[MLB-WIRELESS] Legal power output

[Tony Langdon, VK3JED]

At 05:50 PM 16/07/2002 +1000, Justin wrote:

>  n00b questions time. With all this talk about power outputs and DB gains
>etc, can someone please tell me if I understand it all correctly?
>
>  3 db gain = twice the power. Correct?

Yes.  Check out the PowerAndGainToDecibels page on the wiki for more info. :)

>  All gains and losses are added or subtracted to get a final number?

If expressed in dB, that is correct.

>  The legal power output is about 4 or 5 watts?

4W EIRP

>  This output is a calculated value based on the cards output power x the
>antenna gain?

Yes

>  So, if I have a 30 miliwatt card, plugged into an antenna with a 16 db
>gain, but I lose 1 db in the coax, then my overall gain is 15 db. 15 db /
>3db = 5 x gain. 5 x 30 mw is 150 miliwatts effective power put out through
>my antenna. Did I get that right?

This is a case of making sure you're not mixing apples and oranges.  You 
can do this one of two ways:

1.  Convert everything to dB and add the values.

30mW = 15 dBm  (from PowerAndGainToDecibels)

EIRP = 15 + 16 - 1 = 30dBm, which equals 1W EIRP

2.  Convert everything to "linear" qualnities and multiply out the result.

16dB = 40x, 1dB = 1.2x (approx)

EIRP = 30 x 40 / 1.2, = 1000mW or 1W EIRP.

>  A 100 milliwatt card in the same setup would give me half a watt?

100mW card would give 100/30 (3.33)x the EIRP, or a bit over 3W EIRP.

73 de Tony, VK3JED
http://vk3jed.vk.irlp.net