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CC1120: 868MHz-500mW in reality not better than 434Mhz-30mW ?

GGA
Genius 3985 points

Hi,
a practical question/request for your experience:
we are very familar with 1120/434MHz in transparent and packet mode (2-FSK narrow band).

For heavy duty requirements we setup the 868 power version with 1190, according ref.design and measured good results: 3dB more sensitive, 27dB output power.

Now in reality we took two receivers 434 / 868 (packet mode, both based on plain 1120) and two transmitters 434:15dB / 868:27dB and started to check what maximum distance we can reach, just comparing one to one.
Power supply is stable, no disturber around.

Surprising result is that both systems are equal in max. distance.
Measuring against 1120DK's shows ~10dB RSSI difference being close with TX/RX, but with growing distance (through walls etc.) RSSI becomes more and more equal. This matches to our finding 'max. distance is equal'.

I know that with doubling carrier freq. we should lose ~3dB within a building, but this doesn't explain that result.

Any ideas?

  • 0
    TI__Guru**** 317180 points

    Is this line of sight without buildings or is it within a city?

  • 0 in reply to TER
    Genius 3985 points

    TER, it's within a building and / or with walls in between. Line of sight is not a practical example....

  • 0 in reply to TER
    Genius 3985 points

    TER, it's within a building and / or with walls in between. Line of sight is not a practical example....

  • 0 in reply to GGA
    TI__Mastermind 33080 points

    When calculating a link budget, normally a certain guard band is added for additional link margin (typical 10 dB).

    When calculating the link budget / range for indoor applications, I normally use the following figures in addition to the link margin:

  • 0 in reply to RGW
    TI__Mastermind 33080 points

    I would expect that the 868 MHz link with 500mW output power will have a greater range:

    • 868MHz-500mW : approx. 7500m LOS range expectation with a typical antenna loss of 1dB
    • 433MHz-30mW   : approx.4800m LOS range expectation with same typical antenna loss of 1dB (which is not feasible unless the antenna is very large)
    • 433MHz-30mW   : approx.2200m LOS range expectation with a typical size antenna loss of 6dB

    Regards,

       Richard

  • 0 in reply to RGW
    Genius 3985 points

    Richard,interesting table.

    Do you have something similar for 434MHz?

  • 0 in reply to GGA
    TI__Mastermind 33080 points

    Hi,

    I do not have any data for 433 MHz. If I find any then I will post it here.

    Regards,

       Richard

  • 0 in reply to RGW
    TI__Mastermind 33080 points

    Here is an excellent link which summarizes the attenuation through different building material from 500 MHz to 8 GHz:

    http://www.eiwellspring.org/tech/Shielding_by_building_materials.htm

    Regards, Richard 

  • 0 in reply to RGW
    Genius 3985 points

    Great, thx.

    What I read out is that 500M vs. 1000MHz makes about 4-5dB difference.

    What makes me wonder is that this difference remains the same regardless how thick a wall is. I suspected that the more walls are in between, the less a higher frequency performs.
    This would also explain our findings.

    But this table explains only 4-5 dB, where disappear the remaing ~7dB out of my 12dB higher output?

  • 0 in reply to GGA
    TI__Mastermind 33080 points

    Which antennas are you using ?

    Regards, Richard

  • 0 in reply to RGW
    Genius 3985 points

    Our devices are often used somewhere in a box hidden in a wall. So 'antennas' are often a piece of wire at Lambda/4.

    To get as close to reality: that's how we tested it.

  • 0 in reply to GGA
    TI__Guru**** 317180 points

    Have you tested that your antennas are tuned to the surroundings so you get the expected sensitivity/ output power?

  • 0 in reply to TER
    Genius 3985 points

    we know that these antennas are not optimal, but the perspective is to compare 433 against 868 - both with the same limitations, e.g. antenna.

    We do not have much impact to what our customers lay antennas - they just compare the old 433 system with the new, mucg more powerful 868 system. But if the result is 'same reach', they'll ask us if we know what we do ....

  • 0 in reply to GGA
    TI__Guru**** 317180 points

    My question was more: Could the 868 antennas be poorer than the 433 antennas? To do a 1:1 compare the 433 and 868 antennas have to have the same efficiency.

  • 0 in reply to TER
    Genius 3985 points

    Valid aspect. But being close in line of sight (1m) I can see 12dB difference between both systems. This should delete any antenna aspect.

  • 0 in reply to GGA
    TI__Mastermind 33080 points

    If your antenna is a wire and it is cut to a quarter-wave then this is ideal for a low frequency like 433 MHz. Then as mentioned in an earlier comment; for the same antenna performance, the distance would be:

    • 868MHz-500mW : approx. 7500m LOS range expectation with a typical antenna loss of 1dB
    • 433MHz-30mW   : approx.4800m LOS range expectation with same typical antenna loss of 1dB

    and to compensate for the material absorption between 500 MHz and 1 GHz is 5 dB; then the "compensated LOS" range would be:

    • 433MHz-30mW   : approx.6900m "compensated LOS" range expectation with same typical antenna loss of 1dB

    If your device would of been an hand-held device then the area for the antenna is limited. For 868 MHz, a good antenna can be achieved but for 433 MHz, the antenna losses start to increase due to the limited size.

    Regards,

       Richard