[Lf] Bandwidth and receivers]

[Andre' Kesteloot]

g3ldo wrote:

>  Bill, G0AKY said
>
> > All agreed in principal but there is one nail you forgot the LF band hr is
> > only just over 2Khz wide
>
> I get the impression from Bill's comment that he, and others, think QRSS is
> a wide band mode.
> All other things being equal the bandwidth is inversely proportional to the
> data rate, which means the bandwith for a QRSS signal is much less than a
> standard CW signal. You can check this out for yourself by just looking at
> these signals with QRSS SW.
> When we do the transatlantic tests we often have a group of signals
> transmitting a few Hz apart and each one is perfectly readable (see Ko
> Versteeg's .JPG on page 98 of the LF Experimenter's book).
> The main limitation of having two CW signals closer than, say, 100Hz apart
> is the receiver filter. You can have very narrow filters. I have a level
> meter with a 20Hz filter that certainly selects one signal from the other
> and you can read CW with it provided it isn't sent too quickly. Its main
> problem is that it converts every bit of noise and QRM to a tone the same as
> the signal you are receiving and this is very 'wearing' after a short while.
>
> Most problems with receivers are due to front end non-linearity. The worst
> receiver I have used that has this problem is the Daytong converter.
> Nevertheless, this converter can provide excellent 'ears' (as G3XDV can
> attest) provided steps are taken to reduce the input with an attenuator
> and/or preselector if you are using a large antenna.
>
> The problem often arises when you change from a small antenna to a big
> antenna. You often see it with a special event station using a big antenna
> put out a huge signal but have problems hearing anyone. Seasoned LF
> DXpeditioners, such as the 'Two Daves and a Graham' take with them their pet
> receiver front end signal level box.
> When I operate from GB2CPM the Loran comes pounding in at S9. I have to
> insert some 18dB of front end attenuation plus the AIP before the weak
> signals become readable.
>
> I live only a few miles from Laurie, G3AQC. His signal with me is S9 +35dB.
> (I guess I must be as strong with him). I am able to operate on the band the
> same time he is on provided I use the attenuators and provided I don't get
> too close in frequency!
>
> Regards,
> Peter, G3LDO
>
> <g3ldo at ukonline.co.uk>