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

  1. #11
    I see you... Derrick's Avatar
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    Quote Originally Posted by swillhide View Post
    very good info Derrick! great for me to link to some "More power" guys I know.
    Exactly, that is what spurred my desire to create a thread like this... a one stop shop to educate people who are just starting out.
    When nothing else out there will suit your needs... design and build it yourself.

  2. #12
    Engineer for Jesus Christ IBCrazy's Avatar
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    One of the main reasons many of us radio guys express things in db is because it is additive. Every 6 db is double (or half if subtracted) range. You find adding antenna gain works really well... up until you fly into an area of higher noise than your video transmitter is producing...

    The most common source of video issues is noise floor (now that circular polarization is the industry standard). Sadly, you can't measure it easily.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  3. #13
    Noise floor and interference is why my RF explorer is a part of my normal flying kit.
    Most of the money I made this year I spent on FPV,the rest I just wasted.

  4. #14
    I can't breathe Hans's Avatar
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    I've used some digital superglue and now this page is stuck at the top of the Subforum, also known as a stickie.

  5. #15
    I see you... Derrick's Avatar
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    Quote Originally Posted by IBCrazy View Post
    One of the main reasons many of us radio guys express things in db is because it is additive. Every 6 db is double (or half if subtracted) range. You find adding antenna gain works really well... up until you fly into an area of higher noise than your video transmitter is producing...

    The most common source of video issues is noise floor (now that circular polarization is the industry standard). Sadly, you can't measure it easily.

    -Alex
    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.

    Quote Originally Posted by Hans View Post
    I've used some digital superglue and now this page is stuck at the top of the Subforum, also known as a stickie.
    Thank you Hans, I was going to ask to have this done once complete... gives me a little extra motivation to get this done.
    When nothing else out there will suit your needs... design and build it yourself.

  6. #16
    The thread gets even better - I am starting to flex my spreadsheet building muscles so i can flick a few switches and look at impact of everything - a link margin / range graph per frequency for my own setup is what i am after. Also building out the spreddy will help me process this all properly (though i am sure you already have one!). Can you confirm my understanding on the example below - my key question is for the RS, as this is -85dB, in the calcs does that effectively become +85 as we are using -RS in the calcs (hence minus of a minus is a plus)? I assume this is correct otherwise i get a negative LM for a system that definitely works with ease.

    My setup is pretty typical for many people on this forum i expect: - a TBS Disco-Pro, lawmate TBS tuned Tx and Rx, Yagi, and rubber ducky Vtx antenna. I'll do my calcs for my EzUHF setup once i am sure i am on the right track.

    My setup against the calcs,
    • RS = -85 dB (TBS 2.4 tuned lawmate - the assumption we should all use i understand from the posts above)
    • RxG = 11 dB (TBS Yagi, 11dB from the website specs)
    • TxP = 28 dB (TBS tuned lawmate, range 750-900mW, so will use 750 to be conservative, read dB off the graph above)
    • TxG = 3 dB (Lawmate rubber ducky, 3dBi - not sure if dB is the same, expect it might be less, but has little impact on my calcs as LM is sufficient, though i will endeavour to understand that properly later)
    • FSL = -105 dB (this is for 2.4Ghz at 2km - the graph is for 2.3, but i assume the figure is very close - 2km is typical range limit on a flight and anyway, 3-4km is not going to make much difference)


    So with the formula LM = -RS + RxG + TxP + TxG -FSL i am coming out with
    LM(22dB) = -RS (--85, essentially +85) + RxG(11) + TxP(28) + TxG(3) -FSL(105)

    Hence with a 22dB Link Margin I should get good video signal (noise floor aside). In fact if my calcs are good then my link margin should only hit zero once the FSL is 127, which should give me 35KM range, never likely with my quad.

    EDIT: In my little spreadsheet i have put in the actual formulas to calculate FSL (by exact 2.4 Ghz frequency) and TxP in dB, with those in place and a quick GoalSeek in Excel i get my LM hitting zero on 24.25KM range.
    Last edited by DiscoTuna; 27th January 2014 at 12:23 AM.

  7. #17
    I see you... Derrick's Avatar
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    Quote Originally Posted by DiscoTuna View Post
    Can you confirm my understanding on the example below - my key question is for the RS, as this is -85dB, in the calcs does that effectively become +85 as we are using -RS in the calcs (hence minus of a minus is a plus)? I assume this is correct otherwise i get a negative LM for a system that definitely works with ease.
    Yes, that is what the formula says. -RS and since it is a negative number this is a minus of a negative making it a positive.

