# ePMP 2.4 GHz Limiting Power [Point to Point]?

When I enter "24 dBi" for the antenna gain, the transmitter output power is limited to 18 dBm (It states the total cannot exceed 42 dBm). I thought I was allowed 24 dBm with that antenna gain.

This is for non-sync ePMP 2.4 GHz radios connected to 24 dBi dishes for point-to-point purposes. This is in the US.

My calculactions:

As a reference point, when using a 6 dBi antenna, I should be limited to 30 dBm. For every 3 dbi increase for the antenna, I should reduce my transmitter power by 1 dB. A 24 dBi antenna is rated 18 dB greater than the 6 dBi reference. Dividing 18 by 3 results in 6 [so I need to reduce my transmitter power by 6 dBm from 30 dBm which results in 24 dBm].

Is there an error in the ePMP interface, or is there something I am missing?

Thanks, Chris

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I thought the rule in the USA/FCC was 36dBm/4watts EIRP total. Your 24dBi antenna and the radio power of 18dBm is actually higher then the allowed EIRP by 6dBm.

@Eric Ozrelic wrote:

I thought the rule in the USA/FCC was 36dBm/4watts EIRP total...

That is the rule for devices setup to connect to multiple other devices (Point to Multipoint). If an antenna is used above the 6 dBi reference, the transmitter power has to be reduced an equal amount so the maximum EIRP is always 36 dBm.

There is a different rule for devices setup to connect to only one device (Point to Point) that allows for a higher EIRP. For every 3 dB increase in antenna gain, the transmitter power has to be reduced by only 1 dB. For example, a 9 dBi antenna is 3 dB higher than 6 dBi, but when used in PTP situations, the transmitter power only has to be reduced by 1 dB (to 29 dBm).

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I did some further checking regarding the FCC regulations and the ePMP 2.4 GHz connectorized radio settings [with the 2.4.3 firmware version].

The right half of the chart below shows the FCC regulation of 2.4 GHz PTP links. The left half shows that the ePMP radio actually allows with country code set to US. It appears the ePMP is limiting the transmit power more than it needs to.

FCC 15.247 states :"...the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi."

It looks like there is an error in the ePMP calculation. It is actually reducing maximum power by 2 dB (instead of only 1 dB) for every 3 dB of gain over 6 dBi. Is this in error, or is there another limiting factor associated with the ePMP radios?

Thanks - Chris

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I have now upgraded to v2.5 of the firmware. The same problem exists. I am still interested to know "Is this in error, or is there another limiting factor associated with the ePMP radios?"

Thanks,

Chris

Are you using TDD-PTP or ePTP mode on the AP?

TDD-PTP since I need the narrow channel capability. I have not checked what happens when set to ePTP, but I will do so later today or tomorrow.

I checked: on firmware v2.5 the same issue occurs (ePTP mode with 20 MHz channel width).

eric the rule of 36db is for ism ptmp not ptp. i think in 2.4 for every 3 dbi in antenna gain above 6gbi you must reduce the output power by 1 dbm. so if you have a setup with an antenna of 6dbi you can put 30 dbm through it. if you change your antenna out to a 12dbi then you must take your power down to 28dbm.

Hi all,

In 2.5 release issue with incorrect Power limitation really exist.