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Author Topic: Fixed Elevation Satellite Tracking Antenna  (Read 3463 times)
N4KD
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Posts: 139




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« on: October 10, 2014, 05:32:00 AM »

I was talking to a friend about antennas and what he was going to do with some stuff lying around the yard. He planned to use it for AMSAT work. Then he went on to say that he would set the elevation to the local latitude angle, around 34 near Atlanta.

What's the magic in that for tracking LEO and MEO satellites? Are our antenna beamwidths that large? Or is there something else that makes this practice work?

Thanks and 73,
Dave N4KD
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N4UFO
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Posts: 223




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« Reply #1 on: October 10, 2014, 10:11:39 AM »

Umm... that there are no MEO sateliites???  Cheesy  Seriously, it has to do with the statistics of passes. There are only a handful of working birds for analog communication: AO-7, FO-29, SO-50 and AO-73 evenings and weekends with a few more waiting to come online for amateur service. All are LEOs with low and fast moving orbits (relative to ground position). I used to have fixed antennas at 22.5 degrees and many ops set them at 15 degrees. Statistically speaking, it is rare to have a bird overhead. I'm not saying it doesn't happen very often, I'm saying the actual time spent overhead is small. Usually a bird is in the 0-45 degree range and well within the beam width of modest antennas. In my case, a 7 element UHF and a 2 element IO on VHF (equivalent to a 3 element yagi) were fine for anything below 60 degrees; I made a number of contacts with European stations on low passes.

When the bird would fly 'over my head' I would see the bird fade, but at that point the bird moves 'really fast' relative to my position as it passes. (Which is also when the doppler correction makes a big shift as well.) On a 10-13 minute pass, I might see fading for a minute or two in the middle; not so much that I would completely lose the bird, but it might make copy a little more difficult. Also remember when the bird is overhead, it is closest to you with less path loss and no horizon obstructions. In other words, it's closer and stronger and easier to copy off the 'side' of the beam.

I'm not sure why your friend would associate earth latitude with a sat elevation angle because latitude is about relative angle of the sun and no one is trying to work the sun that I know of.   Wink Suggest to him that 15-20 degrees is more appropriate. Simply put, you need more signal towards the horizon than you do overhead... the bird spends more time there and the path loss is greater. So maximize your signal where it is needed most. AND also recommend NOT to use big long boom yagis... it's not necessary and will end up working against him since it will have a narrower beam width.

Using modest antennas, some of the birds are workable with 5 watts... and anything over 25 watts is pretty much uncalled for as you will 'hog' the transponder. Same goes for transmit gain... too much, even with fully rotatable antennas, makes for too much ERP and will 'swamp' the AGC circuit on the bird and cause everyone else's signal to get lowered. It's rude, not to mention potentially harmful to the satellite's circuitry! The only need for antenna gain is to improve receive... and if you find that lacking, add a preamp at the antenna/mast before going with longer antennas. Again, with fixed elevations antennas, more antenna gain means WORSE receive on anything outside the narrow range of where the antenna is pointed.

Which leads me to another thought... the higher the antenna gain, the narrower the bandwidth and the more accurate the aiming of the rotator must be. If you are trying to talk, adjust frequency for doppler AND rotate the antennas by hand, the less often you have to adjust something the better!! If the antenna has a wider beam width, you have more time before you have to glance at the computer and tweak the rotator dial. A lot of sat ops use computer control of their antenna positioning, but most of those computer interface devices are designed for AZ-EL rotator systems. There may be some AZ rotator systems with computer interfaces but even then. I'm not sure that the tracking programs know how to talk to those... I've not used a tracking program for rotator control myself yet.

Hope that helps, Dave! - Fedex man just dropped off my new TV antenna... got to go get that puppy put up!  Grin

73, Kevin N4UFO
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N4KD
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« Reply #2 on: October 10, 2014, 02:07:41 PM »

Kevin,
   Interesting. Thanks. MEO is just a carry-over from work. I spent a couple years on a MEO project. Back to AMSAT, are most of the satellites in a equatorial orbit? That seems like the only way to get the fixed elevation angle to work.

vy 73,
Dave N4KD
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KA4NMA
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« Reply #3 on: October 10, 2014, 07:58:01 PM »

Check out work-sat.com.  Either download or use a web based satellite tracking program.  You will see most of them travel over major nations and cities in order to offer as many hams the opportunity to use them.

