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The advantage of the MR42E over the MR42 is that you can select different antennas. I couldn't recommend an antenna unless we knew what environment the AP was going into.
This is the spec sheet for the antenna you asked about:
The blurb says:
"The Panel Omni antenna is ideal for low ceiling aesthetic conscious installations where wall penetration or extended reach are important. Some example uses are low-density deployments like schools and hotels to provide basic low-performance connectivity. These types of best-effort deployments usually have APs installed in the hallway to cover adjacent rooms."
Agree with @PhilipDAth we'd need to know the environment where the MR42E would be deployed, because it's an "it depends" type of answer. For example, if it's fairly high ceilings, the "D" antenna with electronic downtilt might be best. If you're a K12 deploying on drop ceilings in classrooms, I'd just use the MR42 per classroom instead of an MR42E with a "C" panel omni. But if you have low ceilings and/or aesthetics are more important, then you could have an MR42E out of sight (like above a ceiling tile) with only the panel omni being visible.
the points you have mentioned.
But what about the coverage and the signal strength, isn't true that since i am using a modular antenna it means that i have more flexibility about the singnal strength and it should covers wide areas?
Different antennas focus the RF energy in different ways. So if you don't need coverage behind the antenna, it can focus it all forwards. This provides better signal in front of the AP, and much worse behind it.
That is the compromise. To improve signal in one place, you have to take it away from somewhere else.
That is why the antenna's have radiation patterns in their spec sheets, so you can see the gain in each direction.
Happy to help @JihadAqel and you are correct, the advantage with external antennas is that you have much more flexibility with respect to where you want to "point" the RF energy. It's not changing the signal strength, that is, a semi-directional or directional antenna like the wide and narrow patch options only provide passive gain by focusing the RF energy to something less than omnidirectional.
When you say "coverage and signal strength" you might also be thinking about distance from the AP, and there is no single correct answer. That's always an "it depends" answer, and why you generally never see distance specs on any data sheets.
The critical figure or merit there is Signal to Noise ratio, which will be a multi-variable moving target with multiple dependencies involving lots of things from building materials to the type of client device to user density, etc. So there's no way to say (on a data sheet) that you would have a specific signal strength at a certain distance that corresponds to a given MCS rate. The signal strength and coverage for any given client's data rate can vary significantly over time, even in a relatively status quo environment. This is also where the recommendation of "do a site survey" usually comes up, especially if we're talking about more than a handful of APs. A proper site survey will give you a great idea of AP density and antenna type and placement.
Also, side note to remember that you also now have RF Profiles in Dashboard, so it's simple to set up (for example, in a K12 environment) different profiles for classrooms versus auditoriums versus outdoor sports fields, and have the RF settings automatically apply to all APs according to their profile. So regardless of your deployment scenario, you might consider (if appropriate) leveraging RF Profiles for any "E" access points separately from regular non-E APs.