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Old Feb 08, 2011, 08:05 AM
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oh yeah 13k$
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Old Feb 08, 2011, 11:24 AM
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A word of caution about VSWR meas. Lowest SWR does not always mean the best configuration, especially when it comes to circular polarization. You can easily push the antenna from good circular polarization to primarily horizontal or vertical polarization by distorting the elements for lower SWR.

You HAVE to model the antenna to understand what is going on.

OMM
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Old Feb 08, 2011, 11:52 AM
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Old Feb 08, 2011, 01:31 PM
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Quote:
Originally Posted by Old Man Mike View Post
A word of caution about VSWR meas. Lowest SWR does not always mean the best configuration, especially when it comes to circular polarization. You can easily push the antenna from good circular polarization to primarily horizontal or vertical polarization by distorting the elements for lower SWR.

You HAVE to model the antenna to understand what is going on.

OMM
An SWR reading really can be totally meaning less as well. I can put a 50 ohm resistor on the end of a coaxial line and get perfect SWR. We all know a resistor isn't an antenna.

Two antennas come to mind here: Yagis and helicals. Line up a set of yagi elements improperly and you can have a dummy load despite perfect SWR. Same thing with a helical. A poor shunt matched helical will have great SWR, but not resonate at all which I why I didn't cover it in the tutorial.

Still, nothing compares to a flight test (or a ground test). The skew planar wheel does wonderful on cheap RX units. Put it on a Lawmate RX and you have no signal at all. No modeling or SWR reading is going to tell you anything about a phenomenon like that.

-Alex
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Old Feb 08, 2011, 01:34 PM
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Also, note that this antenna also exhibits some opposing polarization. A RHCP skew wheel will also radiate some LHCP. Granted it's low (maybe 12 db or so), but it does do this. If you look at the way the elements look when they are flat (as a figure 8) you will see why this is.

-Alex
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Old Feb 08, 2011, 03:57 PM
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Let me be sure, if I look at the SP from the side, and the rounded part of the elements mimics a standard screw thread then it is RH polarized, correct? Just making sure as when testing my helical + sp I have not been getting good results lately, and have been looking for the problem.

Is it possible I have HAM/ATV interference in my area that is CP in nature, and thus using CP over linear will actually yeild worse results? It seems teh 1st day I tested cp at my field it worked great, the next time terrible, exact same components just different planes and different days. am not sure it is the plane setup, as I think the setup on the second day was actually better in theory (more space, more separation, filtering etc whreas the stryker had none of those)
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Old Feb 08, 2011, 09:29 PM
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Derek - I have had the same problem and am currently investigating the cause. The best way to determine polarization is to curl your fingers around any of the elements towards the center element of the coaxial cable. Thus you curl from ground to the origin so you fingers point to the origin point. What hand did you use? That's the polarization.

I'm certain the problem is in the helical. That microstrip match is fickle as can be. I just developed a more reliable match by using a tapered 1/4 wave metal strip. I will be adding it to the tutorial soon

-Alex
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Old Feb 08, 2011, 09:41 PM
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Oh, nice, I will try changing the matching device.

And it looks like both my antennas are RH, so that shouldnt be it.
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Old Feb 09, 2011, 01:33 AM
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Quote:
Originally Posted by IBCrazy View Post
The best way to determine polarization is to curl your fingers around any of the elements towards the center element of the coaxial cable. Thus you curl from ground to the origin so you fingers point to the origin point. What hand did you use? That's the polarization.
-Alex
Lets take for example this antenna http://www.rcgroups.com/forums/showp...3&postcount=13 If I understand this method correctly - it is right polarisation?
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Old Feb 09, 2011, 05:41 AM
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Quote:
Originally Posted by IBCrazy View Post
An SWR reading really can be totally meaning less as well. I can put a 50 ohm resistor on the end of a coaxial line and get perfect SWR. We all know a resistor isn't an antenna.

Two antennas come to mind here: Yagis and helicals. Line up a set of yagi elements improperly and you can have a dummy load despite perfect SWR. Same thing with a helical. A poor shunt matched helical will have great SWR, but not resonate at all which I why I didn't cover it in the tutorial.

Still, nothing compares to a flight test (or a ground test). The skew planar wheel does wonderful on cheap RX units. Put it on a Lawmate RX and you have no signal at all. No modeling or SWR reading is going to tell you anything about a phenomenon like that.

-Alex
So this design doesn't work well on the lawmate rx's? Is that correct?
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Old Feb 09, 2011, 06:26 AM
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The skew planar wheel does not work with Lawmate RECEIVERS. It works fine with the transmitter, though. I have no idea why this is. Nobody seems to know. It does awesome with every other RX.

-Alex
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Old Feb 09, 2011, 06:27 AM
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Originally Posted by aSa2 View Post
Lets take for example this antenna http://www.rcgroups.com/forums/showp...3&postcount=13 If I understand this method correctly - it is right polarisation?
Yes, that is RHCP. If you curl your fingers around the element with your right hand, they point to the centeral feed point.

-Alex
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Old Feb 09, 2011, 06:29 AM
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So, is it a requirement to ave circular polarisation on both the transmitting and recieving end? What is the disadvantages and advantages of having a CP VTx & LP VRx antenna vs LP VTx & LP VRx antenna?
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Old Feb 09, 2011, 06:40 AM
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Every system has distinct advantages and disadvantages. I will try to explain:

linear polarization on both sides - theoretically this gives the longest over all range under ideal conditions (which is seldom the case). However multipath interference and loss of polarization when banking the airplane cause loss of signal.

Linear on one side and circular on the other - Eliminates the problem with polarization loss due to banking. However there is a constant 3db drop due to polarization mis match and it will not reject multipath interference.

Circular polarization on both ends - Best choice for almost all FPV ventures. Does not lose polarization when banking and has excellent rejection of multipath interference giving the cleanest signal. Also excellent for high altitude. Theoretical range is slightly less than linear polarization, but is seldom realized.

-Alex
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Old Feb 09, 2011, 06:46 AM
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Thanks!

So, for my setup (I'm using Lawmate and Airwave VRx) I guess would be CP VTx antenna and a LP VRx antenna (such as the biQuad or a patch). I'm not going for extreme ranges but I do like to bank hard when turning.

I don't think I'll re-invest in new VRx at this time.

But is the biQuad a closed loop antenna or not, ie will it work with Lawmate VRx?
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