Randy ka4nma
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N4UFO
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Posts: 223




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« Reply #4 on: October 10, 2014, 08:01:36 PM »

Equatorial? No way... that's HEOs and Geo-stationary orbits. LEOs are primarily polar or near polar orbits. Here is an online tracking site displaying the current four analog ham sats.  http://www.n2yo.com/?s=7530|27607|24278|39417

From your questions, it sounds like you are thinking more like a geo stationary satellite. With polar orbits, the birds generally travel a SE to NW, SW to NE or a NE to SW, NW to SE kinda thing, depending on whether it's an ascending or descending pass. It's north to south-ish while the earth spins underneath it. Think more ISS or even a really damn high airplane.  Grin  On an overhead pass, it's gonna come over the horizon, rise slowly, then zip overhead fairly fast, sink in the opposite sky and then drop below the horizon. But, much more likely, it will come above the horizon, rise a little, traverse an arch over one section of the sky, then drop back down and go below the horizon.... all in the space of 10-15 minutes.

The average LEO orbit is something like 100 minutes. At the altitudes these birds fly, the footprint is relatively small and fast moving. MOST of the time, if a bird is in range, it is BELOW 45 degrees from any given position. I just looked at the 5 day prediction for AO-7 at my location. Only about 1 in 4 passes will even PEAK above 45 degrees. And that one in every fourth pass, it will spend only a short amount of time above 45 degrees... because that is when it appears to be moving the fastest relative to your location. You know, like a jet doing a low fly over... you see it coming... see it coming... getting closer... WHOOM! and now it's in the other direction... slowly fading... still going... getting smaller.  How much of that overall time does that jet spend above 45 degrees relative to you? A second or two??? A satellite is a little higher and slower than a jet, but the principle is the same. Instead of a second or two, it's maybe a minute or two. - AND, just like the sound of the engine going from high to low tone as it goes by, the biggest shift in frequency between you and the bird will be during that 'fast moving' part.

When people think satellites, they think straight UP. But with LEOs, due to low altitude and curvature of the earth, not so much...  Cheesy

73 for now! Oh, the TV antenna is up, but I gotta wait on a missing jumper cable... will be here next week.  Smiley

Kevin N4UFO

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N4KD
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Posts: 139




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« Reply #5 on: October 11, 2014, 08:30:42 PM »

Kevin,
   Thanks for your continued help. I'm just trying to figure out why fixed elevation angles seem to work so well. I guess if the peak elevation is only 25 degrees, or so, as would be the case in an orbit where the SV only peeks above the horizon for a few minutes. The near overhead passes are the ones that are the most useful commercially because of the additional time for data backhaul.

   I imagine we could go through celestrak and find a decent number of LEO SVs in equatorial orbits, but I was mainly curious about AMSAT work. The link was very interesting. With Nova, or other stand-alone propagators, one always has to load current ephemeris, but this tool seems to always be providing up to date tracks.

   Anyway, this is the office... https://www.viasat.com/files/assets/Antennas/Antenna%20systems%20facilities/Aerial%20Photo%20Shoot%20033_sml_0.jpg
It's a fairly old picture, but it's just as crowded now.

vy 73,
Dave N4KD

Equatorial? No way... that's HEOs and Geo-stationary orbits. LEOs are primarily polar or near polar orbits. Here is an online tracking site displaying the current four analog ham sats.  http://www.n2yo.com/?s=7530|27607|24278|39417
 ...
Kevin N4UFO


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N4UFO
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Posts: 223




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« Reply #6 on: October 12, 2014, 10:28:11 AM »

FB on all Dave... what an awesome place to work, getting to point antennas all over the place!  Cheesy

I'm just trying to figure out why fixed elevation angles seem to work so well.

I don't know that fixed elevation antennas work well... they work; but not as well as fully steerable antennas. Take me for example... my fixed elevation antennas went away and I'm in the process of putting up yagis on an AZ-EL rotator. The key is that they work FAR better than a set of omnidirectional antennas and are in the same price range. So for those that can't afford a complete steerable array, it's a happy solution.

The simple answer as to why they are able to work, is that the present birds are low enough and loud enough to make it possible. They spend the vast majority of their time within the usable beam width of a modest directional antenna that can copy them. (A 7 element UHF yagi has a 3 dB beam width of over 50 degrees.) If they were any weaker, more gain would be needed and the beam width would be too narrow and if they were in higher orbits, they might also spend more time 'hanging' up higher. You might say they are in the 'Goldilocks's zone'  Grin The Fox-1 satellite set to launch next year is supposed to be much 'louder' than the present only working FM bird, so it should be even easier to copy.

