C-8 -- new Zigbee radio Power option

@mike.maxwell... Can you please give a quick breakdown of the new power options available for the C-8 zigbee radio?

I've heard some folks here say that "8" is similar to the C-7's output, but does that rule-of-thumb include the boost from the C-8's ext antenna or not?

If "8" is a comparable setting, why does the C-8 default to "16"?

Do you have any general recommendations for when to use which setting (or any gotcha's to consider with respect to each setting)?


1 Like

Antennas don't "boost" power. RF energy (power) from the chip is fixed at any of the selected values. An antenna alters the shape of the radiated energy pattern. It "squishes" the shape of the RF from a Sphere to a donut. The energy that was going out the top, is now squished so it shoots outwards in the donut shape. That usually means the edge of the donut is further away from the Antenna than it would be as a sphere.

The antenna affects the shape of the RF direction, it does not alter or amplify the power. The C-7 was fixed at "8" and my testing shows that a C-8 set to 8 works same/better than the C-7 did. My most distant Zigbee device (an Iris v3 Motion sensor) paired without the need of a repeater. I added a few Zigbee devices close to the C-8 to validate it worked then I chose to migrate manually the most distant device. It worked first try. This wasn't a legitimate "open air distance test" it was just a confidence builder for myself.

The C-8 did so because the shape of the RF signal via the external antenna reaches further in the direction parallel to the floors of my house.. It wasn't sending RF energy Up and Down as much as it was with the C-7.

I heard there was discussion about setting the default to 8 but the FCC allows for the greater power, so why not... the why not is that the higher numbers increase receive sensitivity along with power out. That means interference can increase. In other words it's a tradeoff.

I only have one Zigbee mesh but 4 ZWave meshes in my home. I've set mine to be 8 because the hub lives in the same room as several WiFi routers. I don't need the extra sensitivity to interference.


To add to what @csteele said, with great power comes greater responsibility. :slight_smile: The more power you put on Zigbee the higher the chance of interference. I set mine to 16, which is double than what C-7 was able to, and don't have any problems. However we don't recommend going up to 20, as that could cause problems with 2.4ghz Wi-Fi, especially if the router and Hubitat are in close range.


To add to @csteele's comments - antenna gain describes how efficiently the antenna converts the output power of the antenna into radio waves in a specific direction, so it's a combination of the radiation pattern of the antenna and the efficiency of the antenna. Looking at it another way, the squished donut will be larger in an antenna with greater gain. The result is measured as effective radiated power, which is basically the power output of the radio plus whatever gain the antenna provides. In this case the ERP of the hub is definitely improved with the external antenna. The same principles apply in reverse with reception.

(BTW I'm not a radio engineer so the above may not be precise but you get the idea)


Thank you all very much for the additional color -- that helps.

My C8's ~2ft from my router, so I'll try setting "12" and see how that goes.

My C7 was in that same spot for years and I never had a wifi/ZB interference issue (that I know of!), so I don't want to get greedy now and ramp up the new ZB power too much.

Thanks again!

1 Like

Don't forget a Bluetooth. It is also in 2.4GHz spectrum.