    I plugged your values into my spread sheet and came up with pretty much the same result... so it looks like you have it and fully understand. I am glad that this thread is useful!

    Now that you have calculated your Link Margin, you realize how overkill the Yagi really is for the flight performance/capabilities of your aircraft. Reworking the link margin calculation with a 2dBi omni antenna on your receiver you still have a link margin of about 10dBi... and now you can fly anyplace around you (within LOS) to 2.4km.
    When nothing else out there will suit your needs... design and build it yourself.

  8. #18
    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

    EDIT: I have since updated my model to include these features (the dropbox link above takes you to the updated model). To further develop the model I would like to get hold of equipment specs from people. Some of these are already populated (5.8, 2.4, 433). To that aim I am posting further details of this model in its own thread to not clutter this thread
    http://fpvlab.com/forums/showthread....ompares-setups







    Click image for larger version. 

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    Last edited by DiscoTuna; 3rd February 2014 at 10:37 PM.

  9. #19
    It is very exciting to actually start really getting this. It completely opens up my understanding of things and will allow me to address so many aspects of this hobby in a more sophisticated way. For instance it will change my flying - i am more likely to push how far i go when i get what appears to be weakening reception as i will be much more aware of how much further i should be able to go from these calculations. It should also make me better at trouble shooting issues and identifying what to focus on to improve performance.

    I can see your point on the Yagi and i better understand the pointedness of some of your comments on some of my other posts. That said i do have an area where i pretty much lose reception under 2km with my omnis where as my Yagi comes through for me. With my new understanding i think i would debug the problem like this (help me out if i am not thinking this through correctly).

    • Issue: My omnis (lets say 2.4Ghz, 750mW Tx, 2dB Rx antenna and 1.5 dB Tx antenna) should give me a positive link margin up to 7km, and a link margin above 6 dB within 3.5km. This is versus the 11dB Yagi setup that should get me a link margin above 6dB within 12 km. So something is eating up the link margin (duh!).
    • Playing with the equations - even if i assume my gains on my antenna are 0.01 on both ends i should still get 2km on the omnis assuming all else is equal (for a link margin above 6dB), so the issue must be more extreme elsewhere.
    • It could be the transmitter is dud - say it is only transmitting at 100 mW then as the increase in performance is inversely exponential, that would have a big impact as this is a change towards the beginning of the curve (unlike changes at the top of the curve). Assuming the omnis are at their full gain, but if my Tx is only outputting 100 mW that would drop me down to 1.25 km before i hit the 6dB link Margin, my Yagi setup would still be good for 4km - so a dud TX may be part of the problem.
    • If we look at the RX sensitivity then that could have an equal significant impact on performance, say it is not -85dB, but -76 then that would drop my range down to 1.25km (omni) and 4k (Yagi)


    Not sure if anything can impact Free Space Loss - perhaps noise in the area (though i was not sure if this impacted Receiver Sensitivity), moisture, etc, but external environment factors anyhow.

    Anyway, this line of thinking has shown me what to address first (actually the magnitude of your gain on your antenna at sub 2km ranges is not of much importance) and that if there is a lot of trouble in other parts of the system then that could lead to a situation where the Yagi will still get you 4km, but the Omnis may drop down to 1km.

    It also shows that i am not sure how to factor noise in the area into the calculations - which factor gets hit? - Receiver Sensitivity, FSL? Presumably noise can not impact TxP, Tx Gain or Rx Gain....



    Quote Originally Posted by Derrick View Post
    Y....
    Now that you have calculated your Link Margin, you realize how overkill the Yagi really is for the flight performance/capabilities of your aircraft. Reworking the link margin calculation with a 2dBi omni antenna on your receiver you still have a link margin of about 10dBi... and now you can fly anyplace around you (within LOS) to 2.4km.
    Last edited by DiscoTuna; 27th January 2014 at 08:56 AM.

  10. #20
    I see you... Derrick's Avatar
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    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.

    It also shows that i am not sure how to factor noise in the area into the calculations - which factor gets hit? - Receiver Sensitivity, FSL? Presumably noise can not impact TxP, Tx Gain or Rx Gain....
    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.
    Last edited by Derrick; 27th January 2014 at 11:01 AM.
    When nothing else out there will suit your needs... design and build it yourself.

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