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Thread: Link Margin, Transmitter Power, Theoretical Range, and Antenna Gain

  1. #21
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    Quote Originally Posted by DiscoTuna View Post
    Again, thanks Derrick for providing this info. I really think i get it now - to ensure this I have made a model for myself that i wanted to share for feedback and also may help and be useful to others. This spreadsheet model allows automatic comparisons of up to 5 different hardware configurations so you can see the impact on Link Margin and subsequent 'safe' flying distance.

    This should for instance allow you to see the difference between a 5.8 vs 2.4 vs 1.2 setup, or the impact on range of a 3dB gain antenna vs 11 vs 24 or say the impact of different mW power on your transmitter or of course any combo of these and other factors.

    Click here for the model. Any feedback most welcome. Warning to anyone using this: I only read about these formulas this morning on this thread, so the whole model might be wrong! The excel has a small amount of VBA (a macro) in it so you will have to accept enable on start up for the graph rescaling to work.

    If this proves useful then i could load into a lookup table details of specific manufacturers Tx and Rx antenna gains & frequency, as well as specific manufactures transmitter's power so that could be pulled from a drop down rather than people having to find the details...On reflection i would also like to put in some kind of noise factor switch that will allow people to see impact if a site or kit had differing amounts of added noise in it - though not sure the best way to model this







    Attachment 40011Attachment 40012Attachment 40013
    Very cool. I fly with IRC 25mW 5.8 VTX with Circular Wireless omni onboard (guessing 2-3dB) and a 13dB Helical on a tracking gimbal (using the IRC tracker), and at 900m I get just a hint of noise. I use an IRC diversity receiver with an omni on the second channel. If my tracking solution fails when I'm out that far, I'll be forced to use NAZA's RTB function to get home. Any idea how the IRC and TBS 5.8GHz VTX's compare? The TBS is a whole lot smaller and lighter - and cheaper.

  2. #22
    Co-Pilot DiscoTuna's Avatar
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    Thanks again Derrick. Yeah - i have been through the methodical process, but it wasn't in the most efficient order (that i now understand). Conclusion was what you were getting to. The Omnis (most likely working fine as good enough range elsewhere), but were picking up too much noise, and the directionality of the Yagi sorted that.

    I may add a slider in the model to add in local noise for an area and make the RS component of the equation take the hit (like i did in my example) - i will explore that equation also.

    Didn't know there could be issues with different channels on Vtx performance if you are saying this is an electrical thing? or due to noise within the band on certain channels - that i tested for. In my noisy area i had the same issues across the entire 2.4 channels (some were worse than others understandably being 2.4, but my issue was in relation to the best band with the omnis)

    Quote Originally Posted by Derrick View Post
    Again you are spot on with your system debugging, using the formulas and mathematics to understand how changes in one area will affect performance of the entire system.

    Now that you know what the potential issues are, the best way to tackle the problem is methodically.
    - Isolate each of the variables independently and complete a test to ensure that that portion of your setup is functioning as expected. For instance, swap out your video transmitter and see if you get any difference (I recommend ground testing, this tends to stress the system a little more). Then do the same for your receiver.
    - Free space loss is beyond your control, so this just is what it is.
    - One other area to look is the number of connectors, adapters, and length of coax used from the receiver to the antenna. You can have significant losses especially at higher frequencies due to these things. Do a Google search on "insertion loss" to find out more about this, generally speaking you are only talking .1dB for a connector and about that for short runs of coax too.

    Something else to consider (Alex does a good job explaining this in one of his videos) is that the omni is listening in all directions, where the yagi is listening primarily to the plane. If you have bad luck with the omni and good luck with the yagi, this is an indicator that the local noise in the area that you are flying may be significant.



    My understanding is that noise directly affects the receiver sensitivity, so this is the factor that would get "hit". For a receiver to function properly you have to receive enough radio frequency energy... looking at the formula provided here: http://en.wikipedia.org/wiki/Sensitivity_(electronics)

    You can conclude that the receiver sensitivity is directly affected by an increase in the noise floor. Keeping all other variables the same, only increasing (or decreasing) the noise floor will affect the signal to noise ratio and thus the sensitivity of the receiver. I don't quite fully understand equilivent noise temperature yet though to understand how this ultimately effects the receiver sensitivity.

    Also, your Excel spread sheet rocks compared to mine... very well done! Mine just has a few boxes that I vary to spit out a number.


    Edit: One other thing that I just thought of... is the there can be significant changes in video transmitter performance based on what frequency you are using within the band.

