In the early days of 73kHz experimenting the only receiver equipment
I had was a Datong converter and my mobile transceiver or the Drake
4c. The Daytong converter was designed for SWL use, had excellent
sensitivity with a short length of wire but was easily overloaded
when connected to an LF transmitting antenna. Yet most of my 73kHz DX
contacts and some of the early 136kHz contacts were used with this
receiver system. The secret was in the appropriate use of filters and
a variable attenuator.
The first stage in the filtering in my LF receiver is the transmitter
antenna resonating and matching circuits. This was followed by two
parallel tuned circuits, top-coupled; followed by an attenuator
(comprising a potentiometer an a couple of resistors. With this
circuit weak amateur signals could be extracted without any
intermodulation from the high powered RTTY and broadcast stations
However, it is well know that two parallel tuned circuits using
capacitive top-coupling degenerates to a high-pass filter in the
stop-band, i.e. gives good attenuation on the lower side of
resonance but the attenuation is only around 40dB on the high side of
resonance. If problems occur from signals higher than the filter
resonance then it is better to use inductive bottom coupling on the filter.
Since the advent of high performance broadband front-ends and VHF
first IFs in modern receivers we seem to have lost the art of good
receiver front end filter design - however there is a lot of design
information around.
I recommend Solid State Design for the Radio Amateur, by Wes Hayward
W7ZOI. On page 117 (Advanced Receiver Concepts) he describes a
tunable Cohn filter ( which is a four resonator inductive bottom
coupled filter for 1.8MHz that can be scaled for other bands).
Additionally, Reference [1], shows the use of filters in domestic
radios. Fig 7, for example, uses switched filters to cover the long,
medium and short wavebands. On the longwave filter a parallel
bandstop filter is used to reduce interference from a known
high-power transmitter.
Reference [2] gives filter design data in a concise form, with
appropriate formula and characteristics.
There are many computer software filter design packages. Reference
[3] is of one such package, analyzing the performance, for example,
of a double-tuned parallel filter.
The use of filters such as these are essential, particularly when
using large antennas. The out of amateur band signals burned out the
diodes of my MFJ-249 when I was trying to resonate my LF antenna for top-band.
Having said all this, filters cannot help in my case of the in-band
trash that comes from the Loran transmitter at Lessay on 100kHz.
REFERENCES
[1] Classified Radio Receiver Diagrams, E. M. Squire, Sir Isaac
Pitman & Sons, 1943
[2 Radio Engineering, Volume Two, E.K. Sandeman, Chapman and Hall Ltd, 1949
[3] The ARRL Radio Designer. (computer software), The American Radio
Relay League, Newington, CT06111, USA
--
Regards, Peter, G3LDO
<[email protected]>
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