To All from PA0SE
Andy, G4JNT wrote:
I've just made a check of both
Droitwich and Donebach transmissions using
the WatKins Johnson WJ-8711 receiver here at work. Using the
narrowest IF
bandwidth of 56Hz, setting manual gain control, and monitoring the
output
tone using Gram on its widest setting I cannot detect any variation
in
carrier level with modulation on either of the two transmitters.
The level
meter, which indicates actual signal level, accurately calibrated in
dBm,
also shows no significant variation with modulation. There is perhaps 1dB
of
variation on Donebach ( due to fading ?) and nothing on
Droitwich.
I'm very sure (read 99.9% certain) Droitwich has no amplitude modulation
-
viewing it on a vectorscope in the past shows only the phase
variation. So
perhaps you are seeing some secondary effects introduced by the receiver
?
Andy G4JNT
I repeated the reception using the Wandel & Goltermann PSM-5
selective voltmeter and a wire aerial instead of the loop. The
results were identical to the SPM-12 and the loop aerial: variation in
strength as read on the meter on Donebach, Droitwich and Dutch
747kHz and steady readings on other LF and MF transmitters. The
variations follow the modulating signal so it is not a matter of
conversion of a data PSK or FSK signal into AM.
It would be interesting to know what other amateurs find? Or
provide an explanation of the anomaly?
73, Dick, PA0SE
< I am pretty sure
Donebach uses some form of controlled carrier modulation,
>apart from the phase modulation mentioned by Walter.
>Our Dutch MF transmittters at 747kHz and 1008kHz are also made by
>Telefunken (400kW at daytime). The 747kHz transmitter uses
controlled
>carrier to save power and expenses; the 1008kHz one does not. The
carrier
>at 747kHz, as measured by the SPM-12 at 25Hz bandwidth, shows the
same kind
>of fluctuations as the Donebach one, the 1008kHz carrier is
steady.
>Remarkable is Droitwich at 198kHz. There the carrier goes downward
during
>modulation!
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