Topic: Transmit power

I'm about to implement a long-range network using high-gain antennas, and I need to clear out some stuff about the abilities of the radio in the WRT54G. According to most people it's possible to adjust the transmit power by using 'wl txpwr', and I feel quite comfortable that this is actually possible (by examining rssi-values and signal strength from tools like netstumbler).

But, what I'm not feeling sure about is how adjustable this transmit power actually is. Can you set it as low as for example 1 dBm (1.3 mW)? Can anyone scientifically prove that this is an actual change? Someone on the IRC-channel said that it should at least be possible to set it 10 dBm below the default (15 dBm). And, what tool should be trusted when viewing the current transmit power on the WRT? 'iwconfig' always show the same value, no matter what you set 'wl txpwr' to. 'wl curpower' adjusts the dBm according to what you set it to. But, all this is kinda confusing, and I don't feel very confident about it.

Anyone else got any more input on this?

Marius

Re: Transmit power

Marius,

I have confirmed with a spectrum analyzer output from the card at levels 5dbm-21dbm. I could send your snapshots if you need them. The problem that I saw in regards to power settings was that the higher you turned power up on the card the higher the side-lobes got. I have mine set around 14dbm, which looks good. I would recommend using a level around 12-17dbm. How long is your link going to be? You can have 42dbm out of your antennas. If you need help figuring out system gain, etc... Don't hesitate to ask.

Chris

Re: Transmit power

I have confirmed with a spectrum analyzer output from the card at levels 5dbm-21dbm. I could send your snapshots if you need them. The problem that I saw in regards to power settings was that the higher you turned power up on the card the higher the side-lobes got. I have mine set around 14dbm, which looks good. I would recommend using a level around 12-17dbm. How long is your link going to be? You can have 42dbm out of your antennas. If you need help figuring out system gain, etc... Don't hesitate to ask.

It's going to be a link at around 4 km. Regulations in Norway say that you can't output more power than 20 dBm (100 mW), so when using a high-gain antenna, I'm thinking about a 19 dBi, I have to be able to lower the tx power to 4 dBm (by introducing cable loss at 3 dB). Of course, I can regulate this by introducing more cable loss. But, that would decrease the overall quality of the link, since the cable loss has to be the same on each side (only using one antenna on each side). So, basically I need to be sure that the txpwr on the WRTs actually can be set correctly that low.

Re: Transmit power

Hi!

I'm not a law expert, but I think that the regulatory limit (by law) is specified for isotropic radiated power (ie, before antenna). 

Please, make sure you verify that, because I have heard of people at US that have made 15Km links withou braking any law (using 20dBm cards with 19dBi antennas), and I have even heard about people using 24dBi antennas.

I have the planification of a 14.4Km link with 15dBm and 24dBi, I hope it works :-S (this is my first really long range link).

In my country there are people that push 1W AND 18dBi antennas (yes, it is allowed 1W in my country, but I won't use one of these things), the law says that you can use 1W in "base station", and they use it (what a irresponsable people, you can even see telephony cells inside the city with 20W!!!!). 

Btw, does anybody knows about any investigation team that would like to analize the effects of microwave radiation in the human body?, they are invited to my country to do their research (Venezuela).

c-ya!

Ildefonso.

Re: Transmit power

Marius,
You have the right idea. Good luck with the link. Here is a formula to help:

EIRP = Effective Isotropic Radiated Power
Pout = transmitter power output (dBm)
Ct = signal loss in cable (dB)
Gt = gain of the antenna (dBi)

Pout - Ct + Gt = EIRP

Re: Transmit power

Yeah, but my question remains, is it possible to lower the txpower of the WRT54G so far down?

The limit in Norway is 20 dBm EIRP (that is, after the antenna).

Re: Transmit power

I have confirmed with a spectrum analyzer output from the card at levels 5dbm-21dbm.

I will assume that you can take it down one more to 4dbm. My AP is in the field or I would test it to make sure. Are your AP's somewhere where you could hook them up and test before you install them in the field?

Re: Transmit power

Don't forget receive sensitivity!  It's just as important as transmit power and antenna gain, and is part of the overall equation when considering link quality.  And there's no legal limit (that I know of) on how high your RX sensitivity is.

Would I be correct in assuming that lowering the TX power of the WRT's radio does *not* affect it's receive sensitivity?

If so, it might be a good idea to lower the TX power and put a very high gain (25dB or higher) antenna on it.  Then you will get a very good receive sensitivity (radio's RX sens. + antenna gain), but your transmitted EIRP will still be legal.  If your link partner does the same at the other end you should have quite a good quality link.  Two highly directional antennas pointing at each other will be largely immune to the high levels of RF noise we see in the 2.4GHz band these days.

Re: Transmit power

That's actually what I'm thinking on this particular link. Lower the txpwr to around 2-3 dBm, use a high-gain antenna (19 dBi) and then I have to add some cable loss to make the link legal. I've tried out the properties with a link budget calculator, but the unknown for now is how reliable the lowering of the txpower will be.

sldnkarm: I don't know how I can test it. I haven't got a spectrum analyzer laying around. smile

Re: Transmit power

danversj: You are correct in assuming that lowering the tx power will not affect it's receive sensitivity. Receive sensitivity is a level that a radio card will use to block out any low lying noise. If you increase the receive sensitivity value greater than what your received signal is it will not be able "hear" the remote end and not communicate. Did you mean higher as in (-90dbm is higher compared to -60dbm)? Just remember that we are dealing with negative numbers, so the opposite would be true (-60dbm is a higher sensitivity than -90dbm). Do we have control of receive sensitivity in OpenWRT?

mflage: I take for granted that I have a spectrum analyzer! I am not sure how you would verify you power output of the radio cards. I will see if I can come up with something.

Re: Transmit power

I'm really confused by all this.  Having just installed OpenWRT, reset my NVRAM, and installed "wl", I see that my txpwr is 255.  I'm worried about burning out my ap. 

1. What nominal value should I set here?
2. Are there any other settings that could overide this?

12

Re: Transmit power

I'm really confused by all this.  Having just installed OpenWRT, reset my NVRAM, and installed "wl", I see that my txpwr is 255.  I'm worried about burning out my ap. 

1. What nominal value should I set here?
2. Are there any other settings that could overide this?

The same thing happens on the stock firmware.

It's telling you that the target power level is 255 mW. It's not actually transmitting at 255 mW though, instead it's being limited by the driver somewhere around 84 mW.

(There's an override switch which lets you bypass that limit)

Re: Transmit power

it's being limited by the driver somewhere around 84 mW.

Great.  Love your work BTW.

Re: Transmit power

The same thing happens on the stock firmware.

It's telling you that the target power level is 255 mW. It's not actually transmitting at 255 mW though, instead it's being limited by the driver somewhere around 84 mW.

(There's an override switch which lets you bypass that limit)

Can you tell us what is this switch, please?

Thanx.

Re: Transmit power

Don't forget receive sensitivity!  It's just as important as transmit power and antenna gain, and is part of the overall equation when considering link quality.  And there's no legal limit (that I know of) on how high your RX sensitivity is.

You mean "low", yeah? The lower, the better, right?

Re: Transmit power

Well in magnitute, receive sensitivity is said to be 'higher' i.e. more sensitive when it can receive signals at a lower threshold - i.e. more negative.  A figure of -80dBm is a "higher", better level of sensitivity than a figure of -60dBm.  Sure, -80 is more negative than -60, but -80dBm is more highly sensitive than -60dBm.

Hehe, semantics is/are fun!