And yes, I like the N2YO website... I find it easy to use, easy to get predictions, and was all I needed to do manual adjustments of my antennas using a "big knob" TV rotator controller. - Too bad it's raining here today... I could be outside working on my antennas.  Wink

73 for now,

Kevin N4UFO
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K4JK
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Posts: 303




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« Reply #7 on: October 13, 2014, 09:39:08 AM »

Glad to see this thread, I'm working on putting up some fixed elevation antennas. To this point all my sat work has been portable with an Arrow II.

Any conventional wisdom as to which antenna on the boom is polarized vertical and which horizontal? I remember reading somewhere that theoretically vertically polarizing the 440 antenna yields better results but I can't remember where/why.
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ex W4HFK
N4UFO
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Posts: 223




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« Reply #8 on: October 13, 2014, 09:20:53 PM »

I remember reading somewhere that theoretically vertically polarizing the 440 antenna yields better results but I can't remember where/why.

I either read or was told the same thing and used that convention with the set of fixed angle antennas I put up. I think it may have to do with noise on the 2m when vertical... so better 2m reception. This next time, I am putting up an AZ-EL rotor and may start out with both beams vertical for purely mechanical reasons. (I plan to use a metal cross boom.)  I also plan to add filtering before the preamps which will be mounted at the bottom of the mast. If I run into problems, I may look into trying to tilt them both 45 degrees in opposite directions using grey PVC conduit attached onto the ends of the cross boom.

If you plan to use a metal cross boom to mount your antennas, keep in mind that the metal boom will interfere with whichever antenna is horizontal.

73, Kevin N4UFO
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K4JK
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« Reply #9 on: October 14, 2014, 05:30:42 AM »

Kevin thanks for your comments.

I'm planning on using PVC for the boom right now. I will have a diplexer on the feedlines so hopefully that along with the opposite polarization will take care of any desense issues on 440.
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ex W4HFK
N4UFO
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Posts: 223




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« Reply #10 on: October 14, 2014, 06:53:51 AM »

I contacted KB5WIA after reading his blog http://kb5wia.blogspot.com/2011/01/satellite-antennas-again.html and asked about his cross boom... he initially used PVC for a cross boom and ended up using two sizes, one stuffed inside the other and then a wooden dowel inside both of those. At that, I think he only ended up with a 4 foot boom instead of the recommended 6-8 feet. But then again, he was using longer, heavier antennas. I also know that KC0ZHF used PVC, but offset his a bit to center the weight. That 2m beam there is an MFJ-1763 and weighs only 2 pounds!



If you are going to keep it up very long, you might want to use grey electrical PVC or paint it.  Smiley

Instead of a cross boom, I arranged my fixed elevation antennas all inline, as if on a single long boom, if you will. I used a 22.5 degree elbow on top of the rotor mast and then a Tee with reducers. That allowed for two 3/4" PVC antenna booms tilted at 22.5 degrees. The 2m 'IO' antenna came off the the lower 'back' side and the UHF yagi was on the higher 'front' end. However, this placed the 2m beam aiming right into the back of the UHF yagi. And despite being in different polarities, I still got a small amount of desense on UHF. (Of course, diplexers might have helped with that.) It was okay for a temporary antenna arrangement, but cross boom mounting is probably the better way to go.

I notice that you keep saying '440' antenna... Just an FYI, keep in mind that not all UHF yagis are created equal. Some are made for 440 FM coverage and others are adjustable to work the entire band. Make sure you tune the yagi for 432-435 before use... most of the ones designed for FM work, have a high SWR at 435 and don't perform well on satellites.

73 for now,

Kevin N4UFO
« Last Edit: October 14, 2014, 07:03:30 AM by N4UFO » Logged
W5PFG
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Posts: 81




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« Reply #11 on: October 14, 2014, 07:42:05 AM »

Glad to see this thread, I'm working on putting up some fixed elevation antennas. To this point all my sat work has been portable with an Arrow II.

Any conventional wisdom as to which antenna on the boom is polarized vertical and which horizontal? I remember reading somewhere that theoretically vertically polarizing the 440 antenna yields better results but I can't remember where/why.

