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Old Nov 02, 2010, 08:13 AM
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Left/Right circular polarized antenna ?

I'm about to buy my first antenna, but is a bit confused about the difference between left/right circular polarized antenna ?

I can see from my search, that a circular polarized antenna works better if you make some hard banks with your plane, but it can't reach as long a linear polarized antenna.

But when using a 500mW Tx and a circular polarized antenna, how long will I be able to fly ?

And whats the difference between left and right circular polarized antenna ?
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Old Nov 02, 2010, 08:23 AM
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no difference between left and right circular. for your calculations subtract 3dbi from the antenna to gather the effective range. in other words, 8dbi circular polarized patch will get you as far as a 5dbi linear polarized patch assuming perfect polarization.
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Old Nov 02, 2010, 08:50 AM
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Trappy is correct, but I should add to this:

With a linear antenna system, a 45 degree bank results in a 3db drop. A 90 degree bank results in a 26 db drop.

Thus a circular polarized antenna will have the same performance as a linear polarized antenna at a 45 degree bank. Beyond that bank, the circular polarized antenna performs better.

So my advice: Unless you plan on doing stunts or flying a serious pattern plane, get a linear antenna. If going for 3D and serious pattern flying, get a circular polarized antenna.

Most of us fly linear polarized antennas, me included.

-Alex
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Old Nov 02, 2010, 09:50 AM
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The following is NOT recommended as a first antenna configuration.
But there is another configuration.
Using a CP on the plane, one RHCP and one LHCP on ground station connected to a diversity switch.
Reduces multipath distortion and no -3dB loss.
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Last edited by blupp; Nov 02, 2010 at 10:42 AM.
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Old Nov 02, 2010, 10:03 AM
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Originally Posted by blupp View Post
The following is NOT recommended as a first antenna configuration.
But there is another configuration.
Using a CP on the plane, one RHCP and one LHCP on ground station connected to a diversity switch.
Attenuates reflections and no -3dB loss.
what does a RHCP and LHCP look like who sells them?
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Old Nov 02, 2010, 10:30 AM
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L-Com makes L & R CP antennae but where is one to get a small CP omni for the plane?...

Jeff
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Old Nov 02, 2010, 10:35 AM
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I get much better performance (as measured by overall image strength and clarity) with CP antennae than with VP alone. It works better over terrain and through trees than VP and I do not get image distortion or shifting when maneuvering the plane (video plane - right side up and shallow turns only).

I use a high gain CP and a low gain VP omni on a diversity system for most of my work.

Jeff
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Old Nov 02, 2010, 10:40 AM
FPV is my cup of tea
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Originally Posted by JustinONE View Post
what does a RHCP and LHCP look like who sells them?
It's short for Right-Hand and Left-Hand Circular Polarization.
You can make CP antennas yourself.
http://www.ham-radio.com/k6sti/attic.htm
http://sv1bsx.50webs.com/antenna-pol/polarization.html
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Old Nov 02, 2010, 03:57 PM
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Originally Posted by IBCrazy View Post
Trappy is correct, but I should add to this:

With a linear antenna system, a 45 degree bank results in a 3db drop. A 90 degree bank results in a 26 db drop.

Thus a circular polarized antenna will have the same performance as a linear polarized antenna at a 45 degree bank. Beyond that bank, the circular polarized antenna performs better.

So my advice: Unless you plan on doing stunts or flying a serious pattern plane, get a linear antenna. If going for 3D and serious pattern flying, get a circular polarized antenna.

Most of us fly linear polarized antennas, me included.

-Alex
I have not done my first FPV flight yet, but I will be flying my easystar, and it will not be with any 90 degrees turns. My plan is to glide around, learn the feeling of FPV flying while flying slow and stay within about 1-2 km from base the first many flights. But I like to get up high.

So I guess that I should get the linear polarized.
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Old Nov 02, 2010, 06:15 PM
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CP vs. Linear Polarizatrion

Circular polarization at the receive end is only safe way to fly and is especially important when you are pushing the limits of range. Remember, this is an FM system. The video signal to noise does not degrade much until just before the signal disappears.

So if you are flying to where you start to get appreciable noise, you are within a db or 2 of video loss. A slight bank and you are toast. I've seen this on so many crash movies. If you get an RC glitch, get hit with a gust, or even hiccup and bump the levers, you'll lose the signal.

Also, I might mention that most of what I'm reading suggests that there is a misunderstanding of how a diversity system is to be used. Diversity receive system compensate for path problems, not antenna mis-polarization or mismatched gains and patterns. True diversity systems should be used with identical antennas spaced a few wavelengths apart and pointing in exactly the same direction. Do not use a diversity controller as an antenna switch. That is another prescription for catastrophic failure.

Cliff K.
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Old Nov 02, 2010, 06:30 PM
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cliffk, there's not much truth in your above post - in fact the only correct thing is probably your name. don't really want to get into that discussion, just pointing it out.
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Old Nov 02, 2010, 07:50 PM
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Originally Posted by cliffkot View Post
Circular polarization at the receive end is only safe way to fly and is especially important when you are pushing the limits of range. Remember, this is an FM system. The video signal to noise does not degrade much until just before the signal disappears.

So if you are flying to where you start to get appreciable noise, you are within a db or 2 of video loss. A slight bank and you are toast. I've seen this on so many crash movies. If you get an RC glitch, get hit with a gust, or even hiccup and bump the levers, you'll lose the signal.

Also, I might mention that most of what I'm reading suggests that there is a misunderstanding of how a diversity system is to be used. Diversity receive system compensate for path problems, not antenna mis-polarization or mismatched gains and patterns. True diversity systems should be used with identical antennas spaced a few wavelengths apart and pointing in exactly the same direction. Do not use a diversity controller as an antenna switch. That is another prescription for catastrophic failure.

Cliff K.
What a load of rubbish !

First point, CP has 3db less range then linear so wrong.

Second point, your signal slowely degrades giving you plenty of warning so again complete rubbish

Third point, no you are the one who completely misunderstands diversity and the use of it.

Moral of the story, read advice but be carefull who`s you choose to listen too
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Old Nov 03, 2010, 02:50 AM
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But what if I like to go high, about 1 km (3.200ft). When flying level at that altitude, it must be like making a bank, compared to the antenna standing on the ground...I guess.

Is there an antenna for high altitude and one for low ?
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Old Nov 03, 2010, 05:18 AM
FPV is my cup of tea
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Quote:
Originally Posted by SpookiePower View Post
But what if I like to go high, about 1 km (3.200ft). When flying level at that altitude, it must be like making a bank, compared to the antenna standing on the ground...I guess.

Is there an antenna for high altitude and one for low ?
There is many antennas that would fit for high, low or both.
I would place the antenna on the plane horizontally, if you have a rubber ducky or dipole.
Then use a CP antenna on the ground like this one and place it horizontally.
And if you want to go higher there are many other CP designs, like helical, quadrifilar helicoidal, quadix, cross-yagis, cp-patch, to name some.
There are many interesting CP designs like turnstile, spiral, skew-planar wheel, cycloid collinear, cycloid dipole, lindenblad, and more.
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Last edited by blupp; Nov 03, 2010 at 05:47 AM.
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Old Nov 03, 2010, 05:39 AM
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Guys, c'mon, it is all the more constructive to point out why someone's opinion or educated guess is wrong, preferably by offering some evidence to the contrary.

Cheers,

Sander.
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