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happyelectron

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About happyelectron

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  1. The problem with the unpointed antenna approach is that it is hard to get the kind of gain one could get with a well pointed dish antenna (or even patch). I've heard about using a GPS to point the tracking antenna, however, I like the idea of also using the signal strength information like these guys have. Also, I agree that a dish or helical is a real hassle to transport. So maybe a tracking patch setup with 5 patches would have some benefit. Anyway, it is neat to see a tracking system work so well.
  2. Sorry for the bad link, and Mr. RC-Cam, thanks for correcting it! I was wondering how they were getting the "circular" in the system. It makes sense to put a circular patch on the plane so that the linear tracking Yagi's always have something to pick up. However, the only thing I don't like about a circular transmit patch on the plane is that it is pretty directional. Otherwise, this is definitely one of the better set-ups that I've seen. Terry, that's a cool idea to do it with 4 patches. Were you going to physically align the four tracking patch's so that they were off axis from a center recieve patch? That seems like a really interesting way to make a tracking antenna without having to use exotic pc yagi's aligned carefully at the dish focus. It may even be possible to do something like that with Helicals so that the tracking antenna system could be completely circular. That way a circular transmit antenna wouldn't need to be mounted on the plane.
  3. I am copying this from a thread on the rcgroups website. Very Cool!!! http://www.ltu.se/polopoly_fs/1.266...newformat-1.pdf with a video of it working here: The only thing I can’t figure out is they say the video receiver is circularly polarized, but they are using linear PC Yagi’s . So as usual I’m a little confused … - Happy Electron
  4. I just ordered one, and will let you know how well it captures video. The helicopter photography website that I found it on said that it worked very well and they used it for pretty high-end applications. It seems that buying a little memory stick video camera, even on ebay, could set people back ~$200. I just thought it was a super inexpensive way to capture video. I agree that it would not be useful for realtime flying because of time delays. - Happy Electron
  5. http://www.adstech.com/products/PTV-370-EF...?pid=PTV-370-EF Here is a $25 PCMCIA video display and capture card. I saw it listed on a vendor of rc helicopter ariel photography equipment for $110. I have no idea how well it works, but at the price it definitely seems worth looking into. I like the idea of a bigger display out in the field, as well as not having the extra step of transfer the video from my camera to my pc to store and share on the internet. Happy New Year!
  6. Here is an email that I got from Dave today about the mysterious switching polarization. I kind of understand it ... Gobble! BTW, Terry, I am trying out various combinations of 900 mhz antennas to see which ones work best. I'm going to start another thread with this information once I get results. I am still leaning towards using Helixes though ... --------------------------------------------------------- I've been thinking about what you were told and what your friend related to you about circular polarization changing. In essence it does not. The article may have been referring to the Oscar VII satellite which has a canted turnstile antenna system on it for a 2M downlink. In this case the satellite antenna would have exhibited circular polarization - left, right indifferent, not important. This satellite spun on its "Z" axis in order to keep all the solar cells illuminated equally.This was accomplished using two methods; a black on one side white on the other carpenters tape 10M dipole and by magnetically stabilizing the spacecraft using a bar magnet aligned with the "Z" axis. The satellite was sun synchronous. The canted turnstile elements (monopoles) protruded 45° out of the bottom or "Y" axis of the spacecraft. After launch and in a couple of week the spacecraft aligned itself with the magnetic lines of force of the Earth. At the poles it would tumble according to simple laws of astromagnetics. Now,on the polarization of the canted turnstile. The spacecraft was going to be turning on its "X" axis. If looking at the spacecraft coming toward a receiving station the polarization could be RHCP or LHCP it didn't matter. Upon going away from a receiving station the polarization would yes, appear to have shifted by 180° and this is why we all used two antennas to receive the down link - one RHCP and one LHCP or one antenna on which we were able to shift the polarization (be it circular) by 180°. It all came back to me in a blinding flash. We designed and built that spacecraft to have an active life of 8 years. It lasted more than that and when it finally died it came back to life 27 years later. If you have a way of checking the Keplarian Element or "Keps" you will see listed once 'again' the effimerides for AO-7. This satellite was a special case (an exercise in astrophysics) where dual circular polarization at the ground station was required to have complete horizon to horizon reception of this 'bird'. We don't do things that way any more. If playing with the satellites it is always good to be ale to switch polarization as some things can happen that without the ability to switch one can be locked out. Sorry, but my instant recall isn't what it used to be. Regards, Dave...
