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Antenna Booster Analysis

Discussion in 'Pro/Adv Discussion' started by RF Guy, Oct 23, 2015.

  1. RF Guy

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    Hey guys- ( pre-apologize for the length of this- but it might be useful)

    I'm a new P3P owner and it's a rainy day. I'm also an RF engineer. I've spent 4 decades optimizing
    signals / antennas / transmitters / receivers. The Windsurfer style antenna boosters were a great idea for the Phantom series. Thing is, I can tell from it's design that nobody has really put the calculator to optimizing it. I'm going to do so. We can do a full analysis of the antenna's "gain" and "pattern". It's likely that the various basic designs of the ones out there are within 20% of a perfect design. They are close - but not optimum. I'll offer easy to understand theory here too- so folks can grasp exactly what is going on - and use these reflectors to the best advantage.

    Part 1: The current reflector design is whats called a "Corner Reflector" or "Parabolic Reflector" antenna. The added part is the reflector. As you might imagine, it reflects the radiated RF from the controllers antenna - towards a forward direction instead of 360. Antenna designs don't amplify the signal, they concentrate it. And they don't just take the rear 180 degrees of radiated power - and face it forward. It's more complex than that.

    Antennas are rated in "DBi gain". This DBi gain is compared to an imaginary pin-head antenna that has a perfectly even radiation pattern - like a 360 degree sphere. No antenna does this... ALL antennas have a "pattern" that is not equal in all directions. You design them to make the pattern you desire. The more you concentrate the power in one direction, the narrower the signal's "Beamwidth" becomes...like focusing a flashlight or hose. You don't get something for nothing! The signal is reduced to the rear. This is called the "Front to back ratio" - and is also in DB. Most times, you Want a lower signal to the rear. In our case, not so much.

    Part 2: Because antennas are fed with RF energy of a certain frequency, the conducting elements of the antenna have to be "resonant". They have to be the correct length so that the RF wave reaches it's end - at the end of the antenna. If the antenna is not the right length, some of the power will be bounced back towards the transmitter and cause heat build-up. This is what's called SWR.. or Standing Wave Ratio. It need's to be 1 to 1.. or 1:1.Designers optimize an antenna's design for "resonance" and signal "pattern" (as well as it's beamwidth)

    Part 3: In designing antennas, usually only one piece of metal is actually fed with the power from the transmitter. The rest of the antenna interacts with that "hot" element through the air (called "passive" elements). The spacing and size of all the other elements of the antenna have to be exact - to operate optimally. Changing these sizes will change the antenna's "pattern" of radiation. In the case of a corner reflector, the distance from the real antenna element and the reflector is critical. The size of the reflector is also critical. Usually, designers use a reflector with a fold /crease in it (like a "V") - rather than the parabolic reflector (smooth curve) like the windsurfer design. Corner reflector antennas like this can give 12-15DB of forward gain. That's about 13-17 times concentration of the power forward! They reduce the signal to the rear of the reflector by 25DB...That means that less than 1/100th of the original signal is going backwards. Up close, you can get to the Phantom if it was in back of your reflector antenna, but out at any range, if you faced away from it, it Would loose your signal using a "beam" antenna like this.

    Part 4: 2.4 gigahertz is the band used by the Phantom (it uses 5.8ghz for other purposes). It is a microwave band just above microwave oven operating frequency (2.2ghz). It's known to engineers as "S" band. It is a "Line-of-Sight" frequency. Since it is so close to microwave oven operating frequencies, it too will be absorbed by water easily. Trees, leaves, dirt, rain...all convert 2.4 ghz into heat. Thus, your range will be near zero if you are blocked by any significant water-bearing thing. Wet bricks absorb 2.4ghz nicely. Conversely, if YOU are elevated to a good vantage point - above average elevation - and have a clear view of your Phantom over existing terrain, you can have phenomenal range. If you have normal trees and are at ground level, you'll have to increase the Phantom's elevation as you go away- to maintain line-of-sign signal conditions. The Phantom 3 uses a 4 element "Phased Array" with it's antenna elements in the legs. If you put a larger parabolic dish (like 24") on the Phantom controller, you could probably cover a range of 10 miles line of sight.

