FWIW - my memory says that the stock C-8 antennas are are 3 dBi 4G antennas.
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while it's tempting to "bigger is better" in terms of dBi, keep in mind the higher the dBi the more directional the antenna can be. My 2cents says for a ranch home,with a flat-ish type layout, a higher dBi antenna won't hurt, and may even help. But for an older 4 floor colonial you'll get better results with a lower dBi antenna as it can spread the signal better in the vertical direction.
I tend to stick with lower dBi antenna in most cases, as with a higher dBi antenna you can hit the sensor out at the mailbox, but the sensor with it's tiny antenna and low transmit power can never reach back to the hub.
Probably more important than dBi is VSWR, which correlates with the antenna's ability to better respond to weaker signals, the lower VSWR the better the antenna's ability. generally a rating of 2 or under is good.
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The stock antennas have poor vswr: over 2 for z wave with efficiency around 0.5. I got matching antennas: ANT-916-OC-LG-RPS for US z wave and ANT-2.4-OC-LG-RPS for zigbee. Very good efficiency, gain in the 2-3 range, and look alike except for a small colored band near the top that represents which frequency it is for.
Note that if your country (e.g. EU members) uses zwave frequencies in the 860s, you would get the ANT-868-OC-LG-RPS instead of the 916 version.
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Gain equals directionality times efficiency. Stock antenna for z wave has about 0.5 efficiency and 1 dB gain. So a 2 dB 1.0 efficiency antenna has the same directionality as the built in.
The follow-up question, is which post is which?
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FWIW, I upgraded to the C8-Pro from a C7 and could not get Zigbee devices to work in the same room as the hub with 15ft direct line of sight to the device. I replaced both antennas with these Amazon.com and they instantly started working. Hoping to get a little better z-wave range too to connect a few device too far out that I haven't been able to connect but haven't tried that yet. I am on a large ranch style so vertical directionality limiting is not a factor for me as there are no device above the antenna.
This indicates that the antennas themselves were bad. It. There have been several instances of this where a replacement oem antenna fixed the issue.,
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Rick - for future clarity's sake, you may want to fix the quote in your post above?... I thought I was going nuts with "Does Rick realize he has those same antennas", but then I realized it was supposed to be a quote 
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Anyone know what ant is what ? On aa C-8 ?
I was told the one closest to the usb is the zwave and the one near the green light is zigbee
is that true ? because on mine the little testing i did seems like this is untrue.
According to previous queries, Z-Wave is the one closest to the USB-C port.
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Ok .. I just asked bobby .. lets see what he says 
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Fixed.... lol
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I just ordered these .. and bobby was nice enough to send out two new stock ant's
I have a kinda week z-wave mesh most everything i have is zigbee and 7 - zwave
I also just ordered a z-wave lock for my door .. Hope with the new ant this will help.
I will update when I get these and test .. see if they do better then stock
Long Range Z-wave is coming.
It's in beta testing.
I have two Zooz ZSE18 motion sensors. Definitely better range. Have one in mailbox and one in package bin.
This question was asked and addressed a few posts above yours. The link is to a post by another staff member (so you can trust that the answer is correct).
I'll be curious to hear what you think of those... I tried them, and I wasn't impressed -- I'm back to stock for now.
But I realize there are a lot of environment-unique things at play, so maybe they'll be a win for you!
Shoot .. now you tell me ! lol Hope so LOL
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yeah .. I see that .. but like I said with some testing on stock ant I found that my z-wave
mesh seems to not do well with one of the stock ant. I swap them its a seems a little better but still
not good .. so I may have a bad ant. that's why bobby is sending 2 new ones.
but I also thought I would try these other ones see what happens - testing
Edit : Could be my z-wave mesh just sucks LOL - Guess we will see
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Those antennas sound like they should be pretty great -- all specs etc really seem to check out well.
So I had high hopes, but they just didn't yield any noticeable improvement... We don't have a great way to compare zigbee performance, but my ZW #s were consistently less impressive than the stock-antenna #s (not terrible, but definitely less).
Overall, the antennas worked, they just weren't a game-changer for me.
