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Old Feb 11, 2011, 08:19 AM
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Old Feb 11, 2011, 08:23 AM
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Originally Posted by Mictronics View Post
Here we go:
Thanks! So comparing the two antennas:

CPOD = 1.72 dB gain with 19 dB rejection of opposite polarization

PinWheel = 0.92 dB gain with 12 dB rejection of opposite polarization

Of course the PinWheel is much lighter and easier to build than the CPOD.

OMM
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Old Feb 11, 2011, 08:59 AM
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Quote:
Originally Posted by Old Man Mike View Post
Thanks! So comparing the two antennas:

CPOD = 1.72 dB gain with 19 dB rejection of opposite polarization

PinWheel = 0.92 dB gain with 12 dB rejection of opposite polarization

Of course the PinWheel is much lighter and easier to build than the CPOD.

OMM
Could I perhaps talk you into giving me the dimensions for your CPOD? I'd like to try it. By observation alone it appears to have 4 equal length 3/4 wave elements where each run is 1/4 wave before it's next turn. The feedpoint appears to be some sort of lowe pass filter? However I am wondering if there's a 90 degree phase shift in there somewhere or if you have capacitive/inductive linked system? Just trying to take a guess in hopes you'll give me the details. I will not post it without permission.

-Alex
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Old Feb 11, 2011, 09:26 AM
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Michael - Thanks a million! I just ran som analysis on the 3 lobe unit and now have some more thinking to do.

It appears that when the lobes are skewed the resonate as all real at 4-5% longer than their electrical wavelength. They also show a standard 120 ohm impeadance for a full wave loop. Thus the antenna resonates ate 1280 MHz, but at only 38 ohm impeadnce.

So now begs the question: Is it better to use 2 loops .1 wavelength over a ground plane? Should I orient them 180 degrees apart or 90 degrees? My thought is that this would lose circularity.

The other option is to allow the antenna to resonate with a complex impedance. My thought for this is that it would not be very efficient. Any ideas?

By the way, how did you get the circularity plots?

-Alex
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Old Feb 11, 2011, 09:34 AM
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Old Feb 11, 2011, 09:41 AM
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Just fixed my Helical to a tapered strip. All this talk has me excited to try the SPW again this weekend.
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Old Feb 11, 2011, 09:54 AM
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Cloverleaf modification

Ok, now I see. I am a bit confused right now. It appears a 3 lobe wheel is more efficient and has better SWR than a 4 lobe. Circularity appears distorted. I thought it would need the ground plane to acheive proper SWR, but apparently that is not the case. The rejecting characteristics are bizzarre! Look at the horizontal plane! I based the 3 lobe design on the wheel antenna that uses a flat lobes .2 wavelengths away from ground and just bent up the sides for circularity.

Looks like I need to try this new antenna. I got an SWR of 1.2 from my skew planar wheels. Now to a 3 lobe cloverleaf, I might actually get 1.1 or better! Element length appears to be 5% longer than calculated wavelength. Compensating for 70% humidity, I'm guessing this drops to 3% or so.

So the comparisons:

Cloverleaf: 1.3 dbic forward, 9-20 db rejection, SWR = 1.2
Skew planar wheel: .92 dbic forward, 12db rejection, SWR = 1.3
CPOD: 1.72 dbic forwar, 19 dbic rejection, SWR = ?

Now the problem... How in the world am I going to make this one easy to build???

-Alex
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Old Feb 11, 2011, 09:57 AM
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Originally Posted by Derek_S View Post
Just fixed my Helical to a tapered strip. All this talk has me excited to try the SPW again this weekend.
Try the cloverleaf as described in the above post. I can't wait to try this one. I also am building a quadrafiliar helix! FPV meets circular polarization.

Thank goodness for the work of Mictronics! My hat is off to you, sir!

-Alex
KJ4RIV
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Old Feb 11, 2011, 09:59 AM
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I think building it in 3 peices should be no problem.

I built a SPW in 4 peices. The trick was I used a small piece of PCB, and installed them on it one at a time. I made a small triangle jig out of cardboard to hold tehm at the right angle and position as I soldered. It was not bad at all really. I have no way to test it, but physically it looks good.


Dimensionally is it the same as the SPW? If so it would be a matter of minutes to take my 4 piece SPW and reconfig it to a clover leaf.
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Old Feb 11, 2011, 01:29 PM
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Quote:
Now the problem... How in the world am I going to make this one easy to build???
Look how I built mines. I simply slotted the ferrule of a coax plug with a dremel cut-off wheel to insert the four wires. If you want it at the end of a coax, just make your own ferrule with a piece of 1/8 copper tube.
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Old Feb 11, 2011, 06:20 PM
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Originally Posted by IBCrazy View Post
... It appears a 3 lobe wheel is more efficient and has better SWR than a 4 lobe. Circularity appears distorted.

Cloverleaf: 1.3 dbic forward, 9-20 db rejection, SWR = 1.2
Skew planar wheel: .92 dbic forward, 12db rejection, SWR = 1.3
CPOD: 1.72 dbic forwar, 19 dbic rejection, SWR = ?

-Alex
First I'm glad to see the interest in doing antenna models. I would like to offer the follow suggestions:

1) Show the 3D plots. For FPV application the 2D plots are almost worthless. This is especially true when trying to compare the opposite polarity rejection.

2) Make use of variables in the 4Nec2 models so that you can more easily optimize the antenna.

3) The model will almost always show better SWR than the actual antenna where the build will have stray capacitance & inductance usually not simulated (although I have tried to incude it in some of my modeling). Getting SWR below 1.3 provides no significant improvement other than making you feel good seeing the meter reading. (The CPOD antenna model showed between 1.1 and 1.2 depending on the particular optimization)

4) Alex, I'll eventually post the CPOD model in my 3 mile omni thread along with some other models. Been a bit busy lately.

OMM
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Old Feb 11, 2011, 08:51 PM
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Weather is supposed to be good tomorrow so I hope I can test in the air. Ther Stryker is in the shop right now getting ready.
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Old Feb 12, 2011, 06:43 AM
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Old Feb 12, 2011, 01:19 PM
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Alex,

I flew with the SPW and 3 turn Helix today. IMO I got very good results. I beat a personal record and hit 2 miles with ok video. The SPW is to your specs, and the Helix got a new tapered impedance strip per your instructions.

It is not fair to comment fully but I will post the video anyways... I later found out the video wire had a loose/cold connection, so I did have video fuzziness but not likely due to the antennas. Due to this I couldnt test the cloverleaf today.

I can say that I had no apparent multipath problems (at least in comparison to previos experience with the V and a patch), It was very nice video with the regular maneuvers an easy star does. I only had a few blipps, probably temporary nulls and interference in the city. Looks like it is time ot build a higher gain Helix?

FPV 110212 EZ AP and CP Test Edited for CP (4 min 55 sec)
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Old Feb 15, 2011, 06:15 AM
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50 or 75 ohm?

OK I have got some memory wire and have started to bend.
I suddenly have to ask. If the equipment that we use is sometimes classed as CCTV and this is 75ohm.
But the other bits are 50ohm.
What cable and plugs should I be useing, 50ohm or 75ohm?
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