Did I step on your toes, @danabw ? 
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Thanks, I'll message them.
I'm using below for Z-Wave on my hub, works very well, has resulted in more direct connects and better connections overall.
It appears support both Z-Wave and Zigbee, as it lists support for the necessary frequencies (Zigbee 2400, Z-Wave 900).
https://www.amazon.com/gp/product/B09CDJ66K2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Supported Frequencies:
- Frequency Range: 698-960 MHz, 1710-2170 MHz, 2300-2700 MHz; Gain: 7dBi; Direction: Omni-directional; Waterproof: Rainning Proof;
Note - this is NOT the same antenna as the one referred to in the OP, though it looks very similar. That one is a magnetic mount.
The one I'm using is held in place via a bolt and is from a different vendor.
So you could use this for Z-Wave and put the one working antenna you have back on Zigbee duty. Or you could try them both out on Z-Wave and Zigbee.
But also letting @support_team know about this is a good idea. 
Oops - not at all, great minds! 
Thanks, I messaged @support_team. Since I don't have a lot of technical knowledge on the subject, I wouldn't even know what to look for in an antenna, if I had to buy it on my own. The C8 was able to connect to the majority of my z wave devices directly, as compared to the C7, so the stock antenna seems fine to me. I'll wait to hear from support before purchasing something from Amazon or Ebay.
Hence my post.
But I agree w/starting w/Support to see what they can do for you. If that goes nowhere, the antenna I posted a link for has worked well for two of us so far, and I believe it would also work well for you.
You could always reply to the email and provide additional details to your case. Perhaps you selected a radio being offline, which was not observed based on your hub's engineering logs, thus the response you received. You could also provide the additional details via private message, since you are here, in the community, as you already did.
See the PM I just sent you...
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Will do. Thanks!
When significantly more devices were running at 9.6 instead of 100 on the C-8 vs. my C-7 I bought the rectangular version:
https://www.amazon.com/gp/product/B09NPH2X8F/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
I tried them pointing parallel to my home's main N-S axis and at 90 degrees. I gave them each over 2 weeks in each direction and I'm still worse off than the C-7. Below is a link where I posted a screenshot of my charts showing how many are are 100, 40, 9.6 and how many are faster, slower or the same:
BTW, 2 of the faster ones, my (Stuart's) upper and lower hutch lights are plugs that sit right below the C-8. But, at the same time, my Fibaro KeyFob is in that same room and only a couple of feet further away from the hub, but it is slower than before. So go figure.
One thing to note is that virtually all of my devices are in metal boxes, that I can't change, as I live in a condo, but some aren't, such as the free standing Fibaro Key Fob and the Palm Room Table Lamp Plug. While the devices connecting at 100, 40, and 9.6 varies from day to day, on only 1 day since mid-April when I installed the C-8 did I see any improvement over the C-7. Below is my slower/faster devices as of this morning.
I'm now thinking of trying one of the other two antennas on that page. I didn't get the real big ones at first as I'm not really sure I have a good way to mount them, so I was thinking of trying the smaller one first. This one:
https://www.amazon.com/gp/product/B07SRWJDL6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
vs. this one:
https://www.amazon.com/gp/product/B09CDJ66K2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
As I've also read that sometimes the lesser powered ones can perform better. But, before I pull the trigger on either of those I'd like your opinion on whether you think I should try to go with the bigger or smaller one, or if it even matters at this point.
Any ideas why I'm getting consistently worse performance from the C-8 vs. the C-7?
Are you sure you don't have a ghost device messing up your mesh? This does not look like an antenna issue, but rather a mesh issue.
Generally speaking, changing the antenna is not going to improve anything, and more likely will make it worse. The radio's power is constant. All the antenna does is shape the signal. A higher gain antenna may create a shape which delivers a weaker signal to your devices, depending on the physical layout of your devices.
I don't think the ghosts show up in that table, but I'm not a Z-Wave expert. See this topic: Zwave and ghosts again
Here is my floorpan. The hub sits on top of a mostly wood cabinet in Bedroom 2 right by the wall between the bedroom and the Master Bedroom. The room in the lower left with no label is the Palm Room and the Den/Exercise room is the Wegitorium. The Dining room takes up about 1/2 the distance shown and the other 1/2 (ending with the wall to Bedroom 2) is the Family Room (also called the Living Room). BTW, the floorpan shows a closet between the living room and Bedroom 2, but it is really 90 degrees from there on the wall by the 3rd bathroom.
I don't believe you have an antenna issue.
I would suggest that you start a new topic asking for Z-Wave help, about why your devices are slow. This topic doesn't draw those users who can help, because they aren't looking at replacement antennas as a solution. I personally don't know enough to help you. Maybe @csteele, or someone else who is very knowledgable...
Thanks. I do have a thread on the C-7 being worse for me than the C-8, but I'll start a new one pulling some stuff from those 2 into 1.
In my case I also found that C7 (with antenna mod) has better gain and more direct connections than C8
https://community.hubitat.com/t/testing-results-of-z-wave-antennas/118062
I saw that. Seems insane, but you are getting the same worse off results with the C-8 that I am experiencing, and I’ve been running daily tests for about 2 months now. I didn’t do the C-7 mod as I didn’t want to be without my hub for a couple of weeks and was hoping for an official version with antennas.
BTW, I too tried a 3 rd party antenna, which didn’t help either.
If there is some kind of attenuation going on in the C-8 as you suspect in the linked thread will it be possible to fix this via software?
If not, is this perhaps some manufacturing defect or an engineering one or both, and what would be required to fix that?
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One other question: I've notice certain devices vary greatly in their connection speed on a day-by-day basis. One day they will connect at 100 and the next at 9.6. They might stay at one speed for days only to suddenly change. I purposely went with Z-Wave Plus devices as I thought they were supposed to be free from Bluetooth, Wi-Fi and other interference. I'm not moving any furniture or pictures around. So what would be causing them to continually fluctuate like that?
I too have seen this with the C8. My C7 also does this too but to a lesser extent. At first I thought maybe there was some external interference that happens regularly near my house- but it would have to happen at 3AM ( database cleanup time) when RF traffic is at a minimum. Maybe the Silabs chip algorithm flushes the mesh at intervals and starts over.
