> away from the set. BUT: That common mode current CAN
> re-radiate
> into a set that is poorly shielded. In this situation, a
> big choke
> could raises the impedance of that coax sufficiently to
> kill the
> current that is generating the re-radiation.
The problem in the near field of a transmitter is virtually
never reradiation. The very strong field intensity from the
transmitter dominates any conducted emissions that are
reradiated.
The problem is almost always conducted currents or potential
differences between two different paths at the device
subject to problems.
I certainly agree beads can help, but one only needs to
think about what the bead does to realize beads by
themselves are a very unpredictable solution.
The bead adds a certain series common mode impedance. We can
predict that impedance. For example a 73 material might add
about 100 ohms per linear inch of ferrite surrounding a
cable. Say for example we add 500 ohms. That impedance goes
in series with the common mode path impedance at the spot
where the beads are added. It might increase attenuation of
common mode by almost nothing all the way up to several
dozens of dB....all totally dependent on something we can't
control.
If the bead is reactive instead of being primarily
resistive, it might actually INCREASE unwanted common mode!
Now let's assume we do the right thing, and bring cables
through a bulkhead that grounds all shields and bypasses all
lines that can be bypassed at one common point. This is
actually how every single piece of equipment should be
built, but they almost never are. Now virtually none of the
current flows through the device, so it is harmless. Now if
we add a bead on a lead it indeed might reduce common mode
currents, and if our bulkhead was less than perfect it might
increase suppression a tiny bit more, but we are still right
back to the totally unpredictable results because we don't
have any idea what the common mode path impedance is.
Beads are generally a lazy man's band-aid for poor cable
routing or poor equipment design. Certainly they can help,
but because we don't know if the common mode path is 10 ohms
or 10,000 ohms, we have no idea what adding another 500 ohms
might do. Obviously if the common mode is very low a bead
would have a large effect. If the common mode path impedance
is very high it won't do a thing. Worse yet if someone
doesn't pick the beads carefully and gets a bead that is
reactive instead of predominately resistive, it can actually
increase common mode.
On the other hand bringing cables in through a bulkhead or
common point (that can float or be grounded, doesn't really
matter) improves lightning protection and reduces common
mode.
One system sticks in my mind. I inherited an apartment
complex that was built over the ground system of a high
power AM station with a 50kW ERP FM on the same site. The
towers were just feet away from the fence. The CATV company
wasted time and money installing multiple shielded cables
and Drake TVI filters.
I had all the cable abandoned and new single shield trunk
line pulled close to the power feed path. At the entrance of
every building I bonded the power line ground to the CATV
cables. In every apartment a duplex outlet was installed
next to the CATV wallplate, and the grounds were bonded. Out
of hundreds of TV's virtually all of the TVI vanished, there
were only one or two sets that continued to have problems.
That was without a single high pass anywhere in the system.
The only rule was every TV and VCR had to plug into the
outlet next to the CATV wallplate. What I found was the TV's
that didn't clean up generally wouldn't clean up no matter
what was done, they all had RF ingress right into internal
wiring.
My own house is wired this way now, I use cheap outlet
strips with F connectors for the bonding point. I can run
any amount of power on any band with almost no TVI, and I am
receiving off the air Atlanta stations at a distance of 60
miles or so. Not a lowpass on a transmitter, and not a
highpass anywhere except in front of the preamplifier for
lowband TV channels at my system head end.
As a side benefit my 300 foot tower takes a dozen or more
hits a year, and the worse that ever happens is the screens
magnetize. I haven't lost a TV or VCR or computer modem
despite multiple direct strikes.
So this all leads me to encourage spending money where it
will always do good first, and then throwing beads or
filters at the problem. Even if you don't fix the TVI at
least you'll have significantly better lightning immunity,
and as a side benefit emissions from the consumer devices
that screw up your radio gear will be greatly reduced.
73 Tom
73 Tom
The bead isn't the problem, the system is.
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