Hi Jim,
On 03/04/2013 12:57 PM, Jim Brown wrote:
> I'm assuming you've already determined that the bad box is
> not his own (by killing power to it).
Actually we knew it was is own network, which he needs to stay up at
all times. The offending device is either his wired router or cable
modem. I wasn't able to determine which with any degree of
certainty. The interference instantly and completely goes away if
either device loses power but to me (a very NON IT guy) that doesn't
say much.
First, Dale noted a similar carrier at 147.39, and we know that
Ethernet birdie frequencies are driven by the fact that each Ethernet
switch is free-running with its own crystal controlled oscillator. SO --
changing out the physical switch (which may be part of a wireless
router) could move the birdie off of 147.21 MHz.
I loaned him a spare router to try. I'm not comfortable messing with
his network so he will get his son who works in IT to swap them out
and see if that makes any difference.
Third, if the device can be located, my guess is that it MIGHT be
sufficiently suppressed by a 4 - 6 or so 1-inch long #43 cores on each
cable connected to it, and to the other end of Ethernet cables connected
to it. That won't help, of course, with stuff radiated by the box itself.
The cables seem to be radiating more than the devices on the network
from what I can tell.
When thinking about problems like this, it helps to think about HOW MUCH
suppression is needed. Those cores aren't going to get you 40 dB, but
they might get you 10-20 dB, and that might be enough.
A 10 dB reduction would help greatly, if not completely eliminate
the problem.
We disconnected an unused cable this morning, which reduced the
level of interference somewhat. Several feet of the unplugged cable
is still involved in what I can only describe as a rat's next of
cables. As a result (not surprisingly) that cable still seems to be
somewhat "hot" with the offending signal. I recommended he
physically get rid of that cable altogether and check RFI levels
again. If that doesn't do it I suggested he swap routers to see what
that does, if anything.
Lastly, if those measures do not alleviate the issue, I suggested he
invest in a good number of ferrite cores to place on the cables.
We are a small club with limited funds, but at some point I hope we
can invest in a good quantity of several of the more useful RFI
suppression cores to have in a RFI "go kit". That way we can try
them in various RFI situations to see if that is the answer. If they
work, we can leave them in place and have the recipient reimburse
the club for the cost of replacing the cores in our kit. I like this
idea better than suggesting folks go out and buy these things,
particularly for VHF cases where this approach may or may not succeed.
73
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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