On the LinkSys you have two aerials and in the software you can select left or right. but which aerial socket is left and which is right ? looking from front or back ? 

Zl4DK, Your answer can be found in Chapter 10 of WNDW on pages 7 and 8.

Thanks that helps a lot. It explains that the radio simply transmits back on the aerial it last got the best signal on. This would normally work OK but would not necessarily work very well for a station with two high gain aerials pointing in different directions as some have suggested. (well at least not when it was required to pass traffic from one direction to another). It would seem that two radios (one on each high gain aerial) connected by a cable would be a much better system.

For field day this past summer, I setup a UBNT rocket M2:

channel0: 24db grid antenna pointing 8 miles to another node and connecting into mesh.

channel1: 90 degree ~10dB sector panel pointing in another direction to support the field day site.

There was a hill and water tank blocking direct line of sight to the field day site. We were streaming video over this hop link. I didn't notice anything unusual or any unexpected degradation.

Each antenna port is basically a transmit receive chain.

They have always been intended to work in tandem to cover the same area.

The individual ports have been used (abused) by some in the past to create links that do not cover the same general coverage area as it was the only way at the time to link distant networks. This is a habit I'm looking to see to be broken.

The much better and really only reasonable method (based on hardware design) is to use DTDLINK to link nodes together.   Both antenna ports (on those devices that have them) can then be used to cover the same area letting you do dual polarity which can increase the performance of the network.

Also it may be wise for any real deployment to look towards the Ubiquiti gear over Linksys (use Linksys only for in lab experimentation and learning while the real nodes get that better RF performance of Ubiquiti)

We may be all be thinking of 2 different scenarios for this discussion:

1) Production designed solution: Everyone build in DTDLink for this scenario and optimize the mesh infrastructure. Very bad idea to "jury-rig", need to be able to trust our mesh.

2) Incident/Emergency: Using duct tape to get something to work with the tools/parts available.

In the spirit of #2, what can work and what does one need to be aware of? While the relay node of 2 antennas in 2 directions on the 2 node's antenna channels 'works', what do we all need to be aware of? Is there a lot of re-transmission due to lost broadcasts on the wrong channel? 50% extra handshaking traffic? How do we quantify any deficiencies?