Projects and experiments from the shack of KO6TH
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Site last updated 15-July, 2007
email: KO6TH -at- amsat.org
Homebrew AZ/EL Rotor and controller
|AZ/EL controller built around a Basic Stamp-II. Meters display 0-360 AZ on left, 0-180 EL on right. Manual positioning as well as a serial connection to FODTRACK for computer control. A paper describing the design is here; software here.|
|The rotors: The AZ rotor is a light ham-type of unknown brand; the EL rotor is an Alliance TV type mounted on its side. Both report position by contact closure. The controller counts the closures to determine position.||The EL rotor is covered with a plastic bag (removed for the photos) to protect it from the weather, as it is not designed to be mounted sideways. So far, no problems with the bearings. Cross-arm is a wood closet pole.|
|Scenes on the roof. These were taken before the current adventure into AO-40's S-band downlink. The antennas are a 2x8 circular yagi on 70cm, and a 8 element 2m beam positioned vertically. The trees to the back of the house are a problem for AO-40, but other satellites come through fine. They also add camouflage.|
AO-40 S-band downlink experiments
|MMDS "BBQ Grill" antenna and original feed. Mount is a 1x4 board, bolted to the cross-arm, with the antenna screwed to the board. Drake-2880 and Conifer-1033 preamp mounted on back of the board. Made 2 QSOs on AO-40 with this setup, 2 more with wider grill below and original feed.|
|Drake modifications: Changed the crystal to 8203.125 khz, and the chip cap in the PLL to 10pf. No other changes were made (i.e. all of the IF coils remain in place). This moves the local oscillator to 2100 mhz, which puts the Ham 13cm band within the Drake's unmodified IF passband (222-408 mhz). AO-40's 2401.323 mhz S2 beacon is received at 301.323 mhz.; the conversion math is easy! IF receiver is an ICOM R-7000 all mode scanner. 60' of RG-6QS coax links the Drake to the ICOM. The grill measured 15" x 32", later replaced with a wider one (24" x 32").|
|New feed: 3 1/2 turn helix in coffee can. Preamp and Drake mounted directly behind feed, on 1" hardwood pole. High Tech signal source for testing - 5x47K resistors and a 1N4004 diode driven by my HT at 144.1 mhz. The 17 harmonic is 2449.7mhz.|
|Upgraded to a wider grill, covered it with metal window screen. Mount now at balance point on arm, which makes less work for rotor. The effect of close mount on 70cm antenna is unknown at this time, but original grill caused no ill effects.|
|Tweeks: rebuilt the canned helix with the connector soldered to the can for a better ground, and the matching section bent down to offset the "bump' on the back of the N connector. I also shortened the helix to 3 turns, but this didn't seem to matter.|
|Finding the dish focus with foil and the sun. The focus appears to be at about 10". Experiments with the signal source and completed dish gets the best signal strength with the focus at the open end of the can.|
The changes to the antenna and feed have improved things over the original setup. At this point, I am able to receive decodable telemetry from the S2 middle beacon, and carry on a few QSOs on the transponder, with the satellite out to about 30k km and squint angles to about 25 degrees. I've also copied the S1 engineering beacon and had one QSO on its transponder at 55.5k km with a squint of 12 degrees. The S1 transponder is much stronger than S2, and I'm sure I could have successful QSOs on it farther out than the one pass I have tried so far; we exchanged 5x6 reception reports for the SSB QSO. But, I cannot hear Sun noise, nor the weaker signals on the transponder. My IF receiver (ICOM R-7000) is pretty deaf, and the Confier preamp not very strong, and my guess is that between the two I'm limited in what I can hear. Next step is probably to order a better preamp (DEM, likely).
Monitoring the S2 beacon and transponder at MA 93, with a squint angle of about 11 degrees. Altitude is 55.5k km. Beacon is copyable but with less than a 50% good CRC rate. Transponder QSOs can be heard slightly above the noise level. I can hear my own uplink on CW and SSB without meeting Leila with 30-40 watts on UHF. Also copied SSTV signal from WC9C with a P2 picture quality.
|Tower Cam: HSC had a special on mini CCD cameras, so I thought it would be useful to put a camera on the tower to see what (trees) my antennas were pointing through. To protect it from the Sun, a solenoid-driven shutter was rigged over the lens. A 3"x6" box from Radio Shack and some clear plastic from a CD cover makes it weatherproof.||
View from tower cam looking due South at the horizon The end of the 2M beam is at left. The vertical to the right of center is a 2M j-Pole.
