be 10*log10(24+15/24) ~=2.1dB !
Implementing RW3BP’s excellent suggestion, to improve Tx/Rx isolation a metal disk of about 73mm diameter has been placed in the dual mode section of the feed.
The exact position in the dual mode section and the diameter had to be found by trial and error
Later though a simulation of the entire setup in CST Microwave Studio (feed plus 2m dish) confirmed the dimension/position found.
See screen shots below where the radiation pattern of the dish-feed setup is depicted.
Running an optimization of the coin/metal disk dimension and position, revealed that the values found by trial and error were very close to the simulated ones.
Edited by Michael Margaras , SV1CAL