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How to measure 2.4 or 5.8ghz power output

Discussion in 'General Discussion' started by justin00, Apr 19, 2015.

  1. justin00

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    Hey peoples

    I have some 5W 2.4ghz amplifiers I got from Alibaba ages ago... now problem with Alibaba is most things are generally 1/10 of the power advertised hehe.... e.g 2000mw laser is generally only 200mw....
    5W is a crap load of power... so I really don't believe its 5W... It does say 5W on the device and the box, but they could write anything... its from China :/

    I work for a large Telco and previously had access to a device that can measure the output of various frequencies but that tool is no longer available to me which is annoying..

    How do I go about measuring the output of my amps or any other equipment ? and please don't say use a wifi analyzer and look at the dbm levels... :]

    Basically I want to see how many watts the amp is outputting... I can't even remember what the device I used to borrow to measure the output is called... Can anyone refresh my memory of what the device would be called ??

    I'm hoping there is some cheap device / trick I can do to measure the output ?? I know the device I borrowed from work was mega expensive... so doubtful I'll find something, but who knows.. it measured a huge range of frequencies so maybe thats why it was so expensive.

    I have like 2 or 3 of the amplifiers available so I might sell 1 or 2 of them, but I would really like to measure the output as I don't want to tell someone its 5W and find out its really 50mw :/

    I'm going to ask the same question to the folks at my work tomorrow (we have like a private facebook like site where all of us can chat and stuff) but hopefully one of you smart folks can assist before :)

    thanks
     
  2. N017RW

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    There's no cheap or homebrew way.

    You need an RF power meter.

    Any local schools, universities?

    Search for businesses that might have such a meter. Marine, mobile communications companies, etc.
     
  3. mohan

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  4. justin00

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    Yeah like I was saying my work has a few of them (very large telco) but due to people borrowing equipment and screwing it up they arent keen on lending out stuff anymore.. plus I work in network/security eng department and it would be hard to justify borrowing it from the 'comms' department...again :(

    Dont think a random company/uni would lend to me either but good suggestion anyway.

    But yeah RF meter.. thats what it was thanks :)

    I will check out that link now Mohan thanks for that to.

    Btw.... anyone ever done any testing with putting the amp on the quad??
    Im assuming most of the data is sent from the quad (i.e the video feed) to the RE and not much would be sent to the quad from the RE?.. I could probs test this by SSH in and having a look at the stats with netstst/ifconfig I guess.
    Only thing is the amp is pretty heavy, plus needs to be powered.. Obviously placing amp on the RE gives excellant results but yeah... surely the quad sends a crap load more data that in recieves.. perhsps I will play around and see...

     
  5. justin00

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    oh wow thats only $150US!! My works one was like 20k lol.

    I will be honest im not all that knowledgable when it comes to this mw/dbm/blah stuff.... can anyone who is knowledgable on it take a look at the link and see the max Watt it can measure? I can see the frequency is 1 - 8ghz which obviously covers 2.4 and 5.8 but cant figure out the max watts it will measure. I might read the manual or email and ask....but if anyone knows this stuff and can figure it out let me know.

    Thanks again. Looks like a nice little product. For $150 might be worth havibg just to play around with regardless of my amps.
     
  6. justin00

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    Hmm in the user manual it says I need a 40db attenuator for 2000mw... so I guess I would need more for 5000mw (if it really is).

    I guess I need to read up on all this stuff as I dont really understand the attenuator in fu detail... have a vague idea but would like to be sure of what I am doing heh.

    Found a place in Aus that sells it for $180AU and its in stock so thats prettyy good... but yeah will research all of thus more.......
     
  7. SteveMann

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    For as long as I can remember, which goes back most of a century, amplifiers, whether audio or RF always specify the input power. In analog devices it was simply the current being consumed by the amplifier times the voltage. It's the input power, after all. So an amplifier that draws 1 AMP at 5-volts will be advertised as a five-watt amplifier. Inefficiencies in the amplifier circuitry, feedline and antenna will reduce that to an ERP (Effective Radiated Power) of less than 5-Watts. If you could measure the power output of the 5W amplifier you bought, it would probably be more like 3 or 4 watts at the antenna connector. Since you would be adding a transmission line and antenna of unknown efficiency, then you will probably have 2-watts coming from the antenna. And it would all be within specifications. Lasers are similar to how power is measured. If you look at the engineering specs of any laser device you may find two power specifications. One id pulse power and one is DC (or CW) power. Heat is what limits what you can pump into a laser diode. You can run it continuously at 200mw and it will never overheat. And you can pulse it with 2W of power briefly without destroying the diode. So the marketers advertise the pulse rating.

    Some transmitters specify an output in dBm (Decibels per Milliwatt) are measured typically one meter from the transmitting antenna. So an amplifier specced at 5dBm means that at one meter the lab measures 5mw of power. (I know the math is wrong, but the principal is correct). An RF engineer can start with that, calculate a path loss using ideal conditions, such as no penetrations to the Fresnel zone of the transmission path (which is physically impossible on the planet surface), and use the antenna gain at the receiver and the sensitivity of that receiver to produce the advertised range estimates. It's mathematically provable, and though unlikely in real-world use, it's the "up to one mile" you see advertised.

    They aren't lying in the advertising. They are publishing mathematically verifiable estimates. There are few standards in this field so comparing specifications between different manufacturers introduces variance in their testing methods. So one antenna advertised at 5dB gain could possibly perform better than the same design from another manufacturer who claims 6dB gain.
     
  8. justin00

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    Thanka for that. I think I will need to read what you wrote a few times to get a better understanding. Not becausw it wasbt clear, but because I have no clue.

