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Old Sep 11, 2008, 10:22 AM
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Torrance
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How to build a 900mhz yagi antenna?

Hey guys, I was wondering if it would be cheaper to just build my own yagi antenna for longer range in conjunction with the 500mw tx.

where would I find basic plans to make such an antenna? Is there a radio operators design book somewhere that gives dimensions and frequency formulas? something good for a beginner would be nice to start out with.
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Old Sep 11, 2008, 10:59 AM
JettPilot's Avatar
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Yes, this would be very easy.

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Old Sep 11, 2008, 12:35 PM
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Daemon's Avatar
Lakewood, Colorado
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build your own yagi
Found at least two links on the first page that describe how to
make your own yagi, and speak in terms of wavelengths so
they can be scaled to any frequency you want.

I wonder though if you might not get equivalent results with
much less effort by simply scaling up the Windsurfer parabolic reflector to 900Mhz
dimensions and attaching it to your standard 3dB duckie.

ian
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Old Sep 11, 2008, 01:28 PM
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Torrance
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how many dB can I gain using the yagi?

I will try the simple parrabolic but would like something much more rugged than cardboard and tinfoil. unfortunatly I don't have any metal shaping tools for making a parrabolic refletor, so the yagi might be the way to go for me.
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Old Sep 11, 2008, 02:09 PM
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All you need is a pair of tin snips or heavy duty scissors and you can make
the parabolic reflector out of a sheet of aluminum, and the support for it out
of thick plastic or something. That's what I plan to do with my 2.4Ghz
reflector after I've tested the tinfoil/cardboard version properly.

As for the Yagi.
If you dig through the links on that first page of Google hits above,
you'll see one where they have a graph that shows what is needed to get
specific gain with a specific number of cross beams as it relates
to main boom length. Their graph tops out around 12dB gain
with 6-7 crossmembers on a 1.4 x wavelength boom.

The parabolic reflector above, has a theoretical 9dB gain at original
size, and 12dB with doubled dimensions. Dunno if that's
in combination with a typical 3dB gain of a dipole antenna or not.
You might get 12dB gain with a 3dB dipole concentrating its energy
into the 9dB reflector.

ian
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Old Sep 11, 2008, 02:19 PM
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Miami
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You would find a parabolic reflector very large at 900 MHZ to give you much gain. Also, its difficult to get the shape accurate enough to get the full benefit of the reflector. If you have any questions on the size and gain of a 900 MHZ parabolic antenna, look at the one HYPERLINK sells. As a matter of fact, I would just buy the antenna if you want parabolic. You are in for a dissapointment trying to build your own parabolic antenna, given low price of buying one, its not worth the days of heartache trying to make something that wont work well.

Building a Yagi is easy though. Be precise as you can in the measurements and it will likely work very well. Compare antennas, thier gain, and radiation patterns at Hyperlink, then decide what you want to do.

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Old Sep 11, 2008, 02:51 PM
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Torrance
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here is the yagi, pretty cheap actually. yet I probably cant use it by myself as the radiation pattern is very narrow, I'll stick with my patch for now.
it also won't work with the Range video Tx channels 2-4, as this one works between 800-960mhz
http://www.hyperlinktech.com/item.aspx?id=1943
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Old Sep 11, 2008, 03:55 PM
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United States, GA, Roswell
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Quote:
Originally Posted by JustinONE
here is the yagi, pretty cheap actually. yet I probably cant use it by myself as the radiation pattern is very narrow, I'll stick with my patch for now.
it also won't work with the Range video Tx channels 2-4, as this one works between 800-960mhz
http://www.hyperlinktech.com/item.aspx?id=1943

How about any these ones.....http://www.astronwireless.com/products.aspx?id=15
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Old Sep 11, 2008, 04:10 PM
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Quote:
Originally Posted by JustinONE
here is the yagi, pretty cheap actually. yet I probably cant use it by myself as the radiation pattern is very narrow, I'll stick with my patch for now.
Of course it's narrow. That's where the gain comes from on any
receiver antenna.

Quote:
it also won't work with the Range video Tx channels 2-4, as this one works between 800-960mhz
http://www.hyperlinktech.com/item.aspx?id=1943
Keep in mind that ch2-4 are not legal for use in the US (and possibly nowhere)
because they fall right in the middle of the aviation industry's
Microwave Landing System DME/P's (radar ranging) 80 control channels.
Now this system is mostly obsoleted by GPS, so many planes
don't have it, and those that do probably use it only as a secondary
system, but it's still an issue if you fly anywhere where commercial
aircraft are around and want to push your luck using those
3 video channels.

ian
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Old Sep 11, 2008, 07:39 PM
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Torrance
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my helper and I still need more practice time operating the patch, maybe after we get some more flights in I'll try long distane flight with the yagi.
The hardest part is finding somewhere to fly long range in Los Angeles area.


