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Old Jan 26, 2011, 08:57 PM
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Ok here it is. A 1280mhz 3 turn (7dbi) RH CP antenna. I used the matching strip since I had the stuff lying around from the reflector. I also sanded the back of the reflector and epoxied a threaded piece of aluminum for a tripod mount. Epoxy holds surprisingly well on metal from my experience on my patch antenna.

This took probably only 1-1.5 hours to make.

Tadah.

BTW, Alex, I got different measurements than you on my 1280. I found that spacing should be 2.128" from your equation (2725/f), so from the starting point, wouldnt the 1st pass of the coil be at 1/2 that measurement (1.064) and the second one at the 1/4 wave (2.128)? Why do we vary?
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Old Jan 27, 2011, 12:41 AM
Fly FPV, sleep; repeat
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Germantown, MD
Joined Mar 2006
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Offtopic:

You should change the sign on your plane from "Experimental UAV" to "Model Airplane". Some people freak out when they see U.. A.. V..
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Old Jan 27, 2011, 06:27 AM
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Derek_S's Avatar
United States, NC, Charlotte
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You are probably right on. I will do that.

Or maybe go the other way and put:

Armed drone! Do not touch! call this number immediatly! Do not Ebay! Ebay results in explosion!
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Old Jan 27, 2011, 08:23 AM
LKJ
Lars k Jensen
Denmark, Central Denmark Region, Tostrup
Joined Apr 2006
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Or maybe add: smile you are on TV.
Ckeers.
Lars.
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Old Jan 27, 2011, 08:51 AM
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United States, NC, Charlotte
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Ok guys, did very limited testing this morining. It was too cold for me to do more, and I have a suspicion my Fox700 hates the cold as it goes black intermittently (edited out in this video).

Note I moved the gear to my Stryker. I decided if there was any way to best test CP in non-FPV, aerobatics might be a good way. I did non-fpv LOS/close range and tried to fly in the back/lobes of the helical at some points. I probably did not exceed 1000ft/300m LOS. I didnt notice any difficulty to fly with the S-P Tx ant on the middle of the Stryker just rear of CG.


Gear:
BOB/FOX 700 w stock RX (CH9/1280mhz)
Skew-Planar Vtx and 7.5 dbi Helical Rx antenna (3 turns w matching strip)
GP CX-161 Camera
eBay 12v stepup w/ 470uf caps on Vi/Vo
Futaba 9cap Tx and RP8D1 Rx
RCTimer 2200kv 2212-06 w/ 30A Hobbywing ESC
HS-82MG Servos (x2)

Hopefully this weekend weather will be nicer and I can push out some. I also did not do a head-to-head of CP vs linear due to the cold but I would like to compare to a V+Patch.

TEST 110127 Stryker 1280 CP Edited (2 min 41 sec)
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Old Jan 27, 2011, 09:06 AM
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So, my impression so far is that the SP/Helical combo is AT LEAST as good as my V/Patch. Certainly not worse as far as short range performance. Unfortunatly I do not have any videos of a linear polarized system that it would be fair to compare to.
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Old Jan 27, 2011, 11:11 AM
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IBCrazy's Avatar
Amherst, VA
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Derek, you need to compress your coil! That stub should be parallel to the reflector, not tapered. On a rough guess you VSWR is around 3:1. Get that stub closer and parallel and watch what happens.

-Alex
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Old Jan 27, 2011, 11:16 AM
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I just watched the video. Welcome to the CP club, Derek. You'll never go back to linear

-Alex
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Old Jan 27, 2011, 11:59 AM
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Derek_S's Avatar
United States, NC, Charlotte
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Quote:
Originally Posted by IBCrazy View Post
Derek, you need to compress your coil! That stub should be parallel to the reflector, not tapered. On a rough guess you VSWR is around 3:1. Get that stub closer and parallel and watch what happens.

-Alex
Ok, I confused tapered with what I would call sloped. I thought taper meant triangular, or decreasing in width as it goes along the length.

So if it needs to be parallel to the reflector, does only the beginning of the coils make contact with the matching strip? Or do I need to flatten out the 1st 2" or so of the helic(copper) coil?

Or maybe more simply, is the 1st pic I posted above as examples the most accurate? The 2nd one appears to be soldered along the strip, thus following its slope upwards.
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Old Jan 27, 2011, 01:19 PM
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Amherst, VA
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Ok, I confused tapered with what I would call sloped. I thought taper meant triangular, or decreasing in width as it goes along the length. - Sloped, tapered, same difference. It needs to be parallel and flat, not sloped.

So if it needs to be parallel to the reflector, does only the beginning of the coils make contact with the matching strip? - Ideally, yes. Or do I need to flatten out the 1st 2" or so of the helic(copper) coil? - You could do that also. Either works.

I will post more pictures soon. Your strip is certainly helping, but it could do better if it were flat along the reflector instead of being sloped.

-Alex
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Old Jan 27, 2011, 01:23 PM
Registered User
France
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657 Posts
I think the short copper tubing lenght is the easier way to make the "balun" (right word?)
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Old Jan 27, 2011, 01:26 PM
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Ok easy fix! Sorry for the confuson... I think as an architecture person I have a different set of semantics in my head so I get confused in engineering situations..
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Old Jan 27, 2011, 05:59 PM
Engineer for Christ
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Quote:
Originally Posted by vulpina View Post
I think the short copper tubing lenght is the easier way to make the "balun" (right word?)
It's not really a balun. The system is still unbalanced. It's just an impedance matching stub.
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Old Jan 27, 2011, 06:02 PM
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I think it is fixed:

BTW I just rewatched my video more closely. It is really a great improvement!
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Old Jan 31, 2011, 04:12 AM
The one and only
dadde's Avatar
Hjo, Sweden
Joined Apr 2004
503 Posts
Wow Alex, you have great timing I've just started making a Helical antenna when you posted this guide. You've probably save me a couple of hours reading nonsense on other pages.

I'm a big fan of your work and I would like to thank you for putting in the effort and writing these guides.
I'm moving over to CP on my multi-rotor platforms to reduce multi-path interference.
I have a quick question however; I have at my disposal both 12 AWG coated copper wire and 3/16" copper tubing. Which of these two would be the better choice for making a 1280MHz Helical?
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