    Ive found a heap of diff testers on ebay and none of them mention the max MW but I suspect the answer I am looking for has been mentioned by you...Just need to read ypur reply a few times and perhaps refer to google too.

    Its 2am here so might go to bed and read it again in the morning. I do appreciate ypu taking the time to write that long reply, so thanks again for that :)

    *edit* actually one thing I will ask before I do alit of research in the morning.... as I said in the user guide it says to use a 40db attenuatir to measure 2000mw... why does it need the attenuator? Does it basically stop to much power going into the measuring device and potentially damagibg it or something to that affect?
     
    #8 justin00, Apr 19, 2015
    Last edited: Apr 19, 2015
  9. SteveMann

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    Those testers on eBay are typically used to measure leakage from a microwave oven - 900 MHz. The important spec you need is their sensitivity. In this case, the lower the number, the better the sensitivity.
     
  10. N017RW

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    Like Mr. Mann said the specs. can be misleading if you make too many assumptions.
    The meter itself can safely read up to 0.0dBm (1mW).


    Let's say your amplifier can theoretically produce 5.5W (37.4dBm).

    The 30dBm attenuator is not enough to safely reduce that amount of power. 37.4dBm - 30dB = 7.4dBm = 5.5mW.
    This 7.4dBm exceeds the 0.0dBm max for the meter.

    Using 40dB of attenuation would take the the 37.4dBm measurement down to -2.6dBm = 0.55mW.
    Now this is within the meter's spec.

    Now take the -2.6dBm from the meter and add 40dB attenuation = 37.4dBm = 5.5W!!!

    The design of this meter will automatically display the proper measurement/number when you enter the amount of attenuation you are using in to it.
     
  11. N017RW

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    Like Mr. Mann said the specs. can be misleading if you make too many assumptions.
    The meter itself can safely read up to 0.0dBm (1mW).


    Let's say your amplifier can theoretically produce 5.5W (37.4dBm).

    The 30dBm attenuator is not enough to safely reduce that amount of power. 37.4dBm - 30dB = 7.4dBm = 5.5mW.
    This 7.4dBm exceeds the 0.0dBm max for the meter.

    Using 40dB of attenuation would take the the 37.4dBm measurement down to -2.6dBm = 0.55mW.
    Now this is within the meter's spec.

    Now take the -2.6dBm from the meter and add 40dB attenuation = 37.4dBm = 5.5W!!!

    The design of this meter will automatically display the proper measurement/number when you enter the amount of attenuation you are using in to it.
     
  12. N017RW

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    Like Mr. Mann said the specs. can be misleading if you make too many assumptions.
    The meter itself can safely read up to 0.0dBm (1mW).


    Let's say your amplifier can theoretically produce 5.5W (37.4dBm).

    The 30dBm attenuator is not enough to safely reduce that amount of power. 37.4dBm - 30dB = 7.4dBm = 5.5mW.
    This 7.4dBm exceeds the 0.0dBm max for the meter.

    Using 40dB of attenuation would take the the 37.4dBm measurement down to -2.6dBm = 0.55mW.
    Now this is within the meter's spec.

    Now take the -2.6dBm from the meter and add 40dB attenuation = 37.4dBm = 5.5W!!!

    The design of this meter will automatically display the proper measurement/number when you enter the amount of attenuation you are using in to it.
     
  13. justin00

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    Thanks again guys I am starting to get it
    Kind of misleading how they say 5watt but due to the way Steve mentioned its not really..well not in the way I would of hoped... As I said though, I'd expect that from most of the stuff from Alibaba hehe.. They were pretty cheap and I did get a few of them to sell.. back in another life... then I remembered I had some left in the cupboard hehe..

    So say the amp can only do 0.5w and I think it can do 5w and add the 40db attenuator.. it won't damage it or anything would it ? its only if I did the opposite.. ? i.e didn't use a big enough attenuator ? but back to my 0.5w example... with the 40db attenuator it would tell me its only 0.5w yeah ??

    Anywho thanks again..Still need to read Steve's post a few more times but starting to get it now :)
    Not sure if I will get the meter after all this :p $170 is pretty cheap... but I won't get much usage out of it... I know a couple of friends will want to borrow it but besides that...
    Oh well if I don't get it atleast I've learnt a few bit from this thread so thanks again everyone :)
     
  14. N017RW

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    Ok,

    1/2 watt Tx with 30dB attenuator (a.k.a. pad) sold with the meter...

    1/2 watt = 500.0mW = 27dBm.

    So 27dBm power from Tx device under test - 30dB attenuator value = -3dBm = 0.501mW.

    That's within the meters spec of 0.0dBm maximum so you would not need additional pads.
    Again, entering the 30dB attenuation value into the meter's set-up parameters would automatically display the correct value in dBm or mW.

    (There's also cable and connector losses but that's not worth worrying about now.)
     
    #14 N017RW, Apr 19, 2015
    Last edited: Apr 19, 2015
  15. justin00

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    very informative thread for me, so thanks heaps guys :)

    Haven't measured the output but I'm sure its atleast like 1W like most of the other kits...
    Might attach it to my RE and play around... took apart my Cisco 877 router (guts inside had a UFL connector luckily) so was able to attach to my amplifier and made a huge differance... obviously you need the device on the other end to be as powerful to send back, but I could see on my wifi analyzer on phone, while the phone couldn't TX back it could still pickup my SSID from a far way away...

    so yeah will attach to RE... Totally O/T but the RE has 2 attenan's... which of the 2 antennas do people hook the amplifier into ?? or does it not matter as its using diversity and as long as 1 antenna is stronger ??