Hi Dan,
First, forget about "ground" -- a connection to the earth is NOT part of
a cure for any form of RFI, nor does it make antennas work any better.
Second, I can't even begin to imagine what an "end fed multiband dipole"
is. What DOES matter is how close the antenna you are using on 30M is to
that cable modem, AND to the cables connected to the cable modem.
Now, getting to your cable modem. it helps to think about how your TX RF
is getting into that cable modem. There are several possible ways.
1) The cable modem itself is badly shielded, so wiring inside the modem
is picking up the signal. The cure -- throw it away and get one that
does not have that problem. If it belongs to the cable company, tell
them it is defective (and HOW it is defective) and tell them to either
fix it or replace it with one that works.
2) The RF is getting INSIDE the cable itself because somewhere in the
cable system there is a bad shield connection. This is a VERY common
problem, and can happen when the cable installer does a bad job
installing one or more connectors on the cable. It can also happen when
a CUSTOMER makes his/her own connection (because he/she doesn't want to
pay the cable company) and does it badly. This is the most common way RF
gets onto the cable, and ALSO how their TV and computer signals leak out
of the cable to cause us interference. This is also the cable company's
problem, and THEY must fix it.
3) The other most common way RF gets into equipment is by the cables
connected to the equipment acting as receiving antennas, and coupling
the signal it receives into equipment by a manufacturing defect called
"The Pin One Problem." There's no easy fix for Pin One Problems in most
equipment, BUT it's easy to kill the antenna action by adding the RIGHT
ferrite choke to the cable(s).
The most likely antennas are the coax, the CAT5 cable that connects the
cable modem to your Ethernet switch or wi-fi base station, and the power
supply cable (between the power supply and the modem).
You MUST wind multiple turns through ferrite cores to make a dent in RFI
on 30M. Just clamping a core on a wire won't do anything useful. For
detailed advice on how to wind them, study
http://audiosystemsgroup.com/RFI-Ham.pdf
A good start for 30M would be 4-5 turns through a 1-inch long #31 or a
#43 core, or 7-10 turns through a #31 or #43 toroid. If you're lucky
and the problem is not #1 or #2, this ought to fix it. It's the cable
company's problem, and you'll have to get them to fix it.
73, Jim K9YC
On 3/25/2013 7:43 PM, Dan Butler wrote:
I live on the 15th floor of a 17 story apartment so "proper" grounding is not
possible. I use an end fed multiband dipole which theoretically does not require RF
grounding. However, I do have a multiple wire counterpoise hooked to the tuner. I
recently converted from a 20/40 end fed dipole to the 6-80 end fed dipole, so have just
started using the other bands. Now when I TX on 30 M (and occasionally 17 M) the internet
cable modem which is in an adjacent room about 20 feet away, cuts off - all the lights go
out, then it starts once the TX is finished. I placed a steel bowl over it and wrapped it
in foil but it still cuts off. I disconnected the router cable and the cable input and it
still cuts off. I presume I have some sort of power supply issue but not sure how to
address that.
Thanks in advance for any useful suggestions.
73, Dan 9V1DB
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