Antenna Projects

53 posts in this topic

Posted (edited)

Nice work, Devonian. I'm most interested in hearing how much rejection you get between RHCP and LHCP.

Given that the weather is so nice for a couple of days, I also decided to build and test a 1280 Mhz version before doing the 4nec2x modeling. I built mine in a LHCP configuration on a male SMA connector using gold plated memory wire and a small PWB used for the solder points:

PinWheelElementsPWB.jpg

The resulting antenna weighs 0.2 oz (6 grams) and even with the small diameter (.025 inch) memory wire is very solid once every thing is soldered. Here it is mounted on a PWB microstrip filter which is connnected to the 1280 Mhz video TX via a right angle connector:

PinWheel1.jpg

You can also see the larger CPOD antenna in the lower right which weighs 7x more and is much more difficult to deal with on the quadcopter. In the mounted configuration shown, the SWR was 1.4:1 (Ref Coeff of 16) as shown on the spectrum analyzer:

PinWheelSWRa.jpg

The very light weight PinWheel antenna (that's what it reminds me of) allows it to sit higher above the quad resulting in less signal obstruction. I did a flight test with the quadcopter doing a 360 degree rotation maintaining aprox 30 meters height at a distance of 1.3 Km. This was a non line of sight test with several rows of trees blocking the path. Normally this will cause a lot of multipath:

PinWheelTestPath.jpg

PinWheelTestPlay.jpg

As you can see, there is perfect video for the entire rotation and the signal variation on the three receiving antennas (Stacked LHCP CPODs) varied only a few dB.

I only had time to do one measurement of the RHCP vs LHCP ratio. At a near zero elevation angle, there was at least 8 dB rejection of the RHCP which is as good as the CPOD. I need to build a RHCP version of the antenna to do a more accurate ratio test and I still plan to do a detail model of the PinWheel antenna to see if additional optimization can be done. Overall I am VERY impressed with the performance.

OMM

Edited by Old Man Mike

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Posted

Very exciting results!

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Posted

Wow, it does show some definite promise Mike.

You're ahead of me plus the Wx is terrible here at the moment, so no chance to test outside of my shack.

It reminds me of the 'cloverleaf' antenna.

I notice that to achieve the half wave part of the element, the quarter wave arm elements end up at around 100 degrees and when installed on the antenna and looking down in plan view, the quarter wave arms are not at 90 degrees to each other - did you observe the same Mike?

Nigel.

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Posted

Wow, it does show some definite promise Mike.

You're ahead of me plus the Wx is terrible here at the moment, so no chance to test outside of my shack.

It reminds me of the 'cloverleaf' antenna.

I notice that to achieve the half wave part of the element, the quarter wave arm elements end up at around 100 degrees and when installed on the antenna and looking down in plan view, the quarter wave arms are not at 90 degrees to each other - did you observe the same Mike?

Nigel.

Hi Nigel,

And sorry I got your 5.8 Ghz build post confused with RCC. I've corrected my post and also gave you credit in the other 3 mile omni system thread.

Yes, I made it around 100 degrees but did not try too hard for an exact angle. I tried compressing and moving the elements around a bit and it did not change the minimum SWR frequency much. The frequency is pretty well controlled by the full wavelength loop. Because of that, I can see a real big advantage for 2.4 and 5.8 Ghz since it is so difficult to get things accurate at those frequencies with soldered quarter wavelength elements used in the CPOD.

I like to think of it as PinWheel because they spin the way I imagine the circular polarization wave. Plus you can construct a PinWheel to rotate in reverse like you can construct this antenna to be LHCP instead of RHCP.

OMM

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Posted

I think the real credit goes to VE3BYT for his thinking and need for CP and HF beacons being received by amateurs on either vertical or horizontal antennas.

I have emailed VE3BYT but haven't had a response as yet.

I get the PinWheel concept now!

I don't have the equipment to validate anything in the way that you and maybe Mr RC-Cam could.

All my results are from gut feel and live testing.

Nigel.

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Posted

"gut feel and live testing" just the way I like it ;)

Terry

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Posted

This aerial looks easy to make for 2.4Ghz, would it be worth using as a horizontally polarized omni for my RC TX ?

Terry

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Posted

The polarisation is circular but the antenna could be oriented to suit.

From the VE3BYT web page, plots are:

Elevation

post-5780-036825000 1289673437_thumb.jpg

Azimuth

post-5780-033267000 1289673471_thumb.jpg

OMM gave a good tip on element material, 'Memory Wire' used for jewelery.

It's spring steel with gold plating, makes it easy to solder.

Nipped into town and the local beads n bangles shop had some...

post-5780-050441000 1289673577_thumb.jpg

Get building Terry !

Nigel.

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Posted

Any thought on egg beater antenna such as:

http://on6wg.pagesperso-orange.fr/Page%201.html

or

Quad Helix Uplook Antenna such as:

http://peakantennas.com/media/docs/QHOQHU2.pdf

Thank you.

-dave

The eggbeater by ON6WG is mostly end fire (for CP) and the clue is at the foot of page 1 of this pdf

http://on6wg.pagesperso-orange.fr/Doc/Antenne%20Eggbeater-Engl-Part1-Full.pdf

I quote:

"At the horizon the polarization is linear and horizontal. As the elevation increases, the polarization

becomes more circular, proportional to that increase in elevation."

It also has the complexity of requiring a quarter wave coax line to achieve phase shift.

