At 06:13 PM 12/11/2003 -1000, Ken Brown <ken.d.brown@verizon.net> wrote:
>And I heard lots
>of varying pulse rates and bandwidth signals around the 60 MHz area.
>Also the spare 3CX800A7s that I have for my
>Titan 425 are pulls from a MRI machine.
I had a "driver" from an MRI machine that I used for a 6m amp. This one was
built by ETO (yes, *that* ETO that builds the Alpha series of amps). I also
had some of the documentation for the whole system. That part that I had
came with a 3CX400A7/8874. This drove a 3CX800A7 stage which drove the
final consisting of a pair of 3CX1500A7/8877's. This whole system was used
in pulse mode with a peak pulse power of 15Kw (No, Not a typo - read
fifteen kilowatts). This was variable from about 30MHz to 70MHz. I don't
have the documentation right here but there were systems to switch various
parts in or out for various power levels depending of 'whole body' or
various limbs or appendages of our bodies. Those machines can crank out
some power!
However, all this comes back to the subject of close proximity to a
lightning strike on a tower. The said strike of around 50Kamps on the tower
produces an orthogonal magnetic field outward from the tower. As this field
crosses a conductor, there is a current produced in that conductor. It
doesn't matter if you have a moving conductor or a moving magnetic field,
you will have a generator of electrical current. Some old character named
Norton says the sum of the currents in a circuit must be zero. Since we
just created a source, it will find a 'user' of this current. If that
current was created in your DC power cord running perpendicular to that
field, the 'user' of the current generated will be your radio, power supply
and/or whatever else you have tied to one of the ends of that wire.
Enter the "halo" ground. This orthogonal magnetic field can be attenuated.
In the commercial end of RF generation (paging, 2-way repeaters,
broadcasting, etc), the sites tend to be located at the top of something.
(For VHF/UHF communication you need 3 things - height, altitude &
elevation!) Since lightning seems to favor high places (but not
exclusively), and since the users of the commercial services really don't
care what kind of weather might be occurring, these sites must continue to
operate during anything and survive. By running a 'halo' of heavy gauge
wire around the ceiling of the shack, this magnetic wave can be intercepted
(and attenuated) by sending the induced current to ground from the halo.
For a more detailed (and MUCH more accurate) description of this halo
system, go to the Polyphaser web site <http://www.polyphaser.com/>, click
on the "Tech Info - Engineering Notes" button and look down for Document
Number PEN1015. Hey - while you're there, check out their many other 'white
papers' on the subject of grounding. These folks make a darn good living
protecting commercial users from the evils of lightning. There products do
work. I'm just a user of them (although I do dream of kick-backs for
pushing their products!!!) and have seen antennas with holes in them from
lightning strikes and the equipment in the shack never noticed the glitch.
Sticking your coax in that Mason jar won't really save your sleek new Orion
or your old reliable PowerMite. Grounding is a system of power, lightning
and RF grounding working in concert together to protect you, your house and
your equipment from damage. Read about it, learn about it, apply it. You'll
be glad you did.
de KD4GT
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