for LF I'm using a commercial US-rig, the Harris RF590. I like it
because of it's extremely high frequency stability, high immunity
against strong signals (IMD), the 1Hz read out and the low tuning rate
(100Hz per revolution) which gives me the illusion of a LF-band as large
as 40m, hi.
But I got the rig also with a Collins 500Hz/455kHz filter (commercial
equipment manufacturers are specifying filter BW often at -3dB and this
gives you the illusion to have a 300Hz filter, but in fact it is 500Hz
For a better separation of the stations in the narrow LF-band, I
replaced it with the FL53A from ICOM. This is a very steep XTAL-filter
(unfortunately also very expensive!) and I took no special measures for
Drawbacks of the Harris RF590 are: operation also a little bit
complicated (commercial radio operators have other needs) and a blow by
in the IF-filtering (at least in my version) due to poor design, but
fortunately it was easy to modify.
I also tried other commercial surplus from Racal, Rockwell-Collins,
Watkins-Johnson R&S and Telefunken. They are all very similar when
developed in the same period (they all had to fulfill the same MIL
standards, I guess).
As an additional audio filter I'm using the DSP599zx from Timewave,
where I can go down to 10Hz BW (-3dB) if necessary and as matter of fact
some CW QSO have only be possible by using this filter and BW between 10
and 35Hz (as the QSO last weekend with G3XDV).
Listening to such a filter is not pleasant. They usually don't ring but
the filter noise is close to the tone of the expected signal.
And of course there is a big gap between theory and practice: the
improvement by such a filter is far from the theoretical value!
Often I use instead of the DSP-Unit a passive home-brew LC-filter
the RX and the headphones. It's a bandpassfilter with a 11 element
Chebyshev LP and a 9 element HP. not so narrow (200Hz), but very
pleasant to listen, even under heavy QRN.
Finally I have to say that it is not necessary to employ commercial
receivers for good results. I've heard most of the UK stations also on
my Drake R4B (old tube rig) with home-brew converter (just LPF, Osc and
Diode ring mixer) and the above mentioned passive audio filter. The
sound is very pleasant (one of the best AGC's I know - better than
Collins and most of the mentioned commercial rigs!). The only
inconvenience is the lack of frequency accuracy,
73 de Toni