    Part 5: Short... Antennas have polarity. They favor horizontal or vertical usually. If the transmit antenna is "cross-pole" or 90 degrees off from the receiving end, at least half the power is lost. The Phantom's antennas are Vertical. Your controller antennas should be too. Here is the catch: The craft rotates in relation to you. It's overhead, then it is in front...it's signal is bouncing all over the place and rotates in space before it gets to you. This was first seen in Spacecraft as satellites rotated and the ground stations had signal fading. The Russians invented the Helix- or circular antenna for this. It is the corkscrew design you see on one of the aftermarket antenna upgrades- AND the new Drone jammers. A helix transmits equally in both planes...half as much power in each though. A short one does not have much gain. FM stations use circular polarization in order to get to moving cars. Once you make the corkscrew longer, you get gain.. and narrower beamwidth.
    (Like over 6 inches on 2.4 ghz)

    Conclusion: Because of battery life, making a simple "Beam" antenna by making a reflector - will result in a range boost that is nicely in-line with your maximum travel range. If you go out in a straight line away from your starting point, you'll be able to get a few miles out there and come back with the stock battery safely. Without increasing your battery capacity significantly, there is no reason to boost range over 2-3 miles. Operating your Phantom past where you can see is is actually against FAA rules anyhow - and puts you at much greater risk of loosing your bird. The Controller is transmitting at 38 milliwatts -not much. But the Phantom is transmitting with around 750 milliwatts and the 4 elements in it's legs provide "gain". It wouldn't matter which side you improved the gain of the antenna on- the communications link between them would improve. The gain of the antennas helps the receiving side equally. Engineers sit there and run all the numbers (antenna gain, TX power, receive sensitivity, computer signal decoding gain, frequency path loss) to figure out weather the system is going to work- and over what distance. And Height is much more important than power and antennas. Get your controller up and in the clear- and you are in control!

    Lastly: The Phantom's engineers have done a fantastic job in this RF system. Think of a router and how far WiFi goes on the same 2.4ghz band. The Phantom uses Spread Spectrum transmission techniques in which the computer decoding the signal can add gain - buy using what is called FFT - of Fast Fourier Transform. It is able to discriminate desired information from background noise by knowing what "should" be real information - mathematically. In other digital signalling systems, it allows decoding of very weak signals which are actually below the background hiss of the earth. Your GPS does this. All said, the Phantom system is highly optimized and certainly to be admired as a piece of engineering. Bob - AB5N
     
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  2. SteveThePhantom!

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    Nice report Bob. I get it! CQ!
     
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  3. Mr.Spock

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    Fantastic job thank u for this. Do u have any instrumental readings and pics that u can share pls?
    Have u been able to measure the P3A /P Tx power after difrent upgrades of FW?
    Is it true that the Tx power of the P3s has been decreased with latest FWs?
    What is the impact of using the chanel hack and at what frequencies do channels 1 to 32 Tx?
     
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  4. Mr.Spock

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    This is my set up by the way. DBS Itelite antenna
     

    Attached Files:

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  5. Ohary

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    So if you were to design an antenna to fit the controller, would it be a corner or parabolic reflector?
     
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  6. Tony Perry

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    Great informative read. Thanks!
     
  7. fastsmiles

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    Thanks for sharing
     
  8. shockwave199

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    Other than having to forgive some misspellings in an otherwise comprehensive piece, what I mostly take away from it is that there really isn't a benefit from the current windsurfer offerings. It's not the first report I've heard about little to no benefit and maybe even some compromise in range. DJI designed the system well - certainly robust enough and balanced enough in whole to provide excellent range without the need to modify it at all. Light bridge and the inclusion of it in the P3 line is one of the biggest reasons why it's so sought after. It certainly played a huge role in my decision to invest in it. Flying the P3 with effortless range within my 3k' needs in stock configuration is liberating and down right fun. I had to spend money and mod the tx myself for my Q500 to get that kind of stable range, and accepting the risks of doing so myself. I think most people should consider not fixing what ain't broke in the first place. In this case, KISS is a good thing, imo.
     
  9. Rocket_Aus

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    Very informative thanks for sharing your knowledge on RF.
     
  10. witold

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    Nice write up. I learned something.

    How hard would it be to convert from 2.4 to something that penetrates better, particularly through foliage? Is there a way to take 2.4 signal and change it to something else on both sides? That would be the ultimate Phantom 3 mod.
     
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  11. 4wd

    4wd

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    It doesn't say that surely?
    "Conclusion: Because of battery life, making a simple "Beam" antenna by making a reflector - will result in a range boost that is nicely in-line with your maximum travel range. If you go out in a straight line away from your starting point, you'll be able to get a few miles out there and come back with the stock battery safely. Without increasing your battery capacity significantly, there is no reason to boost range over 2-3 miles. "

    The point is it's pretty good stock, and going for a more complex (expensive) system such as a beam style antenna with additional elements will give little benefit - because you are already coming close to standard battery range.
     
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  12. JoeyJoe

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    I am sure he greatly appreciates your forgiveness. :)

    It seems clear to me too what the conclusion is. It sounds like the circular style antenna like the one on the FPVLR kit is the best compromise if you cannot tolerate loss of signal while turning or rotating your quad when it is at a good distance.

