Receiving antennas at LF]
Thu, 04 Feb 1999 15:09:18 -0500
Soegiono, Gamal wrote:
> > I would like to add some comments to what John,
> > G4GVC and Peter Dodd, G3LDO has to say on
> > LF receiving antennas.
> In absence of wanted signal and artifical noise, there is only
> the natural noise detectable at the antenna's site. The natural
> noise has statistically varying fieldstrength values depending
> on a lot of parameters. An antenna with specific aperture,
> or antenna factor would convert the fieldstrength of the
> external natural noise into a voltage for the receiver input (say Vnn).
> As long as this voltage is equal or slightly larger than the
> voltage (Vrn) of the receiver's intrinsic noise (for a given bandwidth),
> no improvment can be gained in making the antenna larger
> (increasing the aperture or antenna factor).
> To my understanding the best antenna for receiving _any_ wanted
> signal is the one having an aperture (antenna factor) which
> converts the lowest observeable natural noise field strength into the
> voltage Vrn.
> Smaller apertures will limit the S/N, as will in effect larger
> apertures. Larger apertures will not only cause the wanted signal to have
> higher levels at the receiver's input, but also all the other signals
> captured by the antenna. This will increase the total signal power the
> circuitry of the receiver has to handle, increase
> intermodulation effects which (especially for relatively wideband
> increase the apparent noise level, hence again rendering the S/N ratio.
> LDO> 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.
> 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
> or in other words the level of the weakest readable wanted signal. One way
> improve this would be utilizing a much more narrow IF passband. Lesser
> spectrum lines from LORAN then would fall inside the IF passband, but
> contain all the wanted signal's spectrum. S/N ratio then would increase.
> Another aspect is the effect of desensation of the receiver's
> input due to total signal power at its first mixer or HF stage. This can
> reduced only by better preselection and/or narrower antenna bandwidth.
> An antenna bandwidth of 50Hz does not necessarily mean that it
> rejects frequencies outside that band effectively. The slope of the
> antenna's pass band should fall off rapidly. The stop band attenuation
> should quickly reach values of 40 dB or more and remain below 40dB for
> all more distant frequencies.
> 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 its selectivity rather than its type and principle of
> Other similar important parameters are selectivity of
> preselector, choice of IF BW and overload immunity of the receiver.
> Best 73
> Gamal Soegiono