Comments on narrow filters
Sun, 03 Oct 1999 10:08:54 -0400
Dave Sergeant wrote:
> >From Dave G3YMC
> I was about to reply direct to Alan about his receiver bandwidth suggestions, but other mails now demand a reply to the reflector.
> Nick G4WHO sums it up very nicely when he says that the theoretical improvement is for noise considerations only. This assumes you are listening to a single signal on an otherwise unoccupied band. In this case there will be little difference between the 500Hz signal and 350Hz. However we live in a real world, with often several strong in-band signals and the weak one we want to copy among them. For this purpose the bandwidth and skirt response is the most important thing. Frequently at weekends it is impossible to find a clear frequency in the normal CW part of the band when using normal bandwidth CW filters.
> My 350Hz Yaesu filter is rather lacking in slope response, such that for instance when G3XTZ's signal is on 136.0 I have to tune up to 136.5 to totally clear it. Most HF transceivers are configured for upper sideband CW. An audio filter may help (I have not tried one) but AGC effects will still reduce sensitivity for copying weak signals in the skirts of stronger ones.
> As a long time HF CW operator, both in contests and DX pile up chasing, I could never go back to using an SSB filter for CW, it is just literally as wide as a barn door. Like on HF when it is the norm to have 500Hz or less, it should also be the norm on 136. However though I know some operators have much narrower filters than that, when I transmit I always try to avoid transmitting too close to others, assuming if it is too close with my filter it will be too close for the other amateur's receiver as well. Not that I am a big enough signal to cause much QRM!
> Note that most good CW operators tend to tune for a note of 600Hz or below, as copyability and human ear filtering is better for lower notes. Those of you attending the HF convention next weekend may like to try your hand at the CW pile up competition, which will give you a feel about what real CW operating is like!
> I am also rather puzzled by the emphasis given to DCF39. With a normal CW filter it is so far out of the band that it simply doesn't come into the equation (unless you have receiver overload problems that is).
> 73s Dave G3YMC