Wow, what a difference a better preamp makes! Keeping everything else constant, I swapped out the Confier preamp for a Kuhne MKU 232 A2 TM unit. The test sheet that came with it shows a noise figure of 0.65db, and a gain of 40db; significantly better than the Confier. With a squint angle exceeding 33 degrees, and shooting through several trees, I can still copy the beacon and some faint SSB QSOs. They are about as strong as I remember the signals with the very first setup, under much much better conditions. It also seems that I can decode telemetry better with signals that are close to the noise level, than before. I haven't tried transmitting yet, as I am concerned about the proximity of the 70cm uplink antenna. The Kuhne is rated to only 1mw of RF on the front end, and there have been reports that this is easy to blow. The plan here is to modify the feed antenna to reject 70cm, through some sort of stub. Note the preamp's weather proof cover must be mounted right-side up. Since my rotor supports flipped operation, I also need to figure out some sort of weather covering for the bottom side.
|The preamp is intended to be powered with a bias-T, but under the cover they have also provided an external 12v feed-through, which I brought out through the base plate at the right. The 3-turn helix is to the left, and the Drake 2880 downconverter hangs below.||
View from the Tower Cam. AO-40 is behind these trees!
Y2K didn't all happen in 2000! In 2000, my favorite satellite tracking program, PCTrack version 2.14, had a mild Y2K problem, where the 2-line element "convert" program wouldn't correctly read the year. 2000 was read as 1900, 2001 as 1901. It was easy to go back into PCT's Setup/Satellite menu and fix the year by hand, which is what I have been doing. Then came 2002 which was read, as expected, as 1902. But the editing was refused (came up zero). So, after suffering for 3 months without it's compact screen display, and wonderful pass prediction table (via pctscan), I've written a binary file editor which will patch the objects.dat file for the correct year. The PCT214.ZIP file and PATCH.EXE files are available here, if you wish to give them a try. Run the convert program to update the Keps, and then the patch program to fix the Y2K.
Finally got a chance to rebuild the mount for the S-band grill's feed. The theory was that the wooden pole through the middle of the canned-helix, which touched the uninsulated helix winding in a few places, was absorbing a significant portion of the receive signal. The only solution was to use something off-axis to mount the helix and preamp. It's a little hard to see in the pictures, but the resulting design called for three threaded rods (#10 by 24 thread, about 2 feet long), and a homebrew bracket to tie it all together under the preamp. Two of the rods spread out and hook to the sides of the grill, with the third one to the bottom.
The mount for the grill had been on the on-axis mount. Since this no longer existed, I mounted the dish in the more traditional way, on the back side. This put the whole antenna out of balance, but the Alliance elevation rotor seems to be able to handle it. I placed the Drake downconverter at the back end of the remaining on-axis pole, as a bit of a counter weight. It isn't quite enough, but it helps. A side benefit of the new mount is that it removes the horizontal rotor pole from being in the antenna's field of view, which should also improve the signal level.
To not change too many things at once, I left the canned helix
as-is, including the big hole now unfilled in the back. At this
writing, I have only tried receiving one pass of AO-40, and things look
promising. The portion of the pass received was from about MA 230
to 245, with really bad squint angles (60-90 degrees), and an elevation
of 20+ degrees, through the edge of a tree. Signal strength was
stronger than I had expected (but still S-zero), and I was able to hear
the beacon, and copy 30% good CRC frames, nearly to the end of the
pass. The transponder was off during the time. Next
favorable pass will be in about a week...
|New feed mount, using threaded rods from local hardware store. The threads allow for repositioning the feed/preamp assembly easily, as I expect to be trying out new feeds in the future. The preamp now drives a 4 foot piece of 9913 to the downconverter at the rear of the grill on the pole.||
Close-up of the custom mounting bracket.
Whole setup. Drake is at the end of the central boom, on the left.
Closer image of feed, preamp, and mounting rods.
Good pass this morning. In between checking in to the Amsat Net (14.282 at 1800Z Sundays), I find AO-40 near apogee with a good strong signal. Still not moving the S-meter on my ICOM R-7000 "IF" receiver, but getting real close. For the first time, I am hearing stations that others are having some difficulty hearing. Just missed contacting UA0LMC, as he set before I could get to him. The transponder cut me off twice as the scheduler switched into RUDAK and whatever the other item is. Oh Well. I'm still happy with the results, and the contacts with W6FOG/MM and KG6IAL. But, based on comments on the Amsat-BB, I think I'll play a little with the feed position (relative to the grill's presumed focal point), to see if I can tweek things a bit better.
