RB751U External Antenna
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ddejager
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RB751U External Antenna

by ddejager » Fri Dec 23, 2011 7:29 pm

I've been experimenting with an external 6 dB antenna on the RB751U. I've observed strange behaviour. The documentaion states that to utilize the external antenna on channel 1, you need to set Antenna Mode to "antenna b".

I have configured a second RB751U to act as a client for my testing. I have the external antenna installed on the RB751U that is acting as the AP. With the RB751U client I'm able see both the Tx and Rx signal strengths on both ends of the link. As expected the Tx level on the client end matches the Rx level on the AP end and vice versa.

What is not expected is that when I switch the Antenna Mode from "antenna a" to "antenna b" only the Rx signal level on the AP (or the Tx signal level on the client) improves. In my case the signal level improves from about -30 to -20. (Obviously my client is close to the AP for this test.) The Tx level on the AP (Rx on the client) remains unchanged. When I switch back to "antenna a" both the Rx & Tx levels go back to about -30.

This test implies that the Antenna Mode is only switching the radio receiver over to the external antenna, not the radio transmitter. Is this a bug? It is documented anywhere? Why would this be true?

Can any comment on or confirm my observations?

I'm using version 5.11, RB FW version 2.37.

ddejager
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Re: RB751U External Antenna

by ddejager » Sat Dec 31, 2011 6:21 pm

I posted the question above the support@mikrotik.com. The response I received back was:

Hello,

The client sees also the other antennas, to really test this, try to turn off TX on Chain0 (leave RX). See if this makes a difference.

Also, the signal is much too strong, try to increase distance to get it at around -60. Do the test then. See what happens.

Regards,
Normunds


I sent the following reply:
I ran some additional tests which I will describe here. I still do not think that the external antenna is working properly. As before the client and the AP are both RB751U devices. I increased the distance so that the signal levels were between -56 and -93 (depending on the test and the antenna, see below).

Test 1. AP Settings: Tx, Chain 0 only, Rx, Chains 0 & 1, Antenna set to Antenna A
Tx/Rx chan 0: -83/-56
Tx/Rx chan 1: -77/-52
Tx/Rx CCQ %: 60/40
Btest - both directions simultaneously - 6.4 Mbit

Test 2. AP Settings: Tx - chain 0 & 1, Rx, chains 0 & 1, Antenna set to Antenna B with a 6 dB omni connected to AP
Tx/Rx chain 0: -84/-58
Tx/Rx chain 1: -93/-56
Tx/Rx CCQ: 31/26
Btest - both directions simultaneously - 3 Mbit

The tests above seem to confirm my original claim that when the Antenna is set to "b", the Tx of chain 1 is not connected to any antenna.

As a further confirmation of this I report the following data:

Test 3. AP Settings: Tx - chain 0 & 1, Rx, chain 0 & 1. Antenna set to Antenna A
Tx/Rx chain 0: -57/-56
Tx/Rx chain 1: -68/-54
Tx/Rx CCQ: 61/63
Btest - both directions simultaneously - 7.9 Mbit

clevvernet
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Re: RB751U External Antenna

by clevvernet » Sat Dec 31, 2011 9:16 pm

I'm also seeing similar problems. Didn't know of you made any additional discoveries on this situation?

As you mention it appears that the external antenna isn't working completly. I purchased my 6db antenna and the 751u from Baltic Networks. I was thinking of giving them a call to see if they had seen these problems as well.

Brent

ddejager
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Re: RB751U External Antenna

by ddejager » Mon Jan 02, 2012 6:25 pm

clevvernet wrote:I'm also seeing similar problems. Didn't know of you made any additional discoveries on this situation?

As you mention it appears that the external antenna isn't working completly. I purchased my 6db antenna and the 751u from Baltic Networks. I was thinking of giving them a call to see if they had seen these problems as well.

Brent


I spoke to Brian an Baltic Networks last week. He was not aware of the issue(s).

Jester73
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Re: RB751U External Antenna

by Jester73 » Sun Jan 08, 2012 8:04 pm

The same problem from rb-751, the external aerial works on 60 % more poorly

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chvdr
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Re: RB751U External Antenna

by chvdr » Fri Jan 20, 2012 12:20 pm

Obviously, many people meet same problem.
Can the support make a note, please.
--
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C. G.

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normis
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Re: RB751U External Antenna

by normis » Fri Jan 20, 2012 12:22 pm

like Uldis said in other topic:

One problem could be because of the too much interference received from the external antenna (noise from other wireless sources, for example, maybe could be from the 3g radios) and that could cause the RX radio to lower the signal. It depends on the antenna specification as well how wide frequency range it can receive. One option would be to add a hardware RX filter to boards radio part to filer out the noise from non 2.4gz devices, but that requires soldering and also it might not be the cause in this case.
We usually suggest to use built-in indoor antennas and use the external connector is an option feature.
No answer to your question? How to write posts

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chvdr
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Re: RB751U External Antenna

by chvdr » Fri Jan 20, 2012 1:47 pm

normis wrote:like Uldis said in other topic:

One problem could be because of the too much interference received from the external antenna (noise from other wireless sources, for example, maybe could be from the 3g radios) and that could cause the RX radio to lower the signal. It depends on the antenna specification as well how wide frequency range it can receive. One option would be to add a hardware RX filter to boards radio part to filer out the noise from non 2.4gz devices, but that requires soldering and also it might not be the cause in this case.
We usually suggest to use built-in indoor antennas and use the external connector is an option feature.


That's fine, but:
(1). Our issue is ap-bridge on "fully virgin" area - no wireless 15 km around (nor even non-2,4GHz);
(2). With TP-LINK wireless access point on the same place there is no such problems;
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C. G.

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odge
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Re: RB751U External Antenna

by odge » Thu Sep 06, 2012 7:22 pm

Same problems here. All the 751s we have have continuous wireless problems.... drives me insane.

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Re: RB751U External Antenna

by 321nsan » Wed Dec 26, 2012 8:03 pm

is there any solution for this problem?
I get this problem too.

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