1. Just to comfirm, the 12 dB yagi will give allow me to double my rx range over the patch system correct?

2. what is the general max rx range on the 900 mhz patch?
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Old Sep 11, 2008, 08:23 PM
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Quote:
Originally Posted by JustinONE

The hardest part is finding somewhere to fly long range in Los Angeles area.
I have used the 900 MHZ patch with the Lawmate 500 MW transmitter with the rubber duck antenna from Hyperlink. With a an aluminum ground plane mounted to the back of the Video Receiver 900 patch, I could recieve the video very well at 19,000 feet away, which is almost 4 miles with very good and beautiful video. The video started to get noise beyond 4 miles, and when I moved out to 30,000 feet away from the plane, almost 6 miles before the video was so full of noise that it started to become unusable. This flight was done with a friend flying the RC plane in visual contact from the RC field and I moved the Video Receiver further away from the airplane for each test. See the frame captures below from the different distances. Distance readout is on the bottom right of the screen in feet measured horizontally from the Video Receiver. NOT to be confused with altitude of the RC plane which was low...

Another problem with LA, and even im my test is the huge amount of interference that a close by city Generates. There may be many close in band signals that desensitize your Video receiver, so that its range is reduced greatly, but it just appears and seems as noise in the video. It could easily be mistaken that you just have a weak signal, when the reduction in range is being caused by intereference. I would like to try this in the middle of death valley and see how much more range I get
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Old Sep 11, 2008, 10:39 PM
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Quote:
Originally Posted by JettPilot
I would like to try this in the middle of death valley and see how much more range I get
Probably quite a bit more, which could help out a lot when you had to send your plane out to find help!
Death Valley is an unforgiving place to be. I almost entered the "Furnace Creek 500" 5 years ago, but started reading about the rate of attrition and thought better of it. It's a 500 mile bicycle race in Death Valley...
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Old Sep 11, 2008, 11:06 PM
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Quote:
Originally Posted by JettPilot
This flight was done with a friend flying the RC plane in visual contact from the RC field and I moved the Video Receiver further away from the airplane for each test. See the frame captures below from the different distances.
Strange that a GPS based OSD system would have any way to measure
the distance that the ground based receiver had moved away from it.

Of course the most important part of the above though is that you've
added a large ground plane to the patch. Without it, you won't
see ranges anywhere near that far. Under dreadful near urban conditions
I see about 1/4 mile noise free range with two patches pointed
at the plane. Under optimal conditions I see about a mile of noise free
range, and it's acceptable to fly to somewhere around 2 miles.

ian
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Last edited by Daemon; Sep 11, 2008 at 11:11 PM.
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Old Sep 12, 2008, 12:06 AM
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twinturbostang's Avatar
Germantown, MD
Joined Mar 2006
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Quote:
Originally Posted by Daemon
Strange that a GPS based OSD system would have any way to measure
the distance that the ground based receiver had moved away from it.
Hehehe. I realized the same thing. I wasn't going to say anything though.

Quote:
Of course the most important part of the above though is that you've
added a large ground plane to the patch. Without it, you won't
see ranges anywhere near that far. Under dreadful near urban conditions
I see about 1/4 mile noise free range with two patches pointed
at the plane. Under optimal conditions I see about a mile of noise free
range, and it's acceptable to fly to somewhere around 2 miles.

ian
I'm curious why this is. We should be able to get really long range from the 900MHz systems. In theory, it should be better than 2.4GHz. Perhaps there's a better option for the Tx antenna than that Hyperlinktech 3dbi.
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Old Sep 12, 2008, 11:32 PM
JettPilot's Avatar
Miami
Joined Apr 2005
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Thats easy, you take the plane to where you are going to receive the video from, turn it on, and then let your friend drive it to the RC field and fly it. That way it measures the distance from you I was wondering if anyone was going to catch that

I was expecting better performance from 900 MHZ also, dont know why its not that great. I suspect the receivers are older technology, less advanced, and not as sensitive as the 2.4 GHZ stuff.... Maybe they dont reject near band interference as well also... I tried an expensive preamp on my 900 MHZ receiver, and it actually reduced range I need to try the preamp far away from any city and see what the result is.

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