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I recently tested a little wireless device in a chamber to get its radiated output power. The DUT sat on a pedestal with a ring of antennas all around it. The antenna is a little pcb slot antenna, that's supposed to have a uniform gain of 2dBi all around it in the X-Y direction. It sits off to the side of the product with the pcb behind it.

When I look at my X-Y plane measurements for the TX power I see a lot of the energy shifted to the side away from the PCB. I assume this is either reflected energy or my PCB is changing the performance of the antenna as a whole system, but I'm not sure.

I put in 5dBm of power and after subtracting the loss for free space from the measurements at the antenna ring I got 11dBm on the right side, and 2dBm on the left side (that's blocked by the PCB). Can I assume if I increase my TX power to 20dBm that my output power will also increase by 15dBm. So I'd see 17dBm, and 36dBm accordingly?

I'm assuming I can just do all this as addition and subtraction since it's DB but I'm new to wireless and not entirely sure.

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2 Answers 2

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Yes, you ought to see the increases you calculated since the power in any given direction is proportional to the transmit power. It sounds like the PCB might be reflecting the signal to one side.

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Can I assume if I increase my TX power to 20dBm that my output power will also increase by 15dBm.

Yes, by 15 dB. It should be linear unless your circuit has diodes in it, or gets overheated etc.

So I'd see 17dBm, and 36dBm accordingly?

You would see 17 and 26 dBm eirp respectively.

Your figures sound a bit too good to be true. For an omnidirectional antenna, on a PCB, I'd expect the peak gain to be no more than 0 dBi, so with 5 dBm input you shouldn't see more than 5 dBm EiRP. If it's not omni, more, but to see +6 dBi as you suggest would only happen if the antenna was specifically designed to beam in one direction, like a patch or a yagi.

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