On Fri, 21 May 2010 08:06:59 -0700, Jim Thomson wrote:
>Do we really require 5 k choking Z ??
Considerable experience has shown that this is a good target for
noise pickup. In my analysis, I also showed that a very high value
has the additional benefit of minimizing dissipation. Since I first
published my recommendations about three years ago, I've received
MANY reports from guys who have built and installed chokes like this
and experienced considerable noise reduction. Many of them are
serious contesters like me, who spend entire weekends on the air
trying to pull weak signals out of the noise.
Last winter, I gave an in-person tutorial for several engineers from
the CIA. A few months ago, the lead engineer from that group sent me
a DOD engineering report from 1966 that assumed the 5K ohm value as a
target, came up with the same parallel equivalent circuit that I did,
and came up with a design with three chokes in series that provided
5K from 2 MHz to 30 MHz. The ancients have again stolen our
inventions.
I did not conceive of 5K as the target -- W1HIS did, and published
about 2006 saying so. I read his paper and agreed that he was right.
After reading it, I added something like 3K to an antenna near my
house that already had a conventional 500 ohm choke, and was picking
up QRN from some noisy power supplies. It reduced the noise
significantly.
>Has anybody actually toggled between 2 x identical yagi's
Not an easy test setup. :)
>This 31 material.. with it's mu=1500 looks like something between
>43 material.[mu=800] and 85 material. [mu=2000]
Low frequency mu is not the point. What matters for suppression (and
choking) is RESISTIVE IMPEDANCE, coupled from the core. The #31
material is superior because it exhibits both dimensional resonance
and circuit resonance, broadening the impedance curve.
>will they handle 2.5 to 3 kw...with high SWR ??? After line loss,
>I want to see at least 2250 w at the ant feed point.
I'm an honest and honorable ham who plays by the rules. I have zero
respect for those who don't. That said, the big coaxial chokes I've
described have considerable headroom for dissipation above 1.5kW ham
power limits.
SWR is NOT a factor in performance or dissipation of common mode
chokes. The only thing that matters is the common mode current, and a
sufficiently high value of choking Z reduces that to near zero.
Common mode voltage (and thus current) is the result of IMBALANCE in
the antenna, NOT VSWR. W8JI has written about this as well.
>A simple clamp on ammeter will tell you just abt all you want to
>know.
Nope. It tells you about that current AT THE POINT WHERE YOU INSERT
IT. Common mode current on the feedline varies with the position on
the line, just as with any other antenna. To get meaningful data, you
would need to insert it at the feedpoint of a wire dipole (because
that's where the noise is being coupled) that's 100 ft in the air,
and you would need a great pair of binoculars to read it. And, of
course, if the choke was working well, you would be at the bottom of
the meter scale.
> The Z measurements, I believe were done on a HP vector Z meter,
High impedance measurement on a traditional vector analyzer can be
tricky, as noted in my section on measurement. :) Reflection-based
measurements yield considerable error, because the unknown Z is too
far from the 50 ohm circuit Z. HP addresses this in one of their app
notes on Z measurement. That's the failing of the AIM unit.
73,
Jim Brown K9YC
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