So today I received my new RBD52G (A.K.A. hAP ac²). I did a quick test of WiFi performance in my particular layout (family house, walls made of bricks and concrete, two floors, concrete ceiling between floors). I configured RBD52G in bridge mode, so no routing duties this time. I placed it next to RB951G, which is also configured in bridge mode. All WiFi setup was identical apart from SSID (I wanted to have control over which AP I'm connected to) and 5GHz WiFi on RBD52G. ROS version 6.42.7 on both.
Personally I'm not after last bit of WiFi speed (I'm radio engineer, working for MNO for living, so I believe in hardware - copper and silicon
), so I was focusing on coverage with decent (say 10 Mbps) performance rather than trying to get highest possible speed.
As RB951G doesn't support 5GHz band (and 5GHz has lower coverage after all), I will focus on 2.4GHz performance ...
Specs say that at low speeds (6Mbps or MCS0) RB951G has 3 dB higher transmit power than RBD52G while at higher speed (54Mbps or MCS7) the difference is almost 0. On the other hand, Rx sensitivity is more or less the same at low speeds while it is slightly worse for RBD52G at higher speeds. Which means that RB951G should in theory perform slightly better in DL at low speeds (low signal) and slightly better in UL at high speeds (good signal). As I was focusing on low signal conditions I can't comment on UL performance in good signal.
Now to the observations. I was using my Samsung Galaxy A5 phone with WiFi Analyzer installed. Signal levels showed consistent difference of approx 3dB in favour of RB951G, so specs don't lie. The difference was persistent over whole observed Rx signal strength range (from -55 dB quite near both APs to around -77 dB at the coverage edge). The measured throughput was consistent with this difference. I was using speedtest against internet server, so my DSL line speed (30/5 Mbps) was clearly the bottleneck at decent signal levels. As Rx levels dropped below -70dB, the WiFi became the bottleneck and there observed difference in signal levels showed as difference in acheivable speeds as well. My worst measurements were:
RB951G - signal level -75dB, speedtest result 10/5 Mbps
RBD52G - signal level -77dB, speedtest result 7.5/3.5 Mbps
(both tests conducted at the same spot). Both (difference in signal level and difference in speed) are consistent: difference of 3dB in signal level translates to double (or half) electrical field strength (measured in V/m), 2 dB difference means factor of 1.6 (or 0.6) and 1 dB means factor 1.25 (or 0.8 ) ... the observed speed differ by the same factor.
I can not explain the difference in UL though. Specs say that Rx sensitivity at low signal level should be the same for both APs, so measured UL speed should be the same, but was lower with RBD52G anyway.
Of course I tested the 5GHz AP as well. Specs say that RBD52G has Tx power at low speeds (6Mbps, MCS0) is almost the same as at 2.4GHz. Which I can confirm: close to AP signal levels of both 2.4GHz and 5GHz SSIDs were almost the same (give or take 1 dB). However, wall penetration is much worse at 5GHz than at 2.4GHz, so signal level droped much more when I left the room. And observed speeds dropped with it.
A short note on temperatures: RB951G is not a cool beast either. After a couple of hours, both closed and a small closet and almost idle, both were quite warm to touch. My feeling is that RBD52G is indeed warmer by a few °C (but not much). Case of RB951G has quite a few ventilation holes and my feeling is that after I'll make a few holes in RBD52G case temperatures of both should be almost the same. I imagine that if router's CPU was loaded, the temperature difference would be higher as well (specs say that max power consumption of RBD52G is twice as much as RB951G).
My verdict: if one needs good WiFi coverage, but doesn't need the highest possible speeds, the good old RB951G is still a better choice (and lower price adds to the advantage). However if one needs high speeds, 5GHz ac is a must and RBD52G is a clear winner in this duel. Another use for dual-band APs is small mesh of APs where one band (preferably 5GHz due to higher throughput) is used for "backhaul" ... with single band one simply can not create a decently performing mesh.
As I'm after good coverage at low end speeds, I'll keep using RB951G as AP while RBD52G will move into equipment rack to take over routing duties. I guess temperature will be, despites of CPU being loaded more, still acceptable as WiFi will be disabled.