The far field of a radiated signal, where the reactive E and
H fields can be safely ignored in propagation loss
measurements, occurs at approximately lambda / 2.pi
At this frequency, with a wavelength of 33.5km this occurs at
5.3km so you are still well within the near field. Inside
this region the reactive E field falls off much faster than
1/R^2 and the magnetic field H even faster still, so the
decrease of 20dB going from R = 1.5 to 2.5km does not sound
too unreasonable.
As a point to work from, "Reference Data for Radio Engineers"
gives groundwave loss at 10kHz for a 100km path as 33dB. The
curve does not go much below 30dB loss so can't read off
smaller distances, but if you assume 1/R^2 as a first
approximation for the radiated field (although not strictly
true for groundwave propagation) it should give some very
rough indications of what to expect from the 5km range by
extrapolating back from 100km
The RSGB are still pursuing my 9kHz NOV application with the
authorities, eventually I hope to be joining in at these VLF
freqs one day !
Andy G4JNT
> -----Original Message-----
> From: Rik Strobbe [mailto:[email protected]]
> Sent: 2001-07-12 10:42
> To: [email protected]
> Subject: Re: LF: LF/Hyper test
>
>
> At 09:02 12/07/01 +0100, DJ2LF wrote:
> >Some weeks before Markus DF6NM and me made some tests and
> found a max.
> audible
> >distance of 1500m and a max. visible distance of 2500m (on
screen of
> laptop).
> >Rec. antenna was 10m vertikal.
>
> Hello Walter,
>
> congrats on the 9kHz crossband QSO.
>
> Did you measure the current you could get into the antenna ?
> Ground-loss
> must be extreme at these frequencies.
>
> It is interesting to note that the signal seems to be audible
> up to 1.5km
> and visible on screen up to 2.5km. If good software (argo,
spectogram
> etc..) was used this would mean that the signal level
> decreased about 20dB
> between 1.5 and 2.5km !
> Or otherwise it would mean that you would have to increase
> power from 12W
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