Receiving antennas at LF
Thu, 04 Feb 1999 09:58:57 -0500
Peter Dodd wrote:
> The message from "Soegiono, Gamal" <firstname.lastname@example.org>
> contains these words:
> >G3LDO> The main limitation to big receiver antenna is noise. My
> >> main noise source is the trash sidebands from Loran from Lessay 160km
> >>distance from me on a sea path. They hold the S-meter at S7 on a
> >>2.7kHz bandwidth and effectively makes me deaf to 136kHz DX
> >>unless the conditions are very good to the DX station.
> >For this bandwidth you definitely do not need any larger antenna
> > having greater aperture.
> True, but I dont operate with this bandwidth (except when listening
> to slow CW) - It would cover the whole band! The 2.7kHz bandwidth is
> used to measure the Loran sidband level.
> >There are a couple of spectrum lines of the LORAN signal falling
> > into the IF passband and even more into the frequency range of both your
> >antenna and the preselectors pass band. This tends to limit the possible S/N
> >or in other words the level of the weakest readable wanted signal. One way to
> >improve this would be utilizing a much more narrow IF passband.
> Loran sidebands occur every few Hz, see Peter Martinez's e-mails of
> the 9th, 14th and 18th of August 1998, so there are many sidebands in
> the narrowest of selected bandpass settings.
> >So regarding S/N ratio for reading a wanted signal in the
> > presence of noise the main parameters are the optimum aperture of the
> > antenna and it's selectivity rather than it's type and principle of
> > operation.Other similar important parameters are selectivity of
> > preselector, choice of IF BW and overload immunity of the receiver.
> My antenna has three stages of preselection, which were definitely
> necessary when using the Datong VLF converter. The receiver section
> of the TS-850s is excellent as regards overload immunity, although I
> still continue to use the preselectors.
> As regards selectivity settings on aural CW, several of us have found
> that a lot of noise is generated by the Loran signals interacting
> with the IF filters if they are set too sharp. Under these conditions
> it is best to use a variable attenuator at the antenna input, set it
> for maximum Loran to wanted signal ratio and leave it to the
> component between the headphones to sort it out.
> Slow CW using a computer seems one way forward - the rattling Loran
> signal is bad for the ears.
> Regards, Peter, G3LDO