The Goldilocks zone? How hot is too hot? How do I determine my coverage area?

We're new to the WISP game and just put up our first tower with 4x 90deg ePMP3k's

My boss first had me setup our office on a SM right on roof (roughly 20ft away from AP) for us to build confidence in the system before selling it. But like I thought, it was giving us problems because the AP was wayyy tooo hot for that cose a distance. I brought it down to 6db and that was still too hot. Looks like about 3-4 db is the hottest I can have it without it choking up. Im trying to explain that to him but he thinks maybe we can angle the SM upward to act as an antennuator to offset the close distance and not limit us on that particular AP of serving people further. Are there any tricks like this people can use?

We also installed another SM (ePMP F-300-16) at one of our warehouses thats about 400 meters away. I noticed when using the LinkPlanner, that it gives me a maxpower of 11dB.

So obviously the AP is gonna have a distance limitation, but are we also limited by our closest customer? And is that how we determine our coverage area?


You should be sure to check out cnHeat heatmapping software.

1 Like

How far away from the AP is your largest coverage area subscriber-wise? How far away are the majority of your subs going to be away from the AP? What's the farthest out you'd want to provide service?

If clients are very very close... you could try using an omni antenna, and lowering the TX power. Ideally you want an RSSI of around -60dBm. Your AP's "Subscriber Module Target Receive Level" should be set to -60dBm as well.

1 Like

Hi.  Well, one of the nice things about the Cambium ePMP lineup is ''Automatic Transmit Power Control" on the SM's.

So - on the Access Point side of things, you go into CONFIGURATION -> RADIO, and program in a '' Subscriber Module Target Receive Level''  (which is usually best around -60 dBm) and then on all your SM's, you simply turn on the 'Max Tx Power' to Auto... and then each SM will do it's best to turn it's transmit power up or down in order to keep the signal at the AP as close to the 'Target' of -60 (or whatever you choose).

So - this will help prevent having your AP swamped with signal from too hot of SM's - which will significantly help the 'near/far' problem - and it'll help not transmitting any more 'noise' than each SM needs to in order to talk to the AP.  Ideally, on the AP, you'll want every SM to be as 'balanced' as possible, and this helps greatly vs products which don't have a feature like this.

Now on the other side of the equation, having your SM's hear the AP a little too hot on their side (how loudly they hear the AP) isn't nearly as much of a concern. I mean.. there is a point where too loud is still too loud, but not nearly as important as the AP side of the equation.  If a SM hears the AP at an ideal -60, or if it happens to be a -45 or -35 on the SM side of things, that's all survivable and from what Cambium has said, should not cause problems at those level.

Yes, you can also deliberately uptilt, but with the "Automatic Transmit Power Control", the SM's should turn their power output up or down to hit the target signal set in the AP, and that's usually enough to equalize signals in my experience.  :)

1 Like

Hi TDJ211,

With respect to coverage area, Cambium has a tool called cnHeat.  It will show you predictions of your coverage via a heat map that is very accurate.  It will tell you where an SM can be installed on a building / property and at what height.  You dynamically control SM Install Height and RSSI level.  This way your customer service representative can tell customers within a couple of minutes anticipated service based on knowing both install location and RSSI level.

The reason why this works is that we use 1m GIS data from LiDAR that accurately represents the earth.  We take this data and do an RF prediction for every square meter out to eight miles radius from your AP site.  Predictions like this used to take days and we have it down to minutes.

You can find more about cnHeat at

You can see a video of a webinar showing cnHeat working at