Allow to select 802.11n channel width (HT)
using `ifconfig -v <wirelessinf> list chan` possible to see supported HT modes:
# ifconfig -v rtwn0_wlan0 list chan Channel 1 : 2412 MHz 11b Channel 7 : 2442 MHz 11g ht/20 Channel 1 : 2412 MHz 11g Channel 8 : 2447 MHz 11b Channel 1 : 2412 MHz 11g ht/20 Channel 8 : 2447 MHz 11g Channel 2 : 2417 MHz 11b Channel 8 : 2447 MHz 11g ht/20 Channel 2 : 2417 MHz 11g Channel 9 : 2452 MHz 11b Channel 2 : 2417 MHz 11g ht/20 Channel 9 : 2452 MHz 11g Channel 3 : 2422 MHz 11b Channel 9 : 2452 MHz 11g ht/20 ...
with `ifconfig <wirelessinf> channel N:ht/WIDTH` you can set the desired channel width, i.e.
# ifconfig rtwn0_wlan0 channel 1:ht/20
see "channel" section of https://www.freebsd.org/cgi/man.cgi?query=ifconfig&apropos=0&sektion=0&manpath=FreeBSD+12.1-RELEASE+and+Ports&arch=default&format=html:
channel number Set a single desired channel. Channels range from 1 to 255, but the exact selection available depends on the region your adaptor was manufactured for. Setting the channel to any, or - will clear any desired channel and, if the device is marked up, force a scan for a channel to operate on. Alternatively the frequency, in megahertz, may be specified instead of the channel number. When there are several ways to use a channel the channel num- ber/frequency may be appended with attributes to clarify. For example, if a device is capable of operating on channel 6 with 802.11n and 802.11g then one can specify that g-only use should be used by specifying ``6:g''. Similarly the channel width can be specified by appending it with ``/''; e.g., ``6/40'' specifies a 40MHz wide channel, These attributes can be combined as in: ``6:ht/40''. The full set of flags specified following a ``:'' are: a (802.11a), b (802.11b), d (Atheros Dynamic Turbo mode), g (802.11g), h or n (802.11n aka HT), s (Atheros Static Turbo mode), and t (Atheros Dynamic Turbo mode, or appended to ``st'' and ``dt''). The full set of channel widths following a '/' are: 5 (5MHz aka quarter-rate channel), 10 (10MHz aka half-rate chan- nel), 20 (20MHz mostly for use in specifying ht20), and 40 (40MHz mostly for use in specifying ht40). In addition, a 40MHz HT channel specification may include the location of the extension channel by appending ``+'' or ``-'' for above and below, respec- tively; e.g., ``2437:ht/40+'' specifies 40MHz wide HT operation with the center channel at frequency 2437 and the extension chan- nel above.