Dear Jacek , LF Group,
----- Original Message -----
From: "Jacek Lipkowski" <[email protected]>
To: <[email protected]>
Sent: Saturday, December 25, 2010 4:41 PM
Subject: Re: LF: Receivers vs Transmitters etc. = "Mal'isms"
it's also quite remarkable that once the range was limited mostly by
receiver sensitivity, while right now by the ability to reject qrm. SAQ
was once in regular commercial service with New York with simple receivers
at the other end (btw. anyone know what rx was used?)...
You can see pictures of the receivers used in the USA for reception of SAQ
etc. at http://earlyradiohistory.us/1922RCA.htm. For large point-to-point
installations of this era, when low-power valves/tubes had become well
developed, the receivers were clearly quite elaborate. The article on the
website gives very limited technical details about the circuit used, but I
found the attached schematic for a similar receiver in an old book "Wireless
Telephony and Broadcasting" by H.M. Dowsett (1924). This was used for
inter-European LF point-to-point services. The antenna is a phased
vertical/loop combination, used to null interference. It has a 4 stage tuned
RF amplifier, local oscillator and mixer down-converting to a 2.5kHz "IF"
frequency, and four stages of selective 2.5kHz "note filters". It is
interesting to see that, although an audio output is provided for monitoring
purposes, the main output is via a telegraph relay in the "bridge circuit".
The book explains that the telegraph output goes via landline to "Radio
House", where it drives a paper tape perforator, and this tape is used to
generate text output via a "Creed" telegraph printer. A similar system at
the TX generates machine-morse at around 100wpm. So remarkably, even in the
1920s, the encoding/decoding system is fully automated and digital!
Passive crystal-set type receivers were normal before 1914, used with giant
spark transmitters. Technical descriptions are not easy to find, but
references to "balancing antennas" and so on suggests that external noise
was more of a limitation than sensitivity for these big installations. Very
large receiving antennas were used, no doubt due to the non-linear,
square-law response of such detectors at low levels, e.g. the 100m high,
1500m long inverted L at the Marconi Belmar receiving station in
Massachusetts - see http://infoage.org/html/hen-masts.html. Heterodyne
reception, as pioneered by Fessenden for his early alternator transmitters,
is much more efficient in this respect, being an essentially linear
down-conversion process. I can confirm this from experience with my
electromechanical RX project (see http://www.wireless.org.uk/mechrx.htm ),
which has about 20dB loss between the antenna and headphones. Even so, using
a 10m high, 40m long wire antenna, the sensitivity at VLF is limited by the
QRN, which is always audible here. SAQ is easily copied, and my best DX so
far with this receiver is VTX3 in southern India.
Another reason CW transmission / heterodyne reception replaced spark
transmitters was because it allowed increased RX selectivity, and so better
rejection of QRM and QRN. So it would seem external noise of some sort has
always been a limitation at low frequencies, at least for big communications
projects where very large antennas could be used.
Cheers, Jim Moritz
73 de M0BMU
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