The wifi performance on a couple of 2850 routers and a 2820 was so erratic on 802.11g/n mode that I'd turned them back to "802.11g only" in order to get reliable connection rates.

Prior to the change below, my test devices (within 5m of the routers) were connecting at 36-54Mbps (Wireless-g), and if 802.11n was enabled, connections would initially start at reasonable speeds, but then drop off and either settle around 5-11Mbps or disconnect/reconnect...rinse...repeat etc.

While doing some testing, I changed the antenna orientations so instead of all three being vertical, one was straight up, the next was at 45 degrees and the last one at 90 degrees.

What a difference! Wireless-g was as solid as ever, but having re-enabled wireless-n mode, I've been connected at 72-144Mbps for several hours, and connections on the adjacent floors are much better.

While not a conclusive test, the effect is noticeable on a laptop, Android tablet and Android phone.

I have an appreciation of antenna polarisation and positioning, but the difference here between 'all vertical' and 'multi-angle' is impressive.

Thanks for tips
An other report (tests by me) after having it about 10 motnhs

First the suggestions with 3 external antennas are always the 2 antennas (left and right) be at about 45 degree and the middle vertical

I have a program called inSSIDer and watching the RSSI (-bd) and i can see sometimes(example a week later) the db changes from -27db to up -47db but then later going back down to normal the decibels

Guessing is not much stable the wifi section or at electric socket affecting if are any other devices connected but also from tests that it depend each reboot as well

Example if i let the 2850 settle down for 2 min then most of cases it restard with arounf -30db (at least for my case) while if i reboot immediately will restart from the lets say -35db was last time

Again ot sure if this is because wifi is not stable or is because at the electric socket it needs the draytek stay by it self connected (not have any extension or plug with other devices as they consume some decibels from that electric socket...as that matters as well for a modem and is important point to care

So in others words if draytek settle down few min the 2850 seems is like having a good "breathing" and start good again the wifi and in modem section

Wow!

As you said - no brainer! I have a Vigor 2850n and 2 Vigor AP-700. Moving the aerials to 45Ëš really boosted output. I haven't quantified the difference, but I'm guessing that the connectivity complaints will drop off from now on. Will keep you posted.