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My friend, Ron, K8AQC, has experienced intermodulation
interference when receiving NAVTEX signals at 518 KHz. NAVTEX means "navigation
text" and is a means of distributing weather and navigation information to ships
using direct printing technology. The modulation mode is FEC (forward error
correction) and can be decoded with software or hardware used for amateur radio
AMTOR work. There's more detail at
http://www.beaconworld.org.uk/files/navguide.pdf if you are interested. And
there's a nice free NAVTEX decoder for Windows computers available at
http://www.frisnit.com/navtex/?id=decoder.
I designed and built a 4-section coupled resonator filter
for Ron, center frequency 518 KHz, design bandwidth 20 KHz. A bandpass input
filter, of course, will only reduce intermodulation created in the receiver. It
won't do anything to remove intermodulation caused externally—the proverbial
"rusty bolt" intermodulation source.
The filter uses the same design approach and printed
circuit board and enclosure as other Z10010
bandpass filters.
The three plots below show the filter turned out quite
well and match the predicted performance. The main difference between predicted
and as-built measured is that I tuned the filter to provide a flatter response
in the passband. The result is a slightly greater insertion loss (less
than 1 dB increase). And, as usual, the ultimate rejection at frequencies far
from the passband differs from predicted, due to stray coupling, distributed
capacitance in the inductors, change in inductance with frequency and other real
world factors absent from the design software. |