  3. #23
    Co-Pilot DiscoTuna's Avatar
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    Quote Originally Posted by jones007 View Post
    Very cool. I fly with IRC 25mW 5.8 VTX with Circular Wireless omni onboard (guessing 2-3dB) and a 13dB Helical on a tracking gimbal (using the IRC tracker), and at 900m I get just a hint of noise. I use an IRC diversity receiver with an omni on the second channel. If my tracking solution fails when I'm out that far, I'll be forced to use NAZA's RTB function to get home. Any idea how the IRC and TBS 5.8GHz VTX's compare? The TBS is a whole lot smaller and lighter - and cheaper.
    That is pretty close to what the model says, i just put the numbers in - with that setup you have a link margin of above 6db (the "green" area) at just about 1 km. That is theoretical of course and local noise could easily be taking away those 6 remaining dB, hence the noise creeping in at around 900m means your kit is likely performing near to what it should. Not sure on the difference between IRC and TBS - i use the TBS kit - though now only for a relay from my 2.4 ground station. It works fine.

  4. #24
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    Great stuff you guys.


    My thoughts:


    Keep in mind that the Ambient noise floor above 460 MHz has been measured to be around the -160 dBm area. The local noise floor can only be measured with a RF measurement system that can go that low. Otherwise you would only be measuring the noise floor of the system you are using.
    Most of the time the temporary interference ( IBCrazy on his bike LOL) will drive the local noise floor up far enough to get into the Rx sensitivity range.

  5. #25
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    More reading on this? Google:
    Results of Ambient RF Environment and Noise Floor
    Measurements Taken in the U.S. in 2004 and 2005
    (Submitted By Robert Leck, USA)



  6. #26
    Co-Pilot DiscoTuna's Avatar
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    Interesting - what does this mean in terms of the calculations? Is this noise floor not already taken into account in the Free Space Loss component of the calculation (essentially an increased dB hit on link margin with distance relative to frequency, that must assume a noise floor in it somehow)? Or is taken into account within the Receiver sensitivity (set at -85dB) component of calculations - already factored in there very conservatively as having it set at -85 rather than -160?.


    Quote Originally Posted by Bama99 View Post
    Great stuff you guys.


    My thoughts:


    Keep in mind that the Ambient noise floor above 460 MHz has been measured to be around the -160 dBm area. The local noise floor can only be measured with a RF measurement system that can go that low. Otherwise you would only be measuring the noise floor of the system you are using.
    Most of the time the temporary interference ( IBCrazy on his bike LOL) will drive the local noise floor up far enough to get into the Rx sensitivity range.

  7. #27
    Navigator Bama99's Avatar
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    Also, check out the LIGO Wave Link calculator. You can put your Tx and Rx antenna on 2 points of the google earth map and it will calculate link margin based on hight of terrain, rain and antenna heights. Real cool.

  8. #28
    Co-Pilot DiscoTuna's Avatar
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    Cool site! It talks about a thermal fade margin, now i wonder what that is getting at? - Added link margin loss in the Free Space Loss calculations due to heat?
    Last edited by DiscoTuna; 29th January 2014 at 12:05 AM.

  9. #29
    Engineer for Jesus Christ IBCrazy's Avatar
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    Practical noise floor example

    Quote Originally Posted by Derrick View Post
    I will be sure to add the gain/range formulas/concepts to the antenna gain section and link margin sections. Your noise floor comment is dead on and I have tried to make it very clear that what is being presented is theory and only valid if the world were ideal. If you have a spare moment, I would love a little help/explanation on noise floor and how it affects range. I am still researching this area and will get there eventually. To my understanding as of right now regardless of how sensitive your receiver is, its sensitivity is dropped to the local noise floor of the given frequency band. As a direct result of this your link margin suffers accordingly.
    Anytime. Noise floor is one of those things that really kills the video link in short order in many case. It is the very thing that keeps us from being able to fly as far in populated areas.

    The short and dirty description of noise floor (there is a lot more to this, but this is quick and easy):

    "The level (amplitude) of undesired RF signal received by your receiver on your channel."

    Here's how it works:

    Let's say your noise floor is -74dbm and is caused by a HAM radio tower. Your receiver's sensitivity is -86dbm. While your RX can "hear" a signal at -86dbm under low noise conditions, the level of noise effectively reduces this sensitivity to -74dbm. This is a 75% reduction in range!

    Now let's add a VTX. Let's say at 1km your signal is received at -68db. This means that you have 1km (double distance = 6db) farther to go and your signal will be -74db... which is equal to that of the noise floor. Thus you will max out at 2km.

    Now let's say that HAM radio tower shuts down and the noise floor falls to -92dbm. At 2km you have -74dbm which your receiver can now detect. You go another 2km and you're at -80. At 8km, you're at -86 which is the minimum level of signal your receiver can detect. Now you're maxed out and limited by equipment, not noise floor. If your receiver had a sensitivity of -92dbm, you could do 16km in these conditions. However even with a -92dbm sensitivity, if that Ham radio tower turns on at -74dbm again, you're back to 2km range...

    -Alex
    Last edited by IBCrazy; 29th January 2014 at 01:13 AM.
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  10. #30
    I see you... Derrick's Avatar
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    Great explanation Alex... this is how I was thinking about it, glad you were here to confirm. I have updated the second post of this thread to include your explanation...


    D
    Last edited by Derrick; 29th January 2014 at 01:25 AM.
    When nothing else out there will suit your needs... design and build it yourself.

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