Topband: Fwd: BCB Interference

[Dennis Egan]

---------- Forwarded message ----------
From: "Dennis" <egan.dennis88 at gmail.com>
Date: Nov 3, 2015 07:24
Subject: BCB Interference
To: <topband-request at contesting.com>
Cc:

I have a difficult BCB interference that I'm trying to resolve for a fellow
ham.  It
is present on both a K3 and a 7800 at the station.

When switching to 160M, there is a constant noise level of about S9.  When
using
a narrow CW filter, it sounds like local noise, but if you use an AM filter
you can
hear different BCB radio stations mixed together.   Any station below S9
can't be
heard, and stations above S9 are difficult to copy as they seem to be
"noise modulated".
When using the K3 with neither the preamp nor the attenuator on, and
switching from
any band to 160M, the attenuator turns itself on; this indicates a signal
at the input
of +13 to +20dBm.  The antenna system is a 160m inverted V through a 403A
High Power LPF for 160M.

When switching to 80M, there is a constant noise level of about S5. Similar
to the
160M noise, when using an AM filter you can hear different BCB radio
stations
mixed together creating the noise.  On 80M, the noise threshold is S5, and
stations
S5-S7 are difficult to copy due to the "noise modulation".  When using the
K3 on 80M,
if you turn the preamp "on" it will quickly turn itself off; this indicates
a signal at the input
of +3 to +10 dBm.  The antenna system is an 80M 3el beam through a 4O3A High
Power BPF for 80M.

On 40M, the noise can be heard, but at much less amplitude.  It only makes
the weakest
signals difficult to copy.  On 20/15/10m, you can also hear the noise, but
it is just
an annoyance.

The station owner purchased an ICE 402x BCB filter several years ago, but
reported
that it made no difference in being able to hear.  A 4-square listening
array was also
tried, but the interference was there also.

I plan to attack this on 2 levels.  I made 2 Rx-only BCB filters at home;
each filter
gives about 55dB attenuation on the BCB.  I have scoped them out on a VNA,
and also
put them on another station and can verify that they provide about 55 dB
attenuation
on an AM signal at 1420KHz.  I also have a step attenuator that can provide
another
60dB of attenuation, so between the 3 devices I have 170dB of available
attenuation.
If the culprit is simple front end overload, I suspect this should be
enough attenuation
to show it.

After reading the article by N0AX in QST early this year, there may be
rectification
going on in one or more pieces of equipment.  Since switching diodes in
different gear
could cause a mixing problem, I'm also planning to remove the feedlines
from the back
of all amplifiers and looking for the problem to go away.  I have a handful
of disc
caps to bypass rotor, stackmatch, and other control cables.  I'm also
bringing an RF
probe, so I can listen for IMD products at various pieces of equipment.

Am I overlooking anything?  Are there other sources that could be feeding
into the problem?
Suggestions appreciated.

Dennis W1UE