[time-nuts] Firmware and antenna for Stanford Research FS700 Loran C frequency standard

[Bob Camp]

Hi

> On Jul 17, 2015, at 8:31 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
> 
> On 16 July 2015 at 23:23, Bob Camp <kb8tq at n1k.org> 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
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