Just putting this here in case it's useful to someone else.
I'm still working on setting up my local nodes and meshing to my buddies houses. Part of my testing involved going to the top of the tallest hill in my town, (1300 feet above ground level, the rest of the ground is relatively flat around here).
The problem was that I couldn't connect to my home node, around 2.5 miles away, good line of sight. Testing showed that my home node received all my messages, but the return confirmations never made it back.
The problem? The tallest hill in town is also home to two fully loaded cell towers, blasting away, presumably deafening my node, overloading the front end with strong out of band signals. 900mhz GSM perhaps?
At any rate, I looked into band pass filters. I'm a ham and I've dealt with poorly filtered front ends by the likes of baofeng, so I know filtering can do a lot, and I wasn't sure how much a standard node came with.
The answer? Some, not much. Enough for most use cases. Oddly, most information I found on the subject, specific to Lora, advocated against using filters, saying they are usually unnecessary, etc etc.
While that's probably true most of the time, it's definitely not true all of the time. I'd be interested to see some a/b comparisons of 20-40 foot high nodes in urban environments, both with and without filters.
From experience I know that a baofeng 144mhz radio (known to have poor filtering) with a 1/4 wave vertical antenna up 40 feet, was mostly deaf to any distant signals, and actually performed better in some instances by just using the stock antenna and standing on the ground. Likewise when using the 40 foot tall antenna and adding a filter, the reception was massively improved.
Add to that experience my most recent test. I added a 915mhz band pass filter to my node and brought it on top of the hill, next to the cell towers, and was able to make full duplex communication with my home node.
I'll be doing more testing, some a/b testing with the filter on my home node to see if it improves my range tests.
Filters probably aren't for everyone. And they aren't free performance gains, you can't forget insertion loss. But don't be afraid to buy one and try it if you think you might be having desense issues.
