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
|
