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I purchased a "hfield Wi-Fire" adapter recently because it claimed extended range. I found it new in the box at a discount store. It's a 2.4 ghz only adapter. It comes with a small clamp to allow you to attach it to the top of your laptop. Otherwise it is pretty standard, connecting to your laptop via USB.

enter image description here

When I plug it in, I can do the following on my Ubuntu desktop to get the transmit power information.

ericu@ericu-desktop:~/Downloads$ iwlist wlan0 txpower
wlan0     unknown transmit-power information.

          Current Tx-Power=27 dBm   (501 mW)

I was intrigued by this, because 501 milliwatts is a huge transmit power compared to most devices. I am pretty sure this number is just reported by the driver in the linux kernel, so it could be programmed to say anything. I opened up the unit, which was quite easy. enter image description here

The design of the internals appears to be a typical WiFi USB stick feeding a yagi antenna. The drive element appears to be a partially folded dipole. I'm really unsure what functions as a ground plane in this design, unless it is the coax cable itself.

There is also a patent number listed which is 6307524. More information on that patent here. The patent appears to cover the PCB design for the antenna only. The description is a little curious, since it describes an antenna with the transmission line impedance matching the driven element. This is a desirable quality in most antennas. I guess they are really just saying this antenna design does not require a matching transformer.

Is it possible that this WiFi adapter actually transmits 501 milliwatts of power? Or does that figure include the effective gain of the Yagi antenna printed on the circuit board?

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  • \$\begingroup\$ It's not legal to use more than 100mw in most places. \$\endgroup\$
    – dandavis
    Commented Oct 10, 2017 at 22:28
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    \$\begingroup\$ Since it's a Yagi antenna, which is directional, perhaps it's 501 mW EIRP? \$\endgroup\$
    – user20574
    Commented Oct 11, 2017 at 1:08

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Yes, standard USB supplies 5V @ 500mA which is 2.5W of power. 2.4Ghz power amplifiers are typically 30-50% efficient so 500mW out is no problem.

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  • \$\begingroup\$ The question is not whether the USB provides enough power or not, although it's a resonable to do a reality check like this. \$\endgroup\$
    – winny
    Commented Oct 11, 2017 at 6:47
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    \$\begingroup\$ USB WiFi adapters are available (legally in the US) at up to 1W power out. Under FCC rules, at 1W power out, it is legal to use an antenna with up to 6dBi of gain (EIRP of 36dBm). For each 3dB over 6dBi, the power out has to be reduced by 1dB. It would be legal and entirely possible that this adapter is 500mW out (33dBm) and the antenna is probably around 5-8dBi resulting in an EIRP of 38+dBm. Since the companies website is gone and this is an obsolete adapter, it will be difficult to determine what this reality of this adapter is, but to answer the question, yes it is possible. \$\endgroup\$
    – Dean Franks
    Commented Oct 11, 2017 at 7:48

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