  7. OK, so here is an update from the Antenna Turkey. (BTW, Mr. RC-CAM I flapped my arms around on Thanksgiving day trying your polarization experinment until my wife threatened to put me in the oven and serve me for diner.) Also, before I get into this arcane, tedious, and probabally marginally useful technical communicade, I must say that it only goes to show that even the experts don't agree as to what is going on with Antennas. So, if they can't agree, we are all probabally doomed to long and painful conversations about the relative merits of different antenna configurations. Anyway, I finally spoke with Dave over at the Olde Antenna Labs. His short answer was "Whoever wrote that must be smoking something." (His words, not mine.) I forwarded him a copy of the ARRL references, and we tried to look it up. Unfortunately, he only had the 20th version of the Antenna Handbook. Most of the section is the same, but we couldn't find the following in the 20th edition: So perhaps it was omitted in the most recent update because it was in error. Dave also felt strongly on theoretical grounds that it could not be accurate. He said that the only time he knew that polarization handedness changed was on moon bounce Radio experinments. (This is when they emit a pulse from the earth and then listen for it to bounce back from the moon.) He said in moon bounce experinments, it does happen that a certain handed polarized pulse would come back from the moon with the other handedness. He said that for hitting certain passively reflected satellites, ie - Sputnik, that the bounce handedness would come back different. However, for any originally handed circularly polarized transmit antenna, ie - those on normal satellites, it would show up on the ground the same way it was emitted. This is true, regardless of how the satellite is oriented. (Thus, we don't have to flap our arms anymore.) To help lend credence to his view he said he had used satellite links 10,000+ times and not had to switch between left and right hand circularly polarized antennas as the Satellite moved overhead. Dave also said he was on the antenna design comittee for the recently retired OSCAR Amateur satellite and personally knew most of the authors who wrote the Satellite section of the 20th edition of the Antenna Handbook. He said that if we could provide a list of the authors, he said he would try and contact them personally and find what was going on. Thats all for now ... Antenna Turkey, signing off ... Gobble, Gobble, Gobble
  8. Mr. RC-cam, all I can say to the above on this great Thanksgiving day is "gobble, gobble, gobble ..." As the big Thanksgiving Antenna Turkey, I am certainly confused how a certain handed xmitted polarized signal becomes the opposite handed polarized signal simply by having the Satellite move across the sky. However, the day I start believing my own personal theoretical speculation over the 19th edition of the ARRL Antenna Handbook is the day I know I've changed one too many diapers ... and should start wearing the old ones on my head ... and taking a number of deep breaths, etc... Also, in support of your point, if you look closely at the picture of the AvalonRF diversity reciever, http://www.avalonrf.com/Products/dvreceive...ingantenna.html, you will see that they have wound the Helical Antennas both left and righ handed on the receiver. This would hint at a kind of handedness polarization switching ability in the unit itself - although I see no description of it in the white paper - and the engineer didn't mention it when I spoke to him. At any rate, I based most of my speculation on a number of conversations I've had with Dr. Dave Clingerman over at the Olde Antenna Labs - http://www.hamtv.com/oal.html. He is an old time HAM who has made almost every type of Antenna for everybody from SETI to NASA. If you look closely at the web site, he makes both Helical Antennas as well as phased Circularly polarized Yagi's of the type you mentioned in your last post. He was the one who brought to my attention the certain handedness polarization emitted from the helical. So I am truely stumped as to why it is different as the satellite moves across the sky, and will have to wait and talk to him after the Holidays to find out what's going on. Meanwhile, gobble, gobble, gobble ...
