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Old Mar 10, 2011, 11:30 AM
Engineer for Christ
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Amherst, VA
Joined Jun 2006
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Not really. There is a bit of confusion when it comes to the 3db drop. I will try to explain:

A CP omni antenna eqivalent to a 1.9 dbi Vee has a gain of about .5 db

A Vee has a gain of 1.9.

Thus if you use the Vee on your plane your realized drop is more like 1.5 db, not 3. The reason is that a circular must polarized in all therr dimensions where the Vee only ploarizes in one.

-Alex
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Old Mar 12, 2011, 01:56 PM
I do all my own crashes
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United States, NH, Rochester
Joined Jan 2011
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I'm gathering parts for my 1280MHz antenna build. The question is for my application would 3/8" copper tubing work? (it's what I have layin' around) or is 1/4" the top end of the spec.

Thanks guys, and Alex! ~Wingnut13
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Old Mar 12, 2011, 08:06 PM
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3/8" would work very well if you can bend it. That's about the biggest that I would go for 1280MHz. I personally like 10 AWG wire because it is easy to bend

-Alex
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Old Mar 13, 2011, 08:24 AM
...just an earth bound misfit
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Pack the tubing with sand before bending, it should make it bend a lot nicer.

PS: Any updates on the quadrafilar helix testing Alex?
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Old Mar 13, 2011, 09:38 AM
Engineer for Christ
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Amherst, VA
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I like that sand idea! What a great way to keep it from kinking up!

I have not done anything more with the quadrafiliar helix, and I probably won't. It is extremely difficult to build and doesn't seem to have any advantage over any other type of antenna. It is a cool looking design, but the radiation pattern is suited for high altitude and that's about all.

Old Man Mike discovered that if you flip two skew-planar wheels and spaced them 1/2 wave apart, 180 degrees out of phase, you get a great moderate gain omni directional antenna. Granted it's complex, but it's easier than the Quad helix.

-Alex
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Old Mar 13, 2011, 10:19 AM
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Yes packing with sand work very well. Also, if you can heat the pipe to a red glow and let it cool, you'll see it'll be much easier to bend.

Alex,
I tried your "current draw method" with my 2.4ghz 500mw lawmate tx and found the stock omni pulled the most, followed by my 11dbi patch, my helix came third and lowest is my CL.

-Hugo
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Old Mar 14, 2011, 03:58 PM
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Here's my sentinel rail-gun
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Old Mar 15, 2011, 11:43 AM
I do all my own crashes
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United States, NH, Rochester
Joined Jan 2011
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Here is what I did this morning before work. I used a 20" section of PVC conduit as alex spec'd in his build. I made all the markings and drilled a hole at the starting end for the copper to be held by. The chucked it up in my lathe and started turning coils. This is what I had laying around for copper so that's what I used. 1/4" and 3/8" tubing. There were no kinks but where the copper was held in the PVC, no sand had to be used, copper is very forgiving. It's a good trick for bending exhaust tubing though! Also it looks like I could make one longer if I had to.

Alex I'll send one of these out if you would like to check it out as a antenna. I just received my TX yesterday and this is all new to me......

~wingnut13
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Old Mar 15, 2011, 01:04 PM
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Amherst, VA
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Go ahead and send one my way. If you want, I will match it for you, give you the SWR, and give you the frequency response data. With the thick tubing, the matching is less critical, but still a good idea. If you feed it very close to the reflector, you might not even need to match it at all with the 3/8" tubing.

-Alex
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Old Mar 15, 2011, 05:52 PM
...just an earth bound misfit
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For some reason I can't stop thinking about moonshine...
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Old Mar 15, 2011, 06:24 PM
Fidler & twidler
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Or revenuerz...
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Old Mar 23, 2011, 08:49 AM
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Does anyone have a helical model for NEC2?
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Old Mar 23, 2011, 11:02 AM
Engineer for Christ
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NEC file

Ask and you shall receive.

-Alex
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Old Mar 24, 2011, 03:21 PM
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Thx Alex,

I made a model based on my construction above, however in 4NEC2 it resonate close to 3ghz instead of 2.5
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Old Mar 24, 2011, 04:25 PM
Pat K
The Netherlands
Joined Dec 2010
382 Posts
Hi Alex,
thanks for the wonderful job you've done on FPV and for sharing your experience with us. What wire size would you recommend for a 5,8GHz antenna? Sorry if this has already been said but I could not find it.
Pat
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