What is the difference in the four RT-AX88U antennas?

[JohnB_123]

tl;dr: what do the 4 different antennas on the RT-AX88U do ?

Background:

I'm strongly considering consolidating my 2x RT-AX88U devices (1x router, 1xAP) to just one.

We live in an 1800 sf single story home with lots of attic access. The layout is sprawling and there are devices on the exterior that need wifi (sprinklers, switches, cameras). A single centralized router isn't enough yet having 2x routers seems like overkill. The thought occurred to me that the antenna placement might be the solution.

So I experimented with coaxifi to put one antenna up into the attic then played around with different antenna ports, using rudimentary test of leaving the extended antenna in the attic then using Wifi Inspector to compare relative signal strength of the various ports. I noticed that the rear left antenna (nearest the power button) seems to extend the 2.4 GHz signal, while the other rear antenna (nearest the USB port) seems to extend the 5 GHz. It wasn't clear if the flanking antennas extended much of anything. I've disabled the wifi on node 2 and now get a solid 5 GHz signal in the otherwise hard-to-reach corner of the home.

A bit of mixed hope here - what would happen if I simply extended all 4 antennas to my attic using a simple RP-SMA Male To Female Wifi Antenna Connector extension (~10M or so)? The difficulty being that I would like to have all 4 antennas broadcast both bands. Assuming there's no insertion loss from the extension, of course....

Does anyone know what each of the antennas does, or do they all broadcast everything?

 

[ColinTaylor]

The RT-AX88U is a 4x4 device on both 2.4GHz and 5GHz. That means that all antennas are used all the time for both bands.

 

[Deleted member 22229]

The cable losses at 2.4 and worse 5 GHz through 10 meter of coax cable would be huge. You would need high quality hard line to even come close to making it work with any power left to transmit. On top of that you must pay close attention to the SWR presented to the radios. Anything more then about 1.5 would further decrease output and possibly damage the final transistors.

 

[JohnB_123]

The cable losses at 2.4 and worse 5 GHz through 10 meter of coax cable would be huge. You would need high quality hard line to even come close to making it work with any power left to transmit. On top of that you must pay close attention to the SWR presented to the radios. Anything more then about 1.5 would further decrease output and possibly damage the final transistors.

How is it that it is working with 10M of RG6 cable, which has higher impedance than SMA? I was thinking that using native SMA cable (5-10M for each antenna) would work well but this isn't my field. Would it work if I used antennas with gain?

 

[Deleted member 22229]

This is with LMR -100 that has better loss numbers then RG6. And remember RG6 is rated 75 ohms you need 50 ohms.

The loss in dB is linear with the cable length. So if you use 30 feet of LMR100 cable, your loss will be 12 dB (i.e. your signal is now about 1/16 the power that it was). With 7.5 feet, loss will be only 3 dB (you lose half of your signal).

All of the above numbers are for the 2.4 GHz WiFi band. For 5 Ghz it will be much worse.

 

[JohnB_123]

This is with LMR -100 that has better loss numbers then RG6. And remember RG6 is rated 75 ohms you need 50 ohms.

The loss in dB is linear with the cable length. So if you use 30 feet of LMR100 cable, your loss will be 12 dB (i.e. your signal is now about 1/16 the power that it was). With 7.5 feet, loss will be only 3 dB (you lose half of your signal).

All of the above numbers are for the 2.4 GHz WiFi band. For 5 Ghz it will be much worse.

so the Coaxifi solution ... no bueno? Assuming it only “works” because the recommended routers put out enough power vs the insertion loss?

 

[Deleted member 22229]

Most routers by law are in the milli watts maybe 1 watt so that's a huge issue when it comes to loss. Directional antennas can help but if there is nothing going in then you get nothing out. High Frequency's are a bitch when it comes to coax runs. Low frequency not so much even at long runs. Also remember the antenna has to receive these signals as well and the loss will make those small signals virtually vaporize on the way back to the receiver.

It can be done but with very expensive hard line coax. Think very thick and bulky hard to work with not really a option unless your feeding a tall outside tower.

 

[thiggins]

AX, like AC and N uses MIMO technology, which relies on multiple antennas working together.

Moving one antenna very far away and changing its path loss characteristics will in no case improve anything.

Stick an AP up in the attic. Leave the antennas alone.

 

[JohnB_123]

AX, like AC and N uses MIMO technology, which relies on multiple antennas working together.

Moving one antenna very far away and changing its path loss characteristics will in no case improve anything.

Stick an AP up in the attic. Leave the antennas alone.

Theoretically, yes. But it looks like in tests, this is not the case… Especially when the clients get more than 20 feet from a MIMO router - transfer performance improves when antennas are divided up and moved closer to clients, even over lossy RG6 wiring.:

https://homewiredshut.blogspot.com/2017/12/woca-wi-fi-over-coax-coaxifi-review.html?m=1

 

[thiggins]

I’m highly skeptical of these results, which appear to defy RF and MIMO theory.

Are you saying that you are getting similar results? You say you are getting a strong 5 GHz signal now in a previously unreachable area. What’s the throughput you’re getting?

 

[JohnB_123]

I’m highly skeptical of these results, which appear to defy RF and MIMO theory.

Are you saying that you are getting similar results? You say you are getting a strong 5 GHz signal now in a previously unreachable area. What’s the throughput you’re getting?

I resisted this approach for several months but with quarantine, I got an opportunity to finally try it. If you do a simple search for coaxifi on Reddit, you’ll see other users essentially saying the same thing: they didn’t believe until they tried it and then they experienced positive results.

I have not done any throughput testing yet, but I can say that the 2.4 GHz improvement is very real. I have devices on the far corners of my house that simply could not connect prior to introducing this method. Those are IoT devices so they do not have 5 GHz radios in them (and I do not have flexibility for the endpoints location). I will find some time to do some testing and report back.

 

[RMerlin]

Wouldn't such a setup end up having clients using only one single stream tho, which means stronger signal, but much lower throughput if the client can support more than one stream?

That'd be a case where you need to do actual benchmarking, not just look at the RSSI.

 

[JohnB_123]

Wouldn't such a setup end up having clients using only one single stream tho, which means stronger signal, but much lower throughput if the client can support more than one stream?

That'd be a case where you need to do actual benchmarking, not just look at the RSSI.

did you read the link I posted? The individual who wrote it did a pretty extensive test that included throughput comparisons isolating the native antennas on the router. The conclusion was that after a certain distance, MU-MIMO performance suffered after ~20 feet of distance from the router and it was better to have attenuated signal closer to the client.

regardless, this is a very inexpensive trial. I already sold our second Rt-AX88U on EBay and thus far no real world degradation in our home.

The model is essentially to have a single router placed in the attic such that streaming and other heavy use clients leverage its full potential while far-flung devices tap into an antenna from the router that’s ~5 meters away. This covers our sprawling one story home very nicely. That said, I am considering removing one more antenna and stretching it to the other end of the home. That would leave two antennas native on the centralized router, and two distributed at the far extremities of the house.