Based on a report that inserting a 6 dB attenuation at the RX input and that
reduced the BCI which was S9 + 20 dB or so tells me that the IM is being
generated in the radio. If it were external then it would have been reduced
by 6 dB.
Keep in mind that IM does require 2 or more signals mixed in a non-linear
environment. Someone commented that a 2nd harmonic of a 50KW AM was the
root cause. That causes me to ask, where's the 2nd harmonic being
generated? It very well could be the junction of the ham radio tower and
the guy attachment point. Very common event. Hence, guy wires should be
bonded to the tower in high foreign RF environments. Just wrapping a guy
around a tower leg is not adequate. If the IM changes in wind and or
weather, you can bet on it being a bonding issue that needs attention.
If the 50KW BC signal then mixed with its own 2nd harmonic the sum of the
two will be heard. (BC freq + 2x BC freq)
Example: 650 kHz + 1300 kHz = 1950 kHz (This is the fundamental plus the
2nd harmonic summed) This is WSM in Nashville, TN and not a fault of WSM
but of the local environment at the ham station. Another local example:
1300 KHz + 1960 kHz + 650 kHz = 3910 kHz (This is the 2nd harmonic of 650
plus the 2nd harmonic of 980 plus the fundamental of 650)
Again a 50 KW of 1510 x 2 + 5KW of 980 = 4000 kHz The modulation plays
havoc with the 3980 TN net freq. in certain areas of Nashville. The
sidebands are 2x width for the 1510 fundamental. Another factual example in
Nashville TN. Being WLAC AM 1510 and WSIX AM 980, again neither the fault
of the BC stations.
In both examples the 2nd harmonic was being generated by the galvanic
junction at the ham tower. The two BC stations were clean so to speak.
Now, to validate the testing, if the IM is heard on one radio but not on
another with the same antenna configuration and location, one might then
presume that the IM is being generated in the radio of question. If adding
6 dB of external input attenuation kills the IM then for sure it is in the
radio. If the IM drops by only 6 dB then the IM is external.
73
Bob, K4TAX
----- Original Message -----
From: "Lee Crocker" <w9oy@yahoo.com>
To: <tentec@contesting.com>
Sent: Friday, May 27, 2005 3:26 PM
Subject: [TenTec] ORION BCI
If the diodes are reverse biased, then it sounds to me
like you lost bias. Could be nothing more than a
loose plug on the board. Why do you presume the
problem is a design flaw and not just a glitch in your
radio? If your TS-930 developed a cold joint that
began acting as a diode would you then say Kenwood
made a design error?
73 W9OY
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