Help please: TX power vs REAL TX power

RouterOS Beginner Basics
1

What is the difference between the TX power and REAL TX power listed in the winbox interface?

Here’s my situation: I’m using a UBNT XR2 w/RB433. I need to supply a MAX of 100 mw of power (19dbm). So I set it to 19dbm and it says the REAL TX power is 9dbm. If I set it to default, it goes to 28dbm TX and a real TX of 19dbm.

Is this always the case, and other hardware just isn’t advanced enough to say this? What should I do? If I provide more than 100mw of power, it will blow my amps… But I don’t want to provide just 9dbm… that’s so little!

2

There is a 10db offset. Be careful when playing with the TX settings. That being said, why use any amp? They create far more problems then they solve.

3

It is too expensive for us to actually put the radios at the top of the tower, for maintainance reasons.

So, why the 10db offset? Can you elaborate on the purpose of this, and which one I should actually pay attention to?

4

proabily you have an antenna-gain <> 0 ?

p.s. follow the hint: never use an amp.

5

pretend he didnt say anything about an amp… he’s asking about how to turn down the power.

6

100mw is 20dbm. so i think you would want to enter 10db, that way the 10db offset makes it 20.

7

It goes the other way. 28dbm = 20dbm real. If I put in 10dbm, it would show practically nothing for real dbm.

8

Exactly. Thank you.

9

i think its the other way around. they put that 10db offset in there because most software wont let you enter a value as big as the radio actually supports, so if you enter 17db in the software you get 27db real tx power. The radios firmware adds 10db. Someone else chime in - am I right?

10

How do you interpret this?:

^^ This is what I’ve been trying to figure out all along. How many dbm is actually coming out of this radio?

11

trust me, i think its broken too : ) I posted a while back about the same thing and got no where. Maybe MT will notice this and chime in.

12

IF ANY MIKROTIK REPS ARE WATCHING, PLEASE RESPOND. THANKS.

13

You can also measure the real power if you have an attenuator of -40dB (at least): connect directly two radio using a rf cable and the attenuator. The power seen by one radio is the effective power sent by the other radio -40.

14

The 10dB offset is due to a 10dB amplifier on board of the card. The real TX Power is the power level coming off of the radio card’s chipset. The TX Power is the power level coming out of the antenna connector. You’re adjusting in RouterOS based upon the post-amplifier power level for the XR2. However, RouterOS is actually setting the chipset to the Real TX power listed.

Basically, all this is telling you is that on this card the software is smart enough to adjust for the card’s built in amplifier and you don’t have to worry about it.

EDIT: Also, that means you can’t set the power output lower than about 14dB. If I remember correctly, this chip needs at least 4dB of output to work (I could be wrong on this point). You absolutely couldn’t set it lower than 10dB as there’s no way to bypass/disable the amp.

15

Thanks for your response. Just to confirm, you are saying that in the example screenshot above, there is 19dbm of RF power leading into my pigtail. Correct?

16

With routeros 4.6 and SR71-15 card I see this:

So are you saying RouterOS does not understand the SR71 cards offset, so its putting 19dbm to the card? Which means with the SR71 offset added I am overdriving it? I can’t even figure out if the SR71 has an offset … I wish someone would give us a straight answer : )

17

Correct.

18

I can confirm that you are correct. I just got a response to the email I sent to Mikrotik support:

“real tx-power means what is is getting sending to the cards eeprom.
So The tx-power setting that you change there means the actuval tx-power. In your case the Tx-power is 19db based on your pictures.”

Thank you all for your help.

19

Try always using the default setting, if you key in the value higher than you wireless card max value.
You will burn the card, another thing is, refer to the technical specification, they should stick there.