  9. You are absolutely right, nobody makes or uses helicals without a ground plane. The ground plane is needed for impedance matching as well as to boost the signal gain from the antenna. I brought up the no-groundplane thought exercise to try to understand, why in Mr. RC-cam's Satellite example, reciever antenna's handedness would need to be switchable to get a strong satellite signal. In particular, I was trying to understand how a satellite with a particular handed helical xmit antenna, can deliver opposite handed radiation to a recieve antenna. Since there is no way for a xmit helical's polariztion handedness to change, regardless of how you look at the antenna, it should always show up at the reciever with the same handedness. This would mean that you should not need a mechanical or electronic handedness switching system as the satellite moves across the sky when using a well pointed helical recieve antenna. However, I suspect the reason for the confusion stems from the fact that most Yagi's are linearly polarized. In this case, the Yagi's orientation, or polarization, *would* need to be changeable. A linearly polarized Yagi would need to be rotatable 90 degrees to get it to line up with the satellites polariztion - even if the satellite had a circularly polarized helix xmit antenna - which produces both a vertical and horizontally output. But this is because a circularly polarized xmit helix will only emit horizontally polarized radiation from the side - not circularly polarized radiation. So if the Yagi was linearly vertical, and the satellites helical xmit antenna viewed from the side was horizontal, the signal would be very attenuated. One should not have to change the orientation or handedness of a helical recieve antenna, if it is used instead of a Yagi, since a helical can recieve both vertical and horizontal components as long as it is pointed straight at the satellite - at the expense of a 3dB signal loss. I sure hope the above makes sense, since this stuff is pretty hard to convey in words.
  10. I forward him an email invitation to join ... I agree that it is not very useful to use a circularly polarized antenna on the transmitter. They tend to be large and cumbersome, and only are circularly polarized if you look at them on-axis, or head on. The main reason to use a circularly polarized recieve antenna is to make sure that the recieve antenna can pick up both vertical and horizaontally polarized transmitted radiation. The price we pay for this is a 3dB loss in receive antenna sensitivity if it is fed by a linearly polarized source like a whip. A circularly polarized antenna fed by a circularly polarized transmit antenna, will not experience this 3dB loss penalty. However, compared to the 10dB or larger losses one experiences trying to pick up a horizontally polarized signal with a vertically polarized antenna a circulary polarized antenna tends to provide a more consistant output - especially when the plane is moving around in a lot of different orientations. However, as to how a helix switches from being left or right hand polarized, I'm a little confused as to how that might happen. I spoke with an Antenna engineer, and they said that a helix that doesn't have a ground plane, emits whatever handed polarization it produces out of both ends. For example, a right hand helix is right handed out of both the front and back. With a ground plane it produces whatever handed polarization mostly out of the front. And if you look at a helix from the side, it goes from being circularly polarized to linearly polarized parallel to the dirction of the coil windings. So relative to the ground plane a helix is horizontally polarized when looked at from the side. So, I can't figure out how a pair of satellite helix's changes their polarization. Any more details about the transmit/recievers system where this happens?
  11. He said they use a standard 1/2 wave dipole whip antenna. I can't remember the power, but I think he said it was less than 5 watts. These systems are used on real helicopters for TV news and police video surveilance. He said on the transmit side they have an arm that they lower down below the skids of the helicopter so their isn't interference with the helicopter. He also said they sometimes use a conical helical antenna on the xmit side. With respect to the null on the whip, he said that when the helicopter is directly overhead, usually the distance is not that far and they don't need as high a sensitivity for the receive antenna. In fact, if you look closely at the picture, on top of the receiver, you will notice that they use a patch antenna to cover the area directly over the unit. The above is true under ideal circumstances, i.e. - the transmitter whip oriented vertically with respect to a similarly oriented and polarized reciever antenna. However, if the plane changes orientation via turning and banking sharply, the transmit antenna will become more horizonitally polarized compared to the reciever antenna. This will cause a dramatic drop in the amount of power picked up by the recieve antenna - much more than the 3dB loss experienced by using a circularly polarized recieve antenna. With my powermeter, I've seen the signal easily go down by 10dB just by trying to feed a horizontially polarized recieve antenna with a vertically polarized transmit antenna. Since the plane is constantly moving around I believe polarization mismatch contributes significantly more to intermittant video drop-outs than the half power loss from going to a circularly polarized receiver antenna. In fact, the HAM radio guys use Helicals to track satellites. Apparently, the polarization changes caused by different sattellite orientations and attitudes creates a similar tracking problem to what we're experiencing here. The other thing I should note is that I've had a hard time finding true circularly polarized receive antennas at 900 Mhz. The best I've been able to find is an "elliptical hemispheric" antenna made by Astron Wireless. I've taken the thing apart and it appears to be a turnstile-like antenna over a ground plane. The ellpitcal characteristics are due to a piece of phasing coax between two dipoles oriented at 90 degrees sitting over the ground plane. The end result is that as the xmit signal moves closer to the horizen, it goes from being circularly polarized to horizontally polarized. This causes a significant drop in the received signal if I use a vertically polarized xmit antenna. I am not seeing as pronounced effect when I use Helicals.
