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Luke

Low Profile Onboard Antenna?

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Hello... I'm working on a small RPV/FPV/UAV; it's a flying wing, so ideally, I'd like some sort of low-profile antenna for the video Tx. (I'm picking up enough drag from the FMA IR sensor blob, plus I'd like to avoid breaking the antenna on landing.) At the same time, I'd like to get good range, say, no less than 1-2 miles.

I'm using a BWAV 500 mW, 2.4 GHz Tx, with a 14 dbi patch antenna on a RangeVideo Std. Rx. (In hindsight, the Rx antenna is a bit too directional, but I hope to work out a GPS-based tracker in the near future... All the same, would it be worth buying an 8 dbi, or make myself something?)

Would the little low-profile disk antenna that BWAV sells be suitable, and if so, what sort of performance (range, dropouts during maneuvering) could I expect?

Also, what effect would fiberglass or polystyrene foam have on transmitting antenna performance? (That is, can I install small foam fairings on the antenna?) I guess that if people are worried about paint, foam would be out..?

And, lastly... Is the RangeVideo Rx decent, or would it be worth replacing? I don't want to move to a diversity Rx just yet, but if my current Rx is really junk, I guess that would sort of force my hand.

Thanks!

~Luke

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I cant help with most of your questions but I can say that adding a balsa fairing and heat shrinking it works great. It will detune the aerial a bit but the effect will not be noticed as it is tiny.

Terry

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Ok, thanks!

So, nobody's tried the BWAV disk antenna? Any other low-profile solutions out there that have been tested? (I know YB2Normal's ground-plane job is popular here, but it seems like a lot of drag, especially if the ground-plane "spikes" have to be outside of the airframe?)

~Luke

Edited: Wrong DIY GP name...

Edited by Luke

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I haven't heard any mention of using wifi antennas with these 2.4 gc systems. Many types are available depending on ones needs. You can even build a Radome for your plane and use a ceiling mounted gain antenna. Keep in mind what radiation pattern you want. The higher the gain the narrower the beam width. Ground planes can be adhesive backed copper strips or a circular piece of adhesive backed copper directly on the planes surface

http://www.sparcotech.com/cgi-bin/commerce...mp;keywords=FQ1

Ron

Edited by W3FJW-Ron

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I seem to remember the WiFi antennas are reverse-polarized (or something?) so some wire-swapping would be necessary... Are they really that small, though?

I guess I should have mentioned that the airframe I'm working on is pretty small, 32" span, 17" long, and about 2" deep, and a belly-lander at that. I could lay the standard whip antenna on the outside of the fuselage, facing fore/aft, but I notice that almost everyone has the whip pointing down. (Makes sense, given the radiation pattern.)

I guess I'll just have to give the disk antenna a try.

~Luke

Edited by Luke

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This would be ideal for a plane.. Wifi antennas for the most part will handle horizontally and vertically polarized equally well from my experience. Could be used inside the airframe against the framework. Unless that is, it's a carbon fiber fuselage.

http://www.sparcotech.com/cgi-bin/commerce...y=MCMI24003FSMA

And this would make a good receiving antenna..

http://www.sparcotech.com/cgi-bin/commerce...key=SP-RBDUCK-9

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RC-CAM, thanks for the link..! Forgot to search RC-Groups for info. Oh well, not going to try that.

Ron, thanks for the antenna link... Very nice low-profile antenna... However, I just ordered a bit of RG-174/U, with an SMA on the end... Going to try to build a YB2Normal GP... I realized it's pretty small, and very inexpensive... I figure I'll try with the ground legs inside the foam fuselage, and just the signal wire in free air. If that doesn't work well, the whole thing goes outside, and I pay the drag penalty.

Hopefully I won't fry the Tx in the process of experimentation.

~Luke

Edited by Luke

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Can you provide a link to the YB2?? I'd like to take a look at it..

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Thanks Luke. Judging from the name I thought it was a special antenna. It was a surprise to see it was a quarter wave ground plane. For the ground plane (the 4 legs), I would use a smaller size of wire than #14. Maybe #16 or #18. Less weight puts less strain on that small coax and less heat when soldering to help prevent melting the insulation. A better length for the antenna (vertical portion) would be 1.156 inches which is resonant at 2.430 gc.

Ron

Sure... http://www.yb2normal.com/antenna3.html

I was searching on these forums, and it comes up often enough... Sounds like people are pleased with the performance it gives, so I'm willing to give it a go. We'll see.

~Luke

Edited by W3FJW-Ron

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Oh, no, it's definitely nothing special; I'm building it only because it's so cheap, and supposedly gives pretty good performance. :P

By the way, the Rx patch I bought has about 48" of coax cable on it... From what I'm reading, that's extremely lossy in the 2.4 GHz frequency range? If so, what can I do? Should I splice the cable, so it's shorter (maybe 4", just enough to reach the Tx)? I'm reluctant, since RF is so infernally sensitive, and it seems doing something like that would just be asking for trouble...

Or should I just make a 8 dbi goof-proof patch, and call it a day?

~Luke

<Edited cable length and grammar>

Edited by Luke

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FWIW, I've found that I can gain access to the PCB, where the coax is attached, so shortening the cable isn't a big deal, if that's what I should do.

Or does antenna tuning/balancing make any changes like this inadvisable?

~Luke

Edited by Luke

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The shorter the coax the better. I think he was using Belden 8216/ RG 174 for the coax (That part he used is no longer available from that vendor) so the length from the ckt board to the ground plane should be a multiple of a 1/4 wave times .66 velocity factor which is for all practical purposes one inch (1.02"), so it should be 1", 3", 5" 7",9", and so forth in length and as short as possible in those increments. and the connections to the ckt board should be as short as possible. That will give you the maximum signal strength possible from your transmitter.

Those of us in the Ham Community who work with these wavelengths usually use a file to trim the elements to avoid cutting our antenna elements too short. I wouldn't worry about this too much though for this use as long as your cutting/trimming is reasonably accurate.

I would look at Mouser Electronics or a Ham equipment vendor for the RG174 or maybe ask a ham for a foot or so. I'd send you some but my stuff is all stowed away and I have no idea where to look. Maybe some other member of this forum will send you a scrap piece.

This is probably more than you wanted to know but I've learned a lot about FPV from this forum and this is just a bit of pay back.

Good luck,

Ron

PS: if you want to connect to the transmitter jack instead of the ckt board (using the same dimensions as above), I would use a piece of coax with the SMA plug (or whatever plug your transmitterjack would accept) already attached as these are buggers to make up manually.

Edited by W3FJW-Ron

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Thanks! Actually, this is just the sort of thing I wanted to know, so this is good.

So, I shouldn't just clip down the existing cable, since (lacking knowledge of what type it is) it's impossible to determine the velocity factor, correct?

Also, just a general-info type question... What's the procedure for cable length measurement? Is it from the connector nut right to the end of the inner conductor, or to the point where you've stripped the shield/outer conductor?

~Luke

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Also, just a general-info type question... What's the procedure for cable length measurement? Is it from the connector nut right to the end of the inner conductor, or to the point where you've stripped the shield/outer conductor?

The cable length would be from point of the connector to the ground plane portion of the antenna

Velocity of propagation = 80% of nominal = Vel. factor of 0.8, right?

That's correct.

The formula for figuring wavelength is 468 divided by the frequency in mega hertz which equals one half wavelength. Half of that is 1/4 wavelength.

For all practical purposes, in giga hertz you would divide by 2430 or 2440 which would be the mega hertz equivalent of 2.3 or 2.4 ghz.

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