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Old Oct 05, 2009, 09:03 AM
pushing the envelope
rcgroupie's Avatar
USA, CA, Los Angeles
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9 dB collinear omni antenna build for 900 MHz video

Decided to build a collinear 900 MHz antenna from coax. The antenna is built from 50 Ohm RG 58 coax cable with a theoretical gain of 9 dB over 360 deg. Since each 3 dB doubles the effective power of the transmitter, this will give about 8 times the power.

This would eliminate the need for antenna switching since it is omnidirectional. I'm not worried about directly overhead, because there should be enough leakage to cover any reasonable altitude.

Another benefit of a good antenna is that by reducing SWR, transmitter heating can be minimized. Thus it would be beneficial to use the antenna on the vehicle as well.

First order of business was to cut the cable into 1/2 wavelength pieces. Then the ends were trimmed to have a short length of center conductor and a short length of shield exposed. The tricky part was to do this without cutting the shield.

EDIT: Quickstart
Dimensions:
Coax segment overall length is 4.6" per segment, overlap to get 4.28" sections
Tube is 3.08 long
Tip is 3.25 long
Ferrites centered 4.28 back from feed.

Anyway, some important notes:
Don't use a CB style PL-259 connectors, they are very lossy above 300 MHz, use N, SMA or BNC

RG 58 cable is lossy so don't run long feeds - use RG 8 to extend the cable with a connector as above.

You will not want a directional antenna on an aerobatic plane, just use it for your ground station.

Don't let anything conductive near the active part of the antenna, and get it up in the air.

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Last edited by rcgroupie; Oct 10, 2009 at 09:26 PM.
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Old Oct 06, 2009, 10:51 AM
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rcgroupie's Avatar
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Here is an image of one segment of the coax portion of the antenna. 4 segments = about 6 dB gain and 8 segments = about 9 dB gain. This was a scrap segment since the ends were not perfect.

The 9 dB 8 segment version is about 3' long so it is intended to be connected to the receiver end.

On the vehicle, the standard 3" long whip that comes with the transmitter is normally used (must be mounted vertically). However, since a 4 segment collinear antenna is < 2 feet long, it is feasible to mount it on some vehicles such as blimps.

Combining 6 dB on the vehicle and 9 dB on the ground gives 15 dB gain, equivalent to 32 times the transmitter power!
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Old Oct 06, 2009, 04:41 PM
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Hi,

This is very interesting. I don't understand the theory but this is very interesting. Please continue to share the development. Any other photos of a finished collinear antenna?

Thanks
Jimmy
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Old Oct 07, 2009, 09:01 AM
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I am also interested in this antenna. Thanks, alessandro
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Old Oct 08, 2009, 04:22 PM
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This sounds like a very good idea. Please continue and keep us informed on your progress.
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Old Oct 08, 2009, 05:34 PM
Frogman
Stoughton, WI
Joined Jun 2009
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This is all intriguing but can someone explain to an electronic idiot like me how you would implement this and make it work. All I see is a length of coax with the ends removed. I can handle that part but wouldn't know what to do next. I just got a 900mhz dowlink setup so this interests me.
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Old Oct 08, 2009, 08:37 PM
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Quote:
Originally Posted by frogman1206
This is all intriguing but can someone explain to an electronic idiot like me how you would implement this and make it work. All I see is a length of coax with the ends removed. I can handle that part but wouldn't know what to do next. I just got a 900mhz dowlink setup so this interests me.
Ditto.....
Bump bump....

I found this on a 2.4

http://martybugs.net/wireless/collinear.cgi
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Old Oct 08, 2009, 09:21 PM
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rcgroupie's Avatar
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OK I'm still trying to get some pics. In essence, you just solder the 8 bits of coax, add a whip at the top and a brass tube and a couple ferrites at the base and you're done. You can also scale it for 2.4
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Old Oct 08, 2009, 09:56 PM
Frogman
Stoughton, WI
Joined Jun 2009
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Cool....I could maybe handle that if it is explained properly. I'm not a total electronics idiot but dang close.
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Old Oct 09, 2009, 11:19 AM
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rcgroupie's Avatar
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Joined Jun 2005
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The center conductor of each section is soldered to the shield of the adjacent one, both sides. Care must be taken not to melt the core insulation. When done, there should be two separate circuits, insulated from each other.

The end gets a bit of copper wire radiator, and the feed point has ferrites and a tube to stop the RF from returning. Tip length and ferrite position are adjusted to minimize SWR.
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Old Oct 09, 2009, 04:16 PM
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Are there any commercial made ones on the market for us lazy guys?
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Old Oct 09, 2009, 04:30 PM
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rcgroupie's Avatar
USA, CA, Los Angeles
Joined Jun 2005
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Quote:
Originally Posted by GallopingGhost
Are there any commercial made ones on the market for us lazy guys?
This lazy guy would have bought one...

Dimensions:
Coax segment overall length is 4.6" per segment, overlap to get 4.28" sections
Tube is 3.08 long
Tip is 3.25 long
Ferrites centered 4.28 back from feed.

Anyway, some important notes:
Don't use a CB style PL-259 connectors, they are very lossy above 300 MHz, use N, SMA or BNC

RG 58 cable is lossy so don't run long feeds - use RG 8 to extend the cable with a connector as above.

You will not want a directional antenna on an aerobatic plane, just use it for your ground station.

Don't let anything conductive near the active part of the antenna, and get it up in the air.
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Old Oct 09, 2009, 08:53 PM
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I must admit I like the concept of more gain than a patch in a Omni design. Only downside I see is getting the stuff and doing it. Waaa waaa waa goes the lazy boy...
For what its worth I built several DIY HD TV antennas out of almost nothing and they outperformed almost any commercial antenna I tried.
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Old Apr 01, 2010, 03:25 AM
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What ever happened to this antenna? do u have any test videos?
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Old Apr 04, 2010, 09:26 PM
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Seems like this would be well suited for close in 360 degree flying or long range low-medium altitude 360 degree flying without needing to move the antenna. The weak areas would be High alt above, and far away with med to high alt.

Sound correct?
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