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Old Feb 21, 2010, 02:01 AM
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Read again what I wrote, I meant a 6dB OMNI, same class as your helix, but far easier to DIY.
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Old Feb 21, 2010, 07:04 AM
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Miami, FL
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Quote:
Originally Posted by RENATOA View Post
Read again what I wrote, I meant a 6dB OMNI, same class as your helix, but far easier to DIY.
An omni is not circular polarized. If you are getting what you want out of your system than that is all that matters. How does that omni do when you fly overhead? I not here to convince you of anything, but there is a reason the military, law enforcement and news helicopters use the antennas I showed you. Watch the pros, you might learn a thing or two...

-dave
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Old Feb 21, 2010, 10:30 PM
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Circular polarized antennas do not perform nearly as well as linear polarized units on a diversity controller. In fact, many truly circular polarized antennas have two inputs and need a special RX to use.

The diversity controller in this thread on a properly designed system will outperform a circular polarized antenna station for most FPV purposes. However, the circular will certainly perform better for an aerobatic aircraft flying fairly close range.
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Old Feb 21, 2010, 10:49 PM
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Don't know if you guys saw this yet. So I thought I would post it here. An explanation of Circular polarization.
http://www.frccorp.com/pdf/Why%20Cir...%20Antenna.pdf
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Old Feb 22, 2010, 04:03 PM
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Miami, FL
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Quote:
Originally Posted by IBCrazy View Post
Circular polarized antennas do not perform nearly as well as linear polarized units on a diversity controller.
I choose to disagree. Attached is a pic from a recent mission using some very expensive professional equipment, but what do I know. Two helos and two fixed wings in the air at the same time.

Quote:
Originally Posted by IBCrazy View Post
In fact, many truly circular polarized antennas have two inputs and need a special RX to use.
Again I disagree. I have seen only a few circularly polarized antenna with dual outputs, one lead for LHCP and the other for RHCP. I have seen many diversity antennas with dual ouputs but they were all linearly polarized.

Quote:
Originally Posted by IBCrazy View Post
The diversity controller in this thread on a properly designed system will outperform a circular polarized antenna station for most FPV purposes.
Could be. I have not used your divesity circuit. One carefully controlled experiment is worth a thousand 'expert' opinions and I would love to see the results.

Quote:
Originally Posted by IBCrazy View Post
However, the circular will certainly perform better for an aerobatic aircraft flying fairly close range
or anytime the antenna is not perfectly alligned with the recieve antenna such as when banking or directly overhead.

Alas, I am only a user, not a designer and can only tell you what works best in life safety situations.


-dave
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Old Feb 22, 2010, 04:13 PM
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Quote:
Originally Posted by Martin Y View Post
Don't know if you guys saw this yet. So I thought I would post it here. An explanation of Circular polarization.
http://www.frccorp.com/pdf/Why%20Cir...%20Antenna.pdf
Thank you. It is precisely the reasons noted in that document that military, law enforcement and new agencies use circularly polarized systems.

-dave
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Old Feb 22, 2010, 07:37 PM
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Quote:
Originally Posted by IBCrazy View Post
Correct me if I'm wrong - Explanations in RED
From left to right:
1 .We see two 100k potentiometers sitting on RSSI inputs. - Yes. This adjusts the RSSI voltage to scale them evenly. RSSI is rarely calibrated, so this allows more accurate selection via calibration
2. On negative side of LM358 we see 15k resistors. - That's not the negative terminal. It is referred to as the inverting terminal. This configuration scales the typical 0-5V RSSI to 0-12V. You could replace the 22K with a 15K and get a 0-10V signal which would work just as well.
3. Followed by two 22k resistors.
4. Followed by two 15k resistors.
5. Followed by two 47 uf capacitors prior to entering LM358.
6. I'm very confused by the 200k resistor looking thing, what is that with an arrow going through it, I'm not familiar with that drawing. Is this supposed to be a potentiometers as well? - The term for that device is a rheostat. It is a variable resistor, very similar to a potentiometer. I actually use a potentiometer and leave one of the fixed terminals disconnected. You could opt to drop in a 180K resistor in place of this and it will work just fine.
7. Followed by 2 LEDs with 2k resistors, I assume these are for LED power and resistor will change depending which voltage the LED is rated for. What I also don't follow, on one LED the resistor sits on GND side and on the other it sits on VCC side, is that actually the case? I understand that's fine just wanted to make sure that is what you have in your drawing. The LEDs are simply indicators. They tell which antenna has been selected. Since the output of the second LM358 will change from 0 to 12V depending on selection, it will source one LED while grounding the other.
8. The manual override is what's also confusing me, it's a SPDT switch. Is the center pin connected to the input onto MAX4547 and the two other poles one goes to VCC and the other to GND. Yep. Quite simple. The MAX4547 selects a signal based on the voltage it sees at that pin. Ground it and it selects one. Give it 12V and it will select the other. Do not forget the 2K resistor or the manual override will cause a short inside the LM358 and damage it. Also you need a switch with a center off position or you will be always in override mode.