"Failed" attempts at a better helix feed. I'm still getting a lot of interference when I transmit, and the plan was to insert a 1/4 wavelength open stub between the helix and the Kuhne preamp. Not finding a N-Tee at a local surplus store, I tried using SMA parts. I tuned the stub on the bench, substituting a 50 ohm dummy load for the antenna and watching for a peak in SWR from my 70cm tranceiver. Bad idea. The notch wasn't at the same frequency when things got assembled, and the result was totally ineffective. No pictures taken.
New helix, 3 1/4 turns, in a large pineapple juice can. Lip is about 1" depth. This one seems to work well.
The new helix mounted on the preamp. The preamp location relative to the focus is unchanged, so the helix is farther from the dish. The new helix was given a light spray of black paint to retard rust later.
A view of the whole setup. From the left are: 70cm antenna, 2.4ghz dish, rotor in the middle, tower cam in small box, and the 2m beam on the right.
An attempt to lower the frequency of the bandpass filter in the Drake 2880 downconverter. Several layers of tape are placed over the traces. I think this helped, but have no measurements to prove it.
After abandoning the stub, at least for now, I returned to the helix and tried rebuilding it (again) without the mangled brass shim. Used a shallower can this time, slightly larger (4 1/8"), looking for a bit better illumination (maybe too much?) and a 3 1/4 turn helix. I also built the helix with a N-male connector, so I didn't need the barrell connector between the helix and the preamp. This should lower the receive noise figure (fewer connectors), but it had the effect of moving the feed point an inch or so further out. Somehow this didn't seem to matter much, and the new feed works very well. Of course, now that I have a good feed, AO-40 squint angles are horrible, and the transponder is off... Also tried to re-tune the Drake downconverter's filter to a lower frequency by placing several layers of tape over the traces. The recommendation on the Web was to use Kapton tape, but I don't have any, so I used some household "Magic" tape (3M). Seems to have helped, based on very subjective tests on the bench.
AO-40 squint angles are excellent, and the beacon is finally strong enough, even at apogee, to just wiggle the S-meter on my R-7000 receiver. Early in the pass, I get a peak reading of maybe S2. But, I'll take it. SSB QSOs are much easier to copy, instead of an ear strain.
Also, I have located the cause of the interference when I transmit. The contacts on the left/right-hand polarization switching relay for the 70cm uplink are going bad, and when they aren't seated well, I get a lot of noise. Cycling the relay a few times will usually get it to settle down, and I can talk without much of an increase in background noise. The antenna remains inches away from the edge of the dish.
Closeup of feedpoint connection and spacers. Antenna helix element is 8-guage aluminum ground wire that used to be sold at Radio Shack.
Another view of feed point. Copper foil is wrapped around center pin of N-connector and screwed to the flattened antenna element, then the foil is soldered to the pin.
Mounted antenna on boom. The perceptive will note that the support beam is longer than the element - I ran out of wire after about 18 turns.
Another view of the mounted antenna. The 2m beam is to the left, then the new helix, 2.4 ghz dish, and the 70cm CP beam at the far right.
AO-51, that marvelous bird, was in mode LS last week. Another chance to play on 1.2 ghz! The Helix antenna, now completed, was pressed into service. The downlink is unchanged from the AO-40 days, so other than age, it should function just fine on a LEO. I only had 3 nights and 4 passes for testing. The first night I heard nothing. No downlink, and no change when I keyed up. There were reports that the change-over to LS might have been delayed, so this could have explained it. Next night, I heard some weak signals, and parts of a couple call signs. Nothing from my side, however, when I keyed up. Finally on the 3rd night, we had a high pass slightly to the west, and I was able to have a nice what with two other hams. They reported my signal was good, though I still have a bit of an issue on the downlink side. Way too much QSB (could be the satellite itself?), but the Helix does seem to function. It's hard to say if the Helix or the Quagi is the better of the two antennas. It will take a number of additional passes, or the discovery of someone within a reasonable line-of-sight of here, to make the comparison. In the mean time, I'm on the hunt for a longer piece of #8 aluminum ground wire, now that Radio Shack is all sold out.
Suggestions are welcome! Please e-mail me if you have a "better" idea.