This topic has been covered before on this list. I believe that my recent
experience is also worth reporting.
After over a decade of operating on 160m, I'd never experienced any significant
problems with BC carriers other than very weak carriers on 1810, 1820, 1830,
1840 kHz etc. The greater Los Angeles area is not at a loss for high power BC
stations. Several weeks ago a very loud and wide signal appeared for the first
time on 1840 kHz in addition to others every 10 kHz. These spurs were a bit
weaker after sunset but still loud and wide. Any legitimate 160m signal within
about 3 kHz of these spurs could not be copied. The spurs' modulation matched
that of a broadcast station six miles from my location. With the assistance of
a local broadcast signal compliance expert, signal strength measurements were
made using professional grade equipment close to the BC station's transmitting
towers as well as at my station. The spurious signals were just out of
regulatory specifications and the modulation was mushy. The station's engineer
was contacted. He advertised that he would perform some s
tandard maintenance. To date, nothing has changed.
All of the possible combinations of equipment at my station were tried in an
attempt to isolate and diagnose the problem. The result was that the spurs
appeared only with an external preamp in the path with a full size receiving
flag. This was observed with not just a single preamp but with a second preamp
of an entirely different design.
I inserted a W3NQN Receive Only BCB filter between the flag and each of the two
preamps, one at a time. The result in both cases was that the spurs were
reduced to very weak carriers daytime and to a just perceptible level when the
BC station switched to nighttime power (10 kW to 490 watts). Something has
likely changed in the BC's station signal, causing the preamps to become
overloaded resulting in IMD problems.
http://www.arraysolutions.com/Products/BCB%20RF%20Filters.htm
This filter has an extremely sharp cut-off below 1700 kHz. I understand that
precision tolerance components are hand selected for custom assembly of each
filter to guarantee the specified response. W3NQN has started to label his
products "Design, Assembly and Testing by W3NQN."
These filters have a well deserved reputation. The BCB is superb.
73 - Steve WB6RSE
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Topband Reflector
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