70cm Backbone

I have done some experimenting with both the Doodle labs cards (no longer made) and Xagyl cards.  My plan was to use them as a tree-penetrating last-mile link. You run these at 5 MHz bandwidth (SISO of course) and probably at less than max encoding, so the throughput is not that great. But it is just enough to stream SD video (in one hop). The main problem I ran into is that the Xagyl card has its maximum sensitivity around 443 MHz right where US repeaters are located, rather than down at 425 MHz where broadband/ATV activity is located. I found that a carrier signal of -70 dBm (at 443 MHz) is enough to cause the reported noise floor to rise (and OFDM signal to drop). Around here, if you poke an antenna up in the air, there will be plenty of signals that strong, in that frequency range.  When the card gets hit with a signal stronger than -70 it tends to drop the link until about 1 minute after the problem signal goes away.  None-the-less, I was able to test a 6 mile hilltop-to ground level link with only occasional drop-outs (using Doodle Labs).  My plan is to use 10 MHz BW helical resonator filters on each end to deal with this but I have been busy with other things recently ...

The filters I am using are from ocicom.com - these are not cheap but about half the cost of the dci.ca filters.  OCI will tune the filter to any center frequency you need. A 10 MHz filter allows a 5 MHz central region with reasonable group phase delay characteristics.