How I got a free barbeque dish...and heard AO-40...
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On Friday, June 6 2003, right after lunch a great thing happened: one of my colleagues, who knows I am working on this, and is playing with antenna's with 2.4GHz for Wifi, gave me a barbeque dish used for MMDS that he had scrounged from a neighbor's garage. |
This is a high gain, probably 20db+ or so, rectangular dish, with built-in feed dipole and small coax pig tail with N-connector. It was originally used in the 2.1-2.7GHz range for wireless isp and the dipole should be fairly broadband. This is perfect! Combine this with a nearly picture-perfect pass Sunday morning roughly 5am-12:30pm (Orbit 1197), as well as a great window tomorrow morning from 10am-12:30pm, and I should be able to hear the middle beacon (2401.323MHz +- doppler) and probably qso's!
Upon inspecting the dish, it turns out to be a Lance Industries MMDS dish, model 2524. Various places have information in this dish:
N9ZIA's page with details and inside view of the dipole feed
K5OE's MMDS page with in-depth analysis and mods to add a helical feedhorn
Nominally a 24dBi dish with a dipole feed, I should be able to hear AO-40 easily with this without any modification. At circular, this should give about 21dBi gain (-3dB for the loss of circular)
Cheap Man's Rotor for about $3
| The following Saturday I made a cheap AZ-EL rotor in the afternoon: basically Iused a rotary tool to hollow out the stops on a 1/2 inch PVC T, such that a piece of 1/2 inch pipe would slide through it. Next, I drilled 2 small holes on the side for screws to go through. Putting this on another piece of pvc pipe on an old tripod, also from a colleague, makes for a cheap rotor (about $3 in parts.) Turning the T on the pipe in the tripod controls azimuth, and loosening the screws in the T allows me to change elevation of the pipe! (Click image to view) |
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Setting Up For Another Attempt
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That Sunday, June 9, 2003, I started setting up in my driveway around 9:45am. I was ready to start listening right before 10am. AO-40 was at 52 degrees Elevation. and 180 degrees azimuth, due south, with a squint of less then 10 deg, i.e. antenna's pointing almost straight at me! |
| I set azimuth with a compass, and aimed elevation with the same compass
by hand. After some tuning around 145.323 ( beacon is at
2401.323,+/-Doppler and converter drift) I heard a very faint signal at
145.322 on the ft-290. Careful listening found it to be computer data
indeed. Then I tweaked the elevation, and it turned out I was too low. The
grid dish turns out to be pretty sharp in the elevation plane, and once I
tweaked it, I could not believe the signal; the middle beacon signal was S7
above the noise floor! Between then and about 11:30am when I packed it up,
the bird stayed roughly at 50 deg. el. and went to 210 deg, so I only
tweaked the dish once! At 11:30am the satellite was at nearly 60000 km at
MA115 (ie almost at Apogee)! During this period, I heard about 15 or so
different stations, some usb, some cw, all US and one UA7 in the end when
the bird hung over the Pacific. I also heard Leila all over the passband, at
seemingly random locations. I guess this is due to the radar interference I
have read about.
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I did some experimenting with my other radio, the ft-100, by using a TV
splitter in the downlink signal, and found that this radio was far better at
dealing with the environmental interference, and gave a clearer signal,
particularly on weak stations, esp. when using the built-in DSP functions. I
guess this is to be expected, about 15+ years newer technology here :-)
(The pictures to the side show both radio's tuned into AO-40's Middle Beacon
during orbit 1197) I tuned outside the passband, and imagined a slight drop in noise volume; this would mean I could hear the transponder noise. Not sure if this is possible with my setup (21dbic dish, with 0.93db NF in the down converter.) I also pointed the antenna to the sun; very easy by minimizing the shadow, and found a slight increase of noise compared to cold sky noise. |
I had a blast, it was very exciting to hear that first beacon signal. Now it is time to build an uplink antenna, and try to hear myself. Also, I want to improve the dish by adding a mesh to it (it is a horizontal grid), and a helical element, to get circular polarization, as well as less fading. (See the great article by K5OE referenced above.)
More later!
