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omarraza
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Joined: Sun Jan 13, 2019 12:21 am

Tx Signal is poor, but Rx Signal is good

Tue Jul 07, 2020 4:29 pm

Hello,
I tried to create PTP which SXT 5HPnD as a bridge and LHG5 as station bridge, Tx Signal is is good which it's value is -66 but the Rx Signal value is -72 which it's not good and also Tx/Rx CCQ is 90/45,
the Question is here why Rx is very poor does it's about the Antenna gain dBi because the SXT 5HPnD is 16dbi but LHG5 is 24dbi ?
or what is the problem please help.
 
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bpwl
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Re: Tx Signal is poor, but Rx Signal is good

Tue Jul 07, 2020 9:19 pm

Well, what happens with the antenna gain, if you are in a regulated domain, that dictates the max EIRP (Effective isotropic radiated power) that can be used?
The EIRP is calculated from the peak radiation (in the direction of the highest antenna gain) and then calculated as if we radiate with that intensity in all (= isotropic) directions.

This means that to obey the legislation the radio power must be reduced so that the EIRP is within the legal boundary.

Lets take one example. FCC regulation states a max EIRP of 30 dBm. (Other countries/regions are even lower, like Europe, the ETSI region, uses 20 dBm as limit.)

The SXT 5HPnD has 16 dBi antenna gain. (Gain in dB compared to i(sotropic)). The TXpower of the radio will be reduced to 14 dBm. The effective isotropic radiated power is 14dBm+16dBi= 30dBm
The LHG5 has 24 dBi antenna gain. The TXpower of the radio will be reduced to 6 dBm. The effective isotropic radiated power is 6dBm+24dBi=30dBm
https://www.everythingrf.com/rf-calcula ... ated-power
This means the top power in the beam for both cases is equal. It is 30dBm.

But the working of the antenna is reciproque. The received signal on the SXT is amplified with 16dBi, the signal received at the LHG is amplified with 24dBi by its antenna.
The signal strength sent was equal, so normally also the received signal strength is equal on both sides, or at least close in power values.
It looks like that signal strength is -90dBm. The LHG5 amplifies this till -66dBm (-90+24).
The SXT does similar and amplifies the signal till -72dBm. (-90+16 ?).

( Not all details are given, if the device is using 2 antenna then the TXpower is 3dBm less, or 3 dBm is TXpower set on the LHG.. If the device is not certified for TPC, then again the allowed EIRP is lowered by 3 dBm. This would mean 0dBm as TXpower. Some chipsets in Mikrotik have a minimum TXpower of 2 dBm, even if you set it to 0 dBm)

This does NOT fully explain the difference in CCQ. CCQ is somewhat related to the received signal strength, but this difference 90/45 most probably has also other factors like interference reducing the CCQ. Both devices see a different panorama of the environment. Interference happens only to the receiver side (not the sending side).
 
omarraza
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Topic Author
Posts: 33
Joined: Sun Jan 13, 2019 12:21 am

Re: Tx Signal is poor, but Rx Signal is good

Thu Jul 09, 2020 2:20 pm

@bpwl Thanks.

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