The quadhelix are also end fire (QFH or QuadraFilar Helix).

Nigel.

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Posted

Thank you.

The eggbeater by ON6WG is mostly end fire (for CP) and the clue is at the foot of page 1 of this pdf

http://on6wg.pagesperso-orange.fr/Doc/Antenne%20Eggbeater-Engl-Part1-Full.pdf

I quote:

"At the horizon the polarization is linear and horizontal. As the elevation increases, the polarization

becomes more circular, proportional to that increase in elevation."

It also has the complexity of requiring a quarter wave coax line to achieve phase shift.

The quadhelix are also end fire (QFH or QuadraFilar Helix).

Nigel.

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Posted

One more I am curious about, what are your thoughts on this (if you do not mind)?

http://www.trivec.com/landbased_av2055.html

Obviously this would be a recieve only antenna and need to be built for your frequency. I like the idea of portability, circular polarization, and light weight. I'd love to build one for 2.5Ghz if I could find some plans on it.

Thanks,

-dave

post-426-0-25416800-1294094736_thumb.jpg

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Posted

It's a cross Yagi (crossed Yagi).

http://sv1bsx.50webs.com/antenna-pol/polarization.html

For portability it will be fragile and long term, probably not the best.

A helical would be easier to produce and more durable as well.

Nigel.

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Posted

Most Yagi for that frequency are mounted in plastic tube to protect them.

I also vote for the helical though as its one of my favorites :)

Terry

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Posted

Helicals are fairly easy to make and are durable, but obtaining good SWR is not easy. The impedance runs about 140 ohms or so. dalbert02 has one of mine for 1280MHz that I made around a 2.5" PVC conduit former. I tried using an impedance transformer to help with the VSWR issue, but the best I could do was 2:1 VSWR. Also the plot was like nothing I had ever seen before. Perhaps dalbert can put the picture of it up here? I'd be interested in what you guys think.

I might make another one to send down to him for a plot curve using a crossed wooden former.

Dave, can you post the plots to the BiQuad, the Skew Planar wheel, and the Helical here?

-Alex

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Posted

I made helical aerials two different ways, both on balsa wood formers. I dont know what the exact VSWR was on either as I just set them for minimum reflected power. The first one was on a rolled former and the second was made by making a copper spring and winding it on to an 1/8" balsa sheet with holes in, if you follow me? This one was a bit floppy until mounted in a tube.

I tried various ways of matching but ended up with the first 1/4 turn made from brass sheet about 5mm wide which distance to the reflector was adjusted for minimum reflected power.

I had good results but find patches easier to transport and mount.

Terry

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Posted

Interesting.. I might have to try that. I used a galvanized steel triangle that is 1/8 wavelength tall and 1/2 wavelength long for the first 1/2 turn. The hypotenuse of the triangle follows the helix pattern while the long leg is parallel to the reflector. Dalbert02 did some testing by compressing the coil a bit and got favorable results with the steel sheet matching stub. I wonder what cutting the center of the refletor out would do so there would be a hole as big around as the helix? Would that lower the impedance?

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Posted (edited)

Interesting idea, I know a helical with a loop reflector was posted on here by OMM but I didnt even think about how it was matched. Now I use 5.8Ghz I have given up making aerials as I have no way to test them except in the air.

Terry

Edited by Terry

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Posted

I am going to ry the series transmission line method that your described, Terry. I was looking into the ARRL handbook and it seems that to get a 50 ohm match, I need to make a strip where:

H/W = .625

I computed that from Zo = 138log(4H/d) for a single wire over infinite ground plane.

So if I keep the input feed short enough that resonation isn't a problem, I can run it a short length vertically and then run the strip 90 degrees perpendicular to it.

My question is whether the whole strip needs to be that height or if it should wind upward and finish at that height? Any idea?

-Alex

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Posted

Hi Alex, I wish I had one of my helicals to hand so I could take a pic but I think they are in the loft somewhere.

Mine both started low and got higher as they went round but not as much as the normal pitch. I dont think it is critical but I had no way to test the effects apart from the SWR. From memory the strip started very close to the reflector, probably 1mm gap, the strip was soldered to the side of the N connector pin and the tip cut off.

Hope this helps.

Terry

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Posted

Sorry for the long delay. Sometimes there is not much activity here so I forget to check often enough. Regardless, here are some pics of IBCrazy's helix antenna. Both before and after compressing the coils. I used a sharpie to trace the original wire path and to use as a guide while compressing the coil.

-dave

post-426-0-16861500-1296252129_thumb.jpg

post-426-0-60809600-1296252137_thumb.jpg

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Posted

Attached are the SWR plots of before and after. T1 was a wideband sweep of the original antenna.

post-426-0-44168800-1296252239_thumb.jpg

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Posted (edited)

T2 is a narrow band sweep of the original.

post-426-0-91870900-1296252363_thumb.jpg

Edited by dalbert02

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Posted (edited)

Finally, T3 is a narrow band sweep of the modified antenna. If anyone is interested, I can post the original data files and you can download the free Anritsu software so you can play with the position of the markers, plot Smith charts, or create a return loss plot.

-dave

post-426-0-63093000-1296252487_thumb.jpg

Edited by dalbert02

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Posted (edited)

Finally, sorry for the low quality graphic. I changed to grayscale and made it as large as I could to fit within the 250k file limitation on this board. :(

-dave

Edited by dalbert02

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