On Thu, 06 Mar 2008 05:53:10 -0500, Roger (K8RI) wrote:
>That's precipitation static, be
>it rain or snow and it is a differential DC voltage that creates
It may start out as a DC voltage, but the CURRENT is RF -- the transient of
the arc -- that's why we hear it. We don't hear DC.
>but using a boradband transformer will turn that DC into a common mode
>voltage that will lessen the noise noticeably.
I suspect a different mechanism. Virtually all ham antennas are unbalanced
by their surroundings, so unless there is a serious common mode choke at the
feedpoint, there will be common mode current from any field that excites the
antenna, including that static. If I'm right, a serious choke at the
feedpoint would kill that part of it. If it's a differential signal, nothing
is going to kill it, certainly not a transformer (voltage balun).
Another VERY important point. Voltage baluns (transformers) put all of the
transmitted power in the core of the ferrite. If you're running much power,
that's going to heat the ferrite. If you're running enough power, it will
saturate the ferrite and cause DISTORTION. That means harmonics and
splatter. Current chokes do NOT see the transmitted power, only the
unbalanced voltage/current, and if they have a sufficiently high choking
impedance, they reduce the current (and their dissipation) to a very low
>Nearby lightning strikes and direct hits are an entirely different
>matter. Installing large chokes (linear or toroid) at the antennas is
>not a satisfactory option mechanically,
Why not? It certainly is the best place to put one electrically for noise
suppression! The coils of coax on toroids I've described can very easily go
up at the antenna feedpoint. So far, I've presented my work to the guys at
NCCC, NCDXA, REDXA, and PVRC. I've had many reports that their antennas are
>but is back by the remote switches which is where I plan on using them.
There's certainly nothing wrong about using a choke there, but it will be
far more effective at the antenna feedpoint (that is, up in the air),
because the length of coax between the feedpoint and the choke will act as
part of the antenna.
>The choke at the antenna
>serves one purpose, but doesn't do much to mitigate lightning when there
>are long runs of coax from the choke to the tower in which induced
>currents can be substantial. The chokes back in front of the remote
>switches and grounding the coax shield to the tower can make a
>substantial reduction in the lightning effects/damage.
Wherever a choke is located, it will act as a very low Q parallel resonant
circuit in series with the COMMON MODE circuit (and a short at DC). If it
sees significant energy from lightning, there will be heating. If it sees
enough, there will be melting of the coax and cracking of the ferrite. :)
But for non-destructive amounts of energy, it will simply reduce the current
(and may divert most of it to a different path by Ohm's law).
>Another reason for the extra protection is I do not have the option of
>disconnecting any of the rigs or even computer networks unless I have
>plenty of warning. Doing so is a major undertaking as is reconnecting
>them. Just the computers can take up to an hour to get back up and
>running on the CAT5e network.
As you have noted, disconnecting antennas does NOT provide lightning
protection. And several engineer friends who work in pro audio but have no
antennas have lost computer networking gear to lightning strikes. It's
voltage/current induced on the computer wiring, and voltage/current induced
on power wiring, and the differences between them.
Jim Brown K9YC
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