May 10, 2003 - Building a test signal generator
I needed a test signal in the 2400 MHz range. I had read in a number of places about using oscillator blocks from old PC's. Since these things basically generate a square-wave, they have lots and lots of harmonics, which can be used as a test signal in a higher band:
As I happened to have some 20MHz blocks laying in my scrap parts bin, that is what I used: 20MHz * 20 = 2400 Mhz, so this should work! I used a 9v battery, a 7805 to get it down to 5v, and I soldered it all into an empty Altoids can, using the can as the ground! I added a little power switch and led, and an RCA connector as antenna jack, and there it was! Here is the hand drawn schematic:
| This is what the final result looks like: | |
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May 10, 2003 - Building my first feed
I decided to put together a helix feed against a square backplane. Since I did have a some #10 copper wire laying around, that is what I used. I built the 4 1/2 turn feed found in Helix feeds for MMDS Dishes. I did not have any metal around to build the coffee-can backplane, so I made a square backplane from cardboard box covered with aluminum foil (this technique inspired from the QST article that got this all started!) I folded the foil around the edges and then taped it on the back!
I made it 125mm by 125mm, which turns out to be 1 lambda (300/2401=0.125; this is in meters, so 125mm!) I carefully measured the center of the square, took into account the radius of the helix (20mm), and located the center of the chassis mount N-connector 20mm away from the center, such that the helix should be centered over the square after soldering it to the N-connector. I put small strips of foil around the edge of the hole to get better continuity.
| You can see the feed here mounted on the converter You can click the image to enlarge it. You can see some of the strips of foil I used to layer the whole. |
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This is a different view of the same concoction. |
May 10, 2003 - Testing what I built so far: the test signal generator and the helix feed
I built a small 1/4 wave antenna for 2.4GHz on an chassis N connector (a 3.1cm piece of copper wire soldered to the center). This I hooked up to my down converter, which in turn was hooked up to my FT-100 radio. I turned on the signal generator, put a little piece of copper (again 3.1cm) in the RCA antenna jack, and could hear the ground frequency around 20.004 MHz. A little high, but good enough. This would mean that my 2.4 GHz signal would be around 2400.480 MHz. With the down converter, this would be around 144.480, give or take some to compensate for the crystal drift of both the test signal and the down converter.
I started tuning around 144.480, and low and behold, there was the test signal at 144.435, drifting down slowly. Ultimately, it seemed to stabilize around 144.417 MHz. I found that I could move the test signal away from the 'antenna' by a few feet before I lost it.
Next, I put my homebrew helix feed on the down converter, using a dual male N connector. And indeed, I got a somewhat longer range, about double, and definite more directional sensitivity then the previously used quarter wave antenna. All of this was very encouraging!
May 17, 2003 - Hooking it up to the dish
| I used a pry plate to connect the feed to the dish's LNB arm: |
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I had also gotten an old, but sturdy, tripod from another colleague. I used
3/4" PVC pipe pieced to fit it all to it: a piece of pipe with a 2 T-coupling
worked perfect. One T on the pipe out of the tripod, to be able to go sideways;
one T screwed into the bottom of the dish LNB arm:
From the above, I had learned that I could use a string to find the focal point of the dish. I made a knot a string, and marked it 278mm on one side (focal length; from W1GHZ's site), and 466mm on the other side (top string length). I used this to bend the dish-mounting contraption into the right place, basically aiming the helix to the center of the dish, inline with the mounting arm.
| This is the completed tripod assembly; you can clearly see the PVC mechanism: |
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This is what the 'helix mount' looked like; basically some tape to hold it to the mounting plate. |
I get the hole gizmo hooked together just in time for the tail end of a pass on Saturday, May 17. Earlier in the day the squint angles were very good (getting as low as about 6 degrees), but by the time I got to trying the squint was up to around 20 degrees.
I tried hard for the middle beacon, but did not here anything :-(
