For a good year or two I had just a HE C7 in my environment (home). Then I added another C7 in the same environment for testing (located about 10 feet from each other on the same LAN). All was good. Then, I continued using both C7's for testing after adding a C8. It looks as though both radios use the same frequencies (C7 is US-908.4 MHz, 916 MHz whereas the C8 is US-LR-908.4 MHz, 916 MHz). Is it possible for these hubs to interfere with each other if they are placed within 3 feet from each other? The reason I'm asking is because I have recently had troubles with the C8 with inclusion and exclusion processes. It seems this started right around update 2.3.9.18x). I'm aware of the potential for Wi-Fi interference with Z-Wave but I didn't know if it is possible for more than one Z-Wave radio to interfere with another. Thank you
I have had 4 hubs wall mounted beside each other for several years dating back to the C3-C4 days and no issues. This said I only have the zwave radio active on only one of the hubs.
Actually WiFI interferes with Zigbee since both are in the 2.4Ghz spectrum. Zwave as you noted is 900mhz and older cordless phones and baby monitors could possibly interfere but I have never had an issue.
You might try swapping your antennas on your C8 to see if maybe you have a bad one. Just unscrew the antennas and put them on the other spot.
I have 9+ ZWave radios splattered in my house. 5 are Hubitat hubs and 4 are ZSticks.
Three of my Hubitat hubs are in a 3 ft sphere of each other, so 3 ft apart but not 4ft. 
I have no interference problems I can pin to their proximity.
That said, it's true that 3ft is probably the minimum distance and anything more is bonus.
Why are you using two hub with active Z-wave radios located within a few feet of one another?
I can see the rationale of having two hubs located in different regions of a large property such as a house and a detached garage/workshop/barn, etc.
If you want to use multiple hubs in nearby locations, I would suggest using one hub to connect Z-wave devices and a second hub for Zigbee devices. Disable the unused radio in each hub to avoid potential interference.
With Zigbee, you can specify the channel used by the radio. I do not know of any way to specify a specific channel for Z-wave, but perhaps I am missing something.
In my work, I help people with disabilities. I'm often testing various different devices with different configurations. My intentions are to see if different devices will assist people with disabilities achieve greater independence in their homes. Their needs are different than mine. My C-8 is for my home and the C7's are for testing (see how well things work and if they work with the Hubitat).Thus far, most of my experience has been in Z-Wave but I think it is a good idea for me to start checking out ZigBee devices as well. Thank you for this suggestion.
My general observation (I could be wrong too) is Zigbee seems to be declining in popularity (and the number of available devise) as matter continues to add more device types. If you look throuh the community you start to see where certain device types that were once widly available in Zigbee are becomng scarce (Zigbee bulbs are one), You may want to look more toward matter.
Myself, I am primarily Zwave and trying to get rif of my last Zigbee devices. I'll admit my first reaction when reading your post was why not just put all your device on the same hub, but clearly you have a much different (and very commendable) use case than the average user. That being said, @csteele suggestion of three foot seperation would probably be sufficiant. If you have all three hubs within inches of each other, I can see risk of interference, But a bit of space and you should be safe. I run three hubitat's, but only one has the radios active. Occasionally I have a 4th that I will fire up for testing new devices, but I have never observed any negative issues with having both zwave or zigbee radios on line smultaneously
I’m not sure how you concluded that.
Zigbee devices are still plentiful when I search for them on Amazon, including bulbs. People continue to discuss zigbee devices all the time here in the forum.
Can you tell me if Matter also dispenses with a mesh, like Z-wave LR? It uses WiFi, correct?
General observation. Bulbs in particular, I see people in the forum all the time asking about Zigbee bulbs becasue they can no longer find them. Theres is an active thread in the last few days looking for outdoor outlets. I'm not saying it's a large problem yet, just that its starting to happen. If I was considering a switch like that, I would take that into account if considering switching to Zigbee.
I beleive it is at least mesh like. In my case the few (three) Matter devices I have are matter over thread using AppleTV for my Thread border router. They are widely dispursed around my home and I don't seem to have any of those issues that tend to pleague small or weak meshes. Generally I'm currently a(nd will remain) predominantly Zwave and as more devices are added or replaced I will first look to ZwaveLR before Matter.
For me (I know im the outlier on this one) Zigbee has always been a flaky PIA, no matter how many repeaters I added. Zwave has always been much more robust, and with Zwave LR, its been even better. I've been working on getting rid of my Zigbee stuff to the point I have about 5 devices left, and will be retiring those by the end of the year.
Z-wave operates in the 900 mHz portion of the spectrum. This signals will penetrate through walls and floors more easily than the 2.4 gHz signals used by Zigbee. However, Z-wave is limited by the number of hops from the hub to the final device. On the other hand Zigbee signals do not penetrate as well, but the strength of Zigbee is its mesh capability.
I use both Z-wave and Zigbee. While I have had issues with specific devices, overall, both protocols work well, especially after Zigbee 3.0 was released.
I have not yet ventured into Matter. I am waiting for the debris to settle. It is still a new platform and adopters are still figuring it out.
If your matter devices use thread as their physical (i.e. radio) communication protocol, then yes it’s a mesh network.
If matter devices use WiFi to communicate, then there’s no mesh.
So it depends. Matter can be both.
