[Lf] [Fwd: LF: A small question of size]

[Andre' Kesteloot]

vernall wrote:

> john sexton wrote:
> >
> > Hi Bob,
> >
> > The Radiation Resistance is proportional to the square of the area of the
> > loop, and therefore the fourth power of the diameter. So 30 feet diameter
> > compared with 20 feet diameter increases Rr by a factor of just over 5.
> >
> > John, G4CNN
>
> That is of most interest for a transmitting loop.  For a receiving
> situation, the radiation resistance is not the paramount parameter.  At
> LF there is significant external noise (QRN) so the received signal
> quality can never be better than the signal to QRN ratio.  The receiving
> loop needs to be big enough so that QRN swamps thermal noise in the
> receiver or pre-amplifier.  I would think that 2 metres diameter would
> be adequate for a loop receiving antenna for any LF band (noting that
> QRN power increases with decreasing frequency).  If there is local QRM,
> then a smaller loop can be sited for best isolation from the QRM and
> outperform a larger loop (that has a larger near field).
>
> So bigger is not necessarily better for an LF receiving loop.
>
> 73, Bob