SECTION 2 — Antenna
Click for the Master Index to this project
Like for amateur radio and SWL DXing, the antenna ranks as somewhere between important and crucial in amateur radio astronomy.
The current most recommenced antenna for monitoring Jupiter and perhaps the galactic background = a pair of dipoles with a power combiner plus a delayed feed line to correctly phase the antennas. The phasing line length establishes the proper impedance match plus obtains the desired radiation pattern. This gives ~~ 3 dB directional gain over a plain old dipole at the same height.
Using a registered version of Radio Jupiter Pro, a program written by Jim Sky I learned that for 2017; at my latitude, a pair of east-west dipoles up 6 meters with a 135 degree phasing line in the south dipole’s feed will give the greatest gain to receive Jovian storms. Jupiter’s peak elevation is ~ 35 degrees above the horizon and this set-up maximizes directional gain to match Jupiter’s path.
Big problem. My lot goes North to South and poses some challenges to this ideal antenna. On August 14, 2016 I erected a single, east-west oriented half-wave dipole up ~4.9 meters high. To change the beam pattern, my only option is to raise the antenna up or down on my 2 anchor poles.
I tuned it to a center frequency of 20.150 MHz with a tracking generator/spectrum analyzer plus a return-loss bridge. When I connected it to my RF filter/preamp, main receiver, and AF amplifier with speaker output, the galactic background was strong, although local noise was much decreased compared to the 40M band 1/4 wave vertical antenna that was in the exact spot.
I built 2 wooden supports from 2X4 lumber and some plywood. They rise 1.22m ( 4 feet ) and hold a 4.37m (14 feet) hemlock pole inserted 0.6m ( 1.5 feet ) down inside each base. Thus the top of my poles lies at 5 meters height ( 16.5 feet ). Wire sag drops the dipole distance from ground down to about 4.9 meters. I can move these 2 supports to change the dipole direction and currently it runs exactly North to South. I painted them “radio observatory white”.
Under the grass below my dipole lies 43 buried radials, some of which are 18 meters long.
Above — Across each of the three 2X4 legs of my support lies a chain with some nylon tent poles driven into the turn. My poles cannot fall down. After installation, I irrigated the lawn and the grass looks lush and green now.
Above —a skyward view of 1 antenna support rope. 1The actual dipole was constructed from 16 gauge copper wire.
Above —a skyward look at the feed point when I initially erected my antenna. I can lift the wire up or down for maintenance by tipping the poles and can tighten the slack by separating the poles. I’ve got 6 FT43-43 ferrites over the coax at the feed point.
Radio Jupiter Pro shows, that at my location, Jupiter will move above the horizon at night-time in ~February 2017. My local RFI levels fall off nicely and I’m able to monitor signals only at night. I’ll keep this antenna up and test it in 2017. I’m hopeful to gain some dipole reference signals to compare to any other antenna schemes I might employ.