Hi > On Jul 17, 2015, at 8:31 AM, Dr. David Kirkby (Kirkby Microwave Ltd) > <[email protected]> wrote: > > On 16 July 2015 at 23:23, Bob Camp <[email protected]> wrote: > >> Hi >> >> Quick and simple: >> >> 1) Signal power is proportional to the area of the loop. Bigger is better. >> 2) Inductance is proportional to the turns squared. Turns do not directly >> affect signal to noise. >> 3) Inductance may be resonated with a capacitor. This gives a bandpass >> function. >> 4) The coil shapes are very common. The many inductance calculators on the >> web will give you an inductance estimate. >> 5) If the inductance is resonated, the system Q (and thus bandwidth) is a >> function of the coil losses and the amplifier’s input impedance. >> 6) More turns gives a power match into a higher impedance ( more voltage). >> 7) *Practical* matching of the amplifier to the antenna will give you an >> reasonable target number of turns. >> >> Bob >> > > It's interesting that > > http://www.vlf.it/feletti2/idealloop.html > > says that sensitivity is set by the mass of copper used. To quote > > "A single turn square loop, 1m side, made with 1kg copper has the same > sensitivity of a 1000 turns square loop made with 1kg copper and same > dimensions. In this context, the sensitivity limit is represented only by > loop thermal noise: >
The *power* into the loop is a function of the area. > noise floor (nV/sqrt(Hz)) = 4 sqrt(R in kOhm) > > It is not immediately obvious where that equation comes from, but > re-arranging the equation for thermal noise power > > P=k T B simply the standard thermal noise equation for a resistor > > (P in watts, k= Boltzmann contant, B is bandwidth in Hz) > > and assuming a temperature T of 300 Kelvin, k = 1.38 x 10^-23 J/K, one > finds the constant is 4.06, so the 4 in that equation is fairly accurate at > 300 Kelvin. > > I'd much rather wind a loop with a few turns than a few hundred turns! But > obviously the voltage rises with the number of turns, so requires less > gain. *but* the load resistance (and thus the thermal noose in that load) goes up at the same time. If you have a very low impedance buffer (common base stage etc) the number of turns will be very different than if you have the input gate of a MOSFET. Bob > > Dave > _______________________________________________ > 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. _______________________________________________ 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.
