Output of radiotap.dbm_antsignal has two readings, what is the first and last?

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I think the driver you have is reporting a maximum and then the actual value. For instance, here is the output from the same frame caught by four different adapters on Linux:

tshark -r beacon.pcapng -T fields -e wlan.sa -e radiotap.dbm_antsignal -c 5
00:01:02:03:ef:72       -58
00:01:02:03:ef:72       -60
00:01:02:03:ef:72       -64,-64,-65
00:01:02:03:ef:72       -61,-64,-61

In order, we have here:

• Realtek Semiconductor Corp. RTL8814AU 802.11a/b/g/n/ac
• Senao EUB1200AC AC1200 DB [Realtek RTL8812AU]
• Intel Corporation Wireless 7260 (rev bb)
• MediaTek Inc. (mt7612u)

For the Intel case, here is what Wireshark shows:

Radiotap Header v0, Length 56
    Header revision: 0
    Header pad: 0
    Header length: 56
    Present flags
    MAC timestamp: 388880163915
    Flags: 0x10
    Data Rate: 18.0 Mb/s
    Channel frequency: 5745 [A 149]
    Channel flags: 0x0140, Orthogonal Frequency-Division Multiplexing (OFDM), 5 GHz spectrum
    Antenna signal: -64dBm
    RX flags: 0x0000
    timestamp information
    Antenna signal: -64dBm
    Antenna: 0
    Antenna signal: -65dBm
    Antenna: 1

I am guessing the first one of the three fields is a maximum and then the next two are the specific antenna values. I would think it is driver specific.

Some different adapters, and I know I have some antenna problems (broken/missing):

tshark -i wlan1 -i wlan10 -i wlan11 -i wlan90 -i wlan104 -T fields -e frame.interface_name -e wlan.sa -e radiotap.dbm_antsignal -c 30
wlan10  00:01:02:03:ef:72       -66,-81,-66
wlan10  00:01:02:03:ef:73       -66,-81,-66
wlan10  00:01:02:03:ef:74       -68,-82,-68
wlan11  00:01:02:03:ef:72       -56,-70,-56
wlan11  00:01:02:03:ef:73       -57,-71,-57
wlan11  00:01:02:03:ef:74       -56,-70,-56

   <cut>

It looks like the first value is the maximum in this small sample. I get two values from an Atheros chipset:

Atheros Communications, Inc. AR9271 802.11n

wlan1    00:01:02:03:97:b9       -24,-24

With Wireshark showing:

Radiotap Header v0, Length 36
    Header revision: 0
    Header pad: 0
    Header length: 36
    Present flags
    MAC timestamp: 2018179335135440602
    Flags: 0x10
    Data Rate: 6.0 Mb/s
    Channel frequency: 2462 [BG 11]
    Channel flags: 0x00c0, Orthogonal Frequency-Division Multiplexing (OFDM), 2 GHz spectrum
    Antenna signal: -79dBm
    RX flags: 0x0000
    Antenna signal: -79dBm
    Antenna: 0