  12. Hi folks! It has been a while since I've posted - it turns out 3 kids are a bit of a handful. It amazes me how much you all have figured out since I was active several years ago!! In the meantime, between diaper changes, I've been pondering the general problem of getting realiable video. It amazes me that this is such a hard problem. In researching the topic on the Internet I ran across a company that had some intersting solutions/white papers that has really taught me a lot. I am not affiliation with them, but I thought their approach could help inform and guide some of the stuff everbody is working on. You can see a picture of their diversity approach at: http://www.avalonrf.com/Products/dvreceive...ingantenna.html A technical paper describing their system can be seen at: http://www.avalonrf.com/Literature/applica...cationNote1.pdf I must confess that when I saw the above system, I thought it was one of the ugliest things I've ever seen. It looks like he piled up a box of springs in a Rube Goldberg design to create an extremely inelegant solution to the problem. However, I spoke with the chief engineer, and the more I spoke with him, the more I have grown to appreciate the difficult and subtle trade-offs that he chose that led to the design. The engineer claims they can get 20 miles without the need for physical tracking with this system. In particular, his use of a helical antenna has not been duplicated in many of the approaches I've seen here. I've gotten a RF powermeter, and a bunch of different 900 Mhz antennas. I've been testing various combinations to see what works best. My initial results are that the helical has very good wide directional characteristics, with no nulls, as well as staying *really* circularly polarized at different transmitter/reciever orientations. I am currently looking at using multiple helicals to create a diversity receiver design similar to the above. I'll post some of my experiences with the antennas later. In the meantime, thanks for helping keep my head in the clouds ... (and out of the diapers)!
  13. happyelectron

    Video of FPV helicopter flight

    Wow! Pretty darn inspiring! The video looks awesome. No drop outs or anything. Are you still using your diversity set-up? If so, I am curious which transmitters/antennas you have mounted on the helicopter. On the website you show the 500 mW 900 Mhz and 2 2.4 ghz transmitters. Is that still what you are using. Also, which transmitter/antenna combination shows the strongest performance? Thanks again for making keeping us up to date with such fun and uplifting stuff. - Happy Electron
  14. The IP5 loks even smaller than the block cameras at a much better price! Good luck and look forward to the results! - happyelectron
  15. Cyber-flyer, Check out the FCBEX780S and the FCBIX10A block cameras from Sony Professional at bssc.sel.sony.com. The FCBEX780S has an image stabilizer built in. It is the smallest block camera from Sony that I found that had the image stabilizer built in it. The link is http://bssc.sel.sony.com/Professional/weba...&sp=10&id=63993 I just bought FCBIX10A for $450 - http://bssc.sel.sony.com/Professional/weba...&sp=10&id=64384 I really like it for the following reasons: It is only about double the size of the Panasonic 171, but for that you get a 10x zoom, automatic focus, better exposure control and much better signal processing. In fact, I have not observed the blooming in direct sunlight that I found with the 171. However, alas, it does not have an image stabilizer built in. Buying these cameras from Sony Professional services was a bit of a challenge. I had to spend about 3 hours to find a distributor that actually stocked them. To save you time I finally used: Judy at Aegis Electronics at 800.362.3447. This is not an endorsement of Aegis, but just a sincere effort to save you guys time since most people at Sony had no clue about where to buy these block cameras. If you do get the FCBEX780S, please let me know how you like it since it looks like a great camera. Best of luck, - Happy Electron
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