[Fwd: LF: Receiving]
Thu, 15 Apr 1999 18:22:02 -0400
Peter Dodd wrote:
> 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 , 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  gives filter design data in a concise form, with
> appropriate formula and characteristics.
> There are many computer software filter design packages. Reference
>  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.
>  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
>  The ARRL Radio Designer. (computer software), The American Radio
> Relay League, Newington, CT06111, USA
> Regards, Peter, G3LDO