On Fri, Sep 26, 2014 at 4:07 AM, Dr. David Kirkby (Kirkby Microwave Ltd) < [email protected]> wrote:
> On 17 Sep 2014 23:38, "Peter Putnam" <[email protected]> wrote: > > > > Greetings, > > > > The link below describes a homemade GPS receiver. > > > > It is presented in a detailed and elegant manner that is certain to > appeal to this reflector's subscribers. > > > > Peter > > > > > > http://www.aholme.co.uk/GPS/Main.htm > > I don't understand the units of signal strength > > "The L1 carrier is spread over a 2 MHz bandwidth and its strength at the > Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is > -111 dBm" > > Then goes on to talk about the signal being 20 dB below the noise. > > Unless the -130 dBm is over the whole surface area of the earth, which I > doubt, the units make no sense to me. The units of signal strength should > be V/m, A/m or W/square metre. > > The noise power should be in Watts or dBm. So taking the difference (19 dB, > which is approximately 20 dB) between these figures seems odd to me. > I think you may be confusing field strength with signal strength. What comes out of the feed-line and reaches the antenna connector at the receiver, i.e. signal strength, is indeed measured in watts and can therefore be compared directly with thermal noise power to produce an effective S/N. The antenna itself subtends an "effective" area (aperture) on the surface of a theoretical sphere where the emitter is at the center. So the units of area found in the field strength, i.e. the m^2 in W/m^2, are canceled by the units of the effective area of the antenna, leaving only units of power to appear at the feedline connection. Every unit tells a story. -- Brian Lloyd Lloyd Aviation 706 Flightline Drive Spring Branch, TX 78070 [email protected] +1.210.802-8FLY (1.210.802-8359) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
