Transmitter Power in dBm

[suniljha]

Hi!
I want to know the power of a RF transmitter specified as 14 dBm in terms of approx. communication distance in meters. Any idea??<p>Sunil

[Externet]

Hi.
If the transmitter puts out +14dBm;
and the coaxial loss is 4dB;
You are left with +10 dBm;
And if the antenna has a gain of 6dB;
The radiated power is +16dBm.<p>The loss trough the air space from transmitter antenna to receiving antenna is:
L(loss)=20LogD + 20 LogF +32.3
Where D =distance in miles
and F = frequency in MHz<p>Substract that L figure from +16dBm and you will get the signal power reaching the receiver antenna.
Example: at 150MHz; 100 miles; L(loss) = 115.7 dB
The example yields -99.7dBm reaching the receiving antenna.<p>If the receiving antenna has a gain of 3dB; add them:
-99.7 + 3 = -96.7 dBm
If the receiving coaxial has 5 dB loss,
-96.7 and - 5 = -101.7 dBm<p>If the sensitivity of the receiver is better than -101.7 dBm; you get communication.<p>In other words, add all gains and substract all losses to find the total signal level.
You will have to translate miles to Km.; I do not have the metric formula.<p>Miguel

[manuka]

8km ~5miles. After such a lucid dBm account too !

[jwax]

Just curious, Externet, where did that formula come from? What range of frequencies does it apply to?
John

[Externet]

Hi Jwax
The formula was in an commercial radiocommunications book, part of a long chapter analyzing Fresnel zone propagation.
It applies to dipoles, non-Fresnel zones and line of sight; was good enough for the reply above. I will have to check the frequency range later; if there is any mention about I will post.<p>There is several other formulas, if I just could remember how to insert a picture onto text here; I would post upon interest.<p>This formula did work as I had to submit applications in my days (1980's) of putting together commercial repeater links, as the studies were accepted by the FCC equivalent entity.<p>Yes, 8Km = 5miles; but that is not the result of the +14dBm range calculation.<p>Miguel<p>[ April 06, 2004: Message edited by: Externet ]</p>

[Will]

5 miles = 8 kilometer is closde enough for all practical purposes. Just as a matter of interest the precise definition comes from 1.0 foot = 30.38 cm which produces 5 miles = 8.04672 International Statute Miles. If you want to be even more picky and insist on US (Surveyuor) miles then you have to make an adjustment in the order of two parts in ten to the seventh and I can't remember which way.
Let me show how little I know about the actual subject (Or how way behind the times I am) I thought that dBa meant DeciBel attentuation and dBM (or dBm ?) Decibel Measurement (dbA means Decibel Measurement for sound measurements based on the 'A' weighted scale) ans that the base of dbM was that 0 dBm = 1.0 milliWatt. If that is really the case i.e. we are talking real power levels then, unless an antenna is active i.e. power consuming, instead of passive, then how can it have an actual gain in dBm ? That's not a rhetorical question - I would really like to know !

[Will]

Oops ! - Additional to the many typos I observed once I read my submission i noticed that one of said typoes quoted 1.0 Ft as 30.38 and not 30.48 as it should properly be (It was atypo - the result is still correct) The 30.48 comes from the definition of one inch i.e. 1.0 inch = 2.540000. . . . cm so that 1.0 ft = 12 X 2.54 = 30.48 cm.

[Will]

What a babbling clown I am (5 miles = 8.04672 International Statute Miles) ! - should obviously read '5 International Statute Miles 8.04772 KiloMeters '

[dyarker]

For dBm the "m" is for milliWatt and 0dBm = 1mW.
Relative numbers like loss or gain are "dB".<p>A perfect monopole antenna is defined as a gain of 0dB, and emits the signal equally in all directions. The shape of equal signal strengths is a sphere.<p>With a real antenna like a quarter wave vertical whip, doesn't emit (much) of the ends. So the power not emited off the ends is added to the horizontal directions. The shape of equal signal strengths is more like a donut. No extra power is added, it just emits it in a more useful direction. If the horizontal power out of this antenna is twice the power of a monopole, then it is said to have 3dB gain.

[Will]

A great explanation ! Thank you