[Fwd: LF: Receiving]
Wed, 14 Apr 1999 16:02:03 -0400
Hans-Joachim Brandt wrote:
> Hello all,
> Petr Maly schrieb:
> > These experiencies give me more hope. Several times I tried to listen to 136
> > kHz on my IC-751A and long wire (80 meters), heard nothing but noise of
> > various kind. One suggestion. Few years ago I came across a schematic
> > diagram of receiver for NAVTEX (I was R/O and /mm that time). Directly on
> > input of rx there was a crystal filter on 518 kHz, where NAVTEX transmits.
> > Would it not be excellent solution to make crystal SSB-wide filter centered
> > on 136.7 kHz and put it between ant and rx? I know there is only one problem
> > - where to get crystals around 136 and 137 kHz..., hi
> > Petr, OK1FIG
> In the easiest way I guess that such a crystal filter would become too narrow
> even for the small ham band, and it would be more difficult to broaden such a
> design. For special receive tests, it could be helpful, of course.
> If a receiver needs some input attenuation at LF on large aerials, as reported
> by ON7YD on his TS440, for instance, the better way should be to realize this
> "attenuation" by the loss of a simple coil filter using one to four resonant
> circuits. But such an arrangement would need less resistive attenuation to stop
> the overload of the receiver, therefore even some additional preamplification
> (and selectivity) might be possible, with resonant circuits carefully arranged
> ahead and behind amplification so that in an empty band the S-meter hangs around
> "S1". Then it should even be possible to receive the commercial LF stations with
> an indication of S9+30dB if the receiver has sufficient AGC range.
> Depending on the Q and coupling realized for the resonant circuits such a
> preselector/amplifier combination would either cover the whole band or would
> even need ganged tuning for best performance. In this respect look at the front
> end designs of those old commercial and military valve receivers before
> (roughly) 1960, using up to five ganged resonant circuits ahead of the first
> mixer! They may lack frequency stability and accuracy and i.f. selectivity and
> other features of today, but they were never blocked when connected to large
> aerials, and as far as I have heard when in good shape they are still very
> useful for amateur LF reception.
> At present I am listening on LF using HF receivers, either a home brew design
> with three passive resonant circuits ahead of a TCA440 IC single conversion rx,
> or employing an old TS830s (the dial of which is rather unusable at LF because
> of its 800 Hz offset in CW receive). The converter LF to 14 MHz is passive,
> consisting of a single resonant circuit coupled to a 74HCT4066 connected like a
> ring mixer, with 14 MHz injection. When the input circuit is tuned, the noise
> increase is noticeable in the receiver!
> The connection of the tuned LF aerial to the converter does not result in a
> noise increase, however, and therefore I plan to add some pre-amplification
> ahead of the mixer, just to overcome the mixer loss. When listening to SAQ at
> 17,2 kHz in August 1998 I already had to employ some preamplification ahead of
> this mixer, consisting of a BF245 and an emitter follower.
> In about 40 km distance of my QTH there is the "Deutschlandfunk" transmitter at
> 207 kHz; therefore I must be careful not to overload my receiver input (when the
> LF aerial is tuned to 207 kHz and connected to a passive detector receiver, 600
> mV eff can be measured at the diode input).
> In former decades it has often been said that building a good amateur receiver
> is much more difficult than constructing a powerful transmitter. Obviously this
> is true even today, and therefore I appreciate the discussion on improving the
> receiving part of our LF equipment.
> 73 Ha-Jo, DJ1ZB