    If I ever decide to upgrade my controller and move away from the Windsurfer reflector, I will surely be choosing the FPVLR mod.
     
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  13. Chris P Duck

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    What does the P3 use 5.8Ghz for?
    You say "other purposes" which got me very curious!
     
  14. Dome

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    Hi Bob AB5N,
    It's a pleasure to get you on here with P3 funs .. I know your work around ICOM mike for IC 7000 (I have one of that).

    I'm curious to know how you measured 750mw (28,75dbm) coming out from P3 bird... I didn't get more than ~20dBm out with a calibrated Agilent Analyzer 8560E, from both P3A and P3P.
    The guys at DJI had a nice idea: Use of OFDM with QPSK modulation (quite like DVB-T) to make good video beam coding/decoding through FPGA chip, but they use a terrible spectrum band like 2.4Ghz (jam packed with blutooth/wifi/zigbee, LOS and so on). If it was on 868Mhz band with 100mW the signal goes to moon. It is amazing how big is RF spectrum and how small is the part for free use to people!

    As side note 5.8Ghz is used on Inspire 1 RC for inter-RC comunication, it's not used on a P3A/P.
     
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  15. Mr.Spock

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    Can you post pictures and more info on your RC and P3 Tx and measurements from your analyser pls? Have u tested the P3 and RC Tx power output with different DJI FWs on the bird and the RC?
     
  16. N017RW

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    You can find much of the RF data from the FCC website.

    It however will not contain any firmware differentiated related data only that which was used to grant authorization.

    Thr firmware mods are more to do with modulation than Tx power.
    Tx power is often overrated when comparing systems as receiver parameters are more dominant.
     
  17. RF Guy

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    OK.. Well, I took a passive RF meter and tried to get readings, but the Controller's 38mw output was too low to register well, and the Phantom's slammed the meter to full! I have a more serious instrument- but I need to go get a couple of 9V batteries to power it. I did not measure the TX power after a firmware update (as I've had my P3P 1 week). There is NO reason for them to lower power- as they are FCC cleared for what they started with. They can tweek the signalling encoding..compression rates.. for better signal lock. The only way they would cause a downgrade of range is if they added more information to the signal. The more info a signal conveys, the more raw signal is needed at the receive end to decode it. I can tell you that the Phantom 3P with current software is cranking out a lot more raw RF than I expected. It IS certainly at least 750MW. The 4 element antenna is 3.5DBi gain (from the FCC application) multiplying that power by more than 2X. As well, I'd be very surprised if they were not doing electronic phase-steering of the 4 element antenna..actually actively tracking (and beaming to) where the controller is. That's the only way I can see 750MW going over a mile to two vertical no-gain controller antennas. A WiFi router is 100MW and goes about 300 ft. I'll have more info as I do some tests. Bob
     
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  18. RF Guy

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    Nice! This is an elegant and effective setup. That design is called a "Patch Antenna"...and uses an etched circuit-board to create the resonant antenna elements. These exhibit forward "gain". Normally, it can be about 9 DB or more. It also gives you the option of adding the Sunhans boosters. If you do, you'll be running 40 watts+ ERP (effective Radiated Power). That means you should not point that antenna at any living thing within 20 ft. It's a 40 watt microwave oven.
    Transmit power and range are not 1 to 1. In fact, it is logarithmic...meaning you need a HECK of a lot more power to make a big difference. Of course you are raising it from .038W to 40W...so I'd expect 3 miles without the Sunhans and 6 miles or more (line of sight) with them. As long as you aren't planning on having any children any time soon!
     
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  19. RF Guy

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    Well, it's a trade-off. From my experience, the sharp-edged "corner reflector" gives more gain than the parabola (smooth curve). That said, it is going to have a narrower beamwidth..meaning you better point the controller at the Phantom. Within a half mile, the RF sprayed out the sides of the main beam "lobe" will be enough to stay locked to the Phantom. Once you are out there a bit, the main "lobe" of the antenna's beam pattern will have to take over...so pointing would be more critical...but since the width of the main beam pattern widens out like a cheap laser - you'll probably be OK. Also, the ideal placement for the controller's antennas are NOT as close to the reflector as they are in ALL the designs available now. There just isn't room in back of the two antennas. This means the main beam lobe will not be focused (less "gain") as it would be if the design was perfect. This is even worse with the rounded design. BUT... since you are going to run out of batteries by going out past 3 miles (and coming home), more beam gain may not be needed. Even with a crappy simple windsurfer design, if you are elevated and are line of sight with the Phantom, you will have great range. It's about not having trees and buildings between you. Bob
     
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  20. RF Guy

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    Yes, their focus will be on signalling format and encoding/decoding improvement with firmware revisions. That's where the "gain" is.