Many of us who work satellites on a regular basis are finding success with the 70cm antenna vertically polarized and the 2m antenna horizontally polarized. 
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K4JK
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Posts: 303




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« Reply #12 on: October 14, 2014, 07:51:08 AM »

I contacted KB5WIA after reading his blog http://kb5wia.blogspot.com/2011/01/satellite-antennas-again.html and asked about his cross boom... he initially used PVC for a cross boom and ended up using two sizes, one stuffed inside the other and then a wooden dowel inside both of those. At that, I think he only ended up with a 4 foot boom instead of the recommended 6-8 feet. But then again, he was using longer, heavier antennas. I also know that KC0ZHF used PVC, but offset his a bit to center the weight. That 2m beam there is an MFJ-1763 and weighs only 2 pounds!

If you are going to keep it up very long, you might want to use grey electrical PVC or paint it.  Smiley

Instead of a cross boom, I arranged my fixed elevation antennas all inline, as if on a single long boom, if you will. I used a 22.5 degree elbow on top of the rotor mast and then a Tee with reducers. That allowed for two 3/4" PVC antenna booms tilted at 22.5 degrees. The 2m 'IO' antenna came off the the lower 'back' side and the UHF yagi was on the higher 'front' end. However, this placed the 2m beam aiming right into the back of the UHF yagi. And despite being in different polarities, I still got a small amount of desense on UHF. (Of course, diplexers might have helped with that.) It was okay for a temporary antenna arrangement, but cross boom mounting is probably the better way to go.

I notice that you keep saying '440' antenna... Just an FYI, keep in mind that not all UHF yagis are created equal. Some are made for 440 FM coverage and others are adjustable to work the entire band. Make sure you tune the yagi for 432-435 before use... most of the ones designed for FM work, have a high SWR at 435 and don't perform well on satellites.

73 for now,

Kevin N4UFO

Oh, I say 440 out of laziness, the UHF yagi I'm using has a gamma match and is tunable down to 420 so I should be good to go.

Both the antennas weigh less than 3 pounds each so I'm not too worried about the PVC but I was thinking of using schedule 80 for more rigidity. But putting a 1 inch inside of a 1.25 inch would probably work too.

Is the minimum recommended spacing 6 feet? I was trying to find info on that but haven't had any luck yet. I was guessing though that 1 wavelength on the lower band would be fine, so that seems to go along with 6-8 feet.

Glad to see this thread, I'm working on putting up some fixed elevation antennas. To this point all my sat work has been portable with an Arrow II.

Any conventional wisdom as to which antenna on the boom is polarized vertical and which horizontal? I remember reading somewhere that theoretically vertically polarizing the 440 antenna yields better results but I can't remember where/why.

Many of us who work satellites on a regular basis are finding success with the 70cm antenna vertically polarized and the 2m antenna horizontally polarized. 
Okay thanks Clayton that's what I thought. Hope to work you again sometime.
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ex W4HFK
N4UFO
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« Reply #13 on: October 14, 2014, 12:56:28 PM »

Is the minimum recommended spacing 6 feet? I was trying to find info on that but haven't had any luck yet. I was guessing though that 1 wavelength on the lower band would be fine, so that seems to go along with 6-8 feet.

To be honest, I don't know for sure... The M2 cross boom that DX engineering sells is 11 feet. But if my calculations are correct, then 1 wavelength at 144 Mhz is about 6.5 feet. If one is mounting both antennas vertical, then a 6.5 foot boom gives complete separation. But if one is mounting the 2m horizontal, you have to add in the elements... So if half of the driven element is 1.625 feet, add that to 6.5 feet and you get 8.125 feet total. That is where I came up with 6-8 feet anyway. I have an almost 6 foot long piece of 1.25" EMT metal conduit (OD of 1.5") that I planned to use unless I find something better. Being that I plan to keep both antennas vertical that should be close enough.

73 for now,

Kevin N4UFO
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K4JK
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« Reply #14 on: Yesterday at 05:25:40 PM »

Well I got mine up, hearing most things FB so far. Here is a bad photo.






The rotator is on a 21' metal mast (really a heavy duty chain-link-fence top rail) and the short rotating mast and crossboom are painted PVC. Bracket is a .25 inch piece of aluminum drilled to accept the 4 U bolts with saddles.

It would probably help to have a another element or two on 430 so I may look around for a 7 or 8 element. I also have some 1/2 inch superflex sitting around that I will use for my main feedlines as soon as I can scrounge up some of those fancy N connectors. (LMR 400 right now)
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ex W4HFK
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