-Alex
Alex,

Another question
LM358 has two independent o-amps, is there a reason not to use one of them instead of two where the RSSI cmes in?
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Old Feb 22, 2010, 07:44 PM
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No. I use one as the buffer for both RSSI's. I just drew the circuit as separate entities when in fact they are on the same chip.
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Old Feb 22, 2010, 07:49 PM
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NJ
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Quote:
Originally Posted by IBCrazy View Post
No. I use one as the buffer for both RSSI's. I just drew the circuit as separate entities when in fact they are on the same chip.
Thanks a lot man! I was confused a bit!
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Old Feb 22, 2010, 07:53 PM
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Quote:
Originally Posted by dalbert02 View Post
I have seen only a few circularly polarized antenna with dual outputs, one lead for LHCP and the other for RHCP. I have seen many diversity antennas with dual ouputs but they were all linearly polarized.
-dave
Dave,

Many of the circular polarized antennas are not truly circularly polarized. However, when you get into expensive right hand and left hand polarized systems, then yes these are truly high performers. Unfortunately these are rare and expensive.

The difference between two on a diversity system an a single circular is that there are two antennas on the diversity system. The dual antennas work based on the signal to noise ratio. Thus if one antenna receives a lot of noise, a diversity system will continue to select the opposing signal. You do not get that capability with one antenna.

This is truly an excellent discussion. I am happy to have a disagreement with another of your caliber. After all, our opinions are based on our experiences. And if we all agreed, then there would be little point to these forums.
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Old Feb 22, 2010, 10:12 PM
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Miami, FL
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IB,

I apologize for derailing your thread.

FYI, L-Com, (previously hyperlink technologies) has LHCP and RHCP patches for $14 http://www.l-com.com/item.aspx?id=21759

As far as building your own antenna and it outperforming commercial designs, it is very difficult to do without some type of antenna analyzer. Attached is a pic of one I have access to. I am all for experimentation but if you are not measuring, you are just guessing. At microwave frequencies a millimeter of difference can change your SWR immensly.

I would love to test your circuit and compare to my current amateur diversity systems. I currently use one designed by Matt Klarich for Black Widow and a beta unit from RC-CAM before it became DPCAV's Oracle. Old, but reliable.

Perhaps you could design a COFDM video transmitter and reciever? That would take this hobby to a whole new level!

Thanks for your time.

-dave
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Old Feb 22, 2010, 10:55 PM
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I think the discussions on what works best, and what might be an issue, are not derailing the thread. That's just my opinion though.

I'm still working on my own designs. The first design (heavily cribbing from IBCrazy's) which I posted earlier worked okay, but I had some issues with part dimensions. I also wanted to go the next step and support four inputs, audio outputs, and lights. I'm using a common pre-packaged microcontroller to do the logic part. I've got this PCB in the fab house now.

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Old Feb 23, 2010, 08:29 PM
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Awesome! You'll have to send me one for a demo. I'll reimburse you the parts and shipping expense.
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Old Feb 25, 2010, 12:04 PM
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NJ
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I feel so dumb, I finished building the controller, now I went to find RSSI pins on my RXs and it doesn't look good. I got 2 RXs one from RangeVideo and the other from DPCAV, both are racewood units, both look identical inside, both have exactly same looking RX unit. On the DPCAV one I found a pin which is 2.2v with no signal and 1.8v with signal and the rangevideo one I cannot find anything... This is very frustrating.
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Old Feb 25, 2010, 03:14 PM
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The RV version will have one. Perhaps you haven't checked all of the pins?

In a pinch, you can buy a new module for $20 or so.

However, once you do find the RSSI pin on the other RX, you are going to have to scale them identically. This is done with the potentiometer on the buffer and adjusting the gain on one (or both) LM358 units in the buffer. Let me know what your RSSI values are and I'll tell you how to set it up.

-Alex
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