All excellent points Bruce and I might add to any beginners out there, the reason I went the way I did with my antennas back when I had my full blown satellite rig is that I "wanted" the challenge of building a homebrew AZ/EL setup. I did enough research to know that it certainly wasn't necessary for LEO's, omni's are fine. I'm considering homebrewing some eggbeaters, groundplanes and turnstiles just to experiment with. I guess I got it from my late father ( the original W4HIJ), I like to play with antennas. :-) Michael Bruce Robertson wrote >> I'm going to elaborate on this discussion, for the benefit of >> beginners who are considering building new stations with tracking >> antennas. The narrower the beam, of course, the greater the gain when >> pointing at the bird, both transmit and receive, and -- this is the >> critical issue -- the lower the gain when you are pointed away from >> the bird. Now with LEO satellites, some of which cross the sky in say >> 15 minutes, you need to have everything spot-on if your beamwidth is >> very narrow: the station clock has to be accurate within a second, the >> keps have to be up-to-date, etc. otherwise, your computer is telling >> the rotors to point in the sky where the satellite is going to be in >> five seconds, or will be in five. Especially with high passes, you can >> be off by enough to not be able to hear the bird at all. So on >> receive, long antennas, besides the additional expense and challenge >> of mounting them, also add the challenge of getting your station >> perfectly aligned, or you'll hear zippo. >> >> On transmit, long antennas present another challenge: they >> 'concentrate' your signal so that it might well be excessively >> powerful for the satellite in question. If my homebrew 7 - element >> 70cm yagi often needs to be down around 5w xmit on VO-52 to be in the >> right range of effective power, how will I deal with things when I >> have a 40-element beam? By all rights, I should put an attenuator >> between the rig and the antenna so that I can get down under a watt! >> It is my opinion, in fact, that a significant proportion of the >> over-powered signals on our birds are from people in just this >> situation: people using HEO antenna systems that simply can't provide >> a small enough signal! >> >> In fact, LEO satellites do not require these sorts of antenna systems >> for reliable use. A beginner will be perfectly happy with, say, four >> elements on 2m and 6-7 on 70cm (assuming the use of low-noise preamps, >> which you are *crazy*to do without on long antennas, too). The beauty >> of this system is that if a high wind knocks it slightly out of whack >> in azimuth, it will not be the end of your satellite work: you'll just >> have weaker signals, not silence. The other beauty of this system is >> that it doesn't require an elevation rotor *at all*. Because the >> elevation pattern of the antennas will more-or-less fill the sky if >> you point the array up about 10-20 degrees (make it 10 if you have a >> clear horizon). Now, suddenly, you've avoided all the hassle of >> another rotor, you've made your array lighter and easier to work with, >> and you have way less of a demand on your pointing system. Heck, if >> you want to go ol' school, you can do the pointing yourself with a >> twist of the dial. >> >> These yagis do not need to be brilliantly built: mine were made with >> welding rod and pine wood. They had very strange lobes off the side, >> and all the rest, but they netted me lots of Q's and were very >> reliable. >> >> To be even more radical, I urge beginners to start with >> omni-directional antennas and low-noise preamps. A wire dipole or a >> vertical, both with almost no coax between them and the preamp, should >> hear 'stuff' really well. Not Q-5, but a start. Then use this as a >> baseline from which to compare the theoretical and real-world >> improvement you get with your yagi array. If you aren't getting >> improvement, then work out what's up. >> >> This is not an argument against long arrays. I'm building some that I >> bought from someone on this list around this time last year. I want to >> do some exotic stuff like work Russia over the pole on AO-07 or hear >> every last beep out of the newest cubesat. But I'm aware that in my >> windy region these are going to be a bear to keep in place. So I'm >> putting as much work into an omni array, too. I plan to transmit from >> the latter when things get too QRO. >> >> I guess in summary I'd say that in my opinion a big antenna array >> isn't like a high-power computer, which works the same as a >> lower-powered one, but has the umph when you need it; it is like >> buying a high-powered plane as a new pilot: significantly more >> challenging, and possibly leading to frustration. >> >> 73, Bruce >> VE9QRP >> >>
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