I have been following the "Hum Coupling" thread, and would like to comment,
from the viewpoint of an engineer who has done a lot of interference
reduction.
My guess is that magnetic fields from power transformers are either 1) being
picked up by L30, or 2) that the magnetic fields are going through the cores
of L30 and/or T5, and modulating the effective permeabilities of these
cores. This will cause a modulation of inductance, and thus a modulation of
the VFO frequency.
There are two general solutions: First, one can make the cores less
susceptible to permeability modulation by using lower permeability cores
(although that would require more wire turns, and perhaps physically larger
inductors to have the same inductance/coupling. Second, one can use
shielding that is effective for low frequency magnetic fields, at the
source, at the VFO, or both. That means, of course, a ferromagnetic
material.
Equipment that uses aluminum case construction offers little to no
protection against magnetic fields below around 10MHz, and it isn't great
until you get above 30MHz.
Regarding shielding at the VFO, mu-metal is great stuff, but it is hard to
apply. If you bend or form it at all, you will have to anneal it to get back
its amazing high permeability and shielding effectiveness. Most people wind
up using mild steel. For a given thickness it isn't as effective as
mu-metal. However, it can be formed without the necessity of annealing. And,
you can simply use more, if you like. To work best, the magnetic shield
should completely surround the susceptible parts.
Sometimes, you don't need the absolute best solution to make things good
enough. For example, I have sometimes reduced interference enough by
wrapping a few turns of hypersil tape around the outside of a toroid (this
is the same thin metal tape that tape-wound transformer cores are made of).
Also, one could try somehow putting a sheet of mild steel between the
interference source and its target.
Regarding shielding the interference source (a large power transformer),
that is a lot harder. Mu-metal is ineffective because the strong magnetic
fields near a high power transformer will saturate the mu-metal (it
saturates easily), rendering it useless. Again, our friend, mild steel, can
come to the rescue. A steel enclosure helps. For an E-I core transformer,
steel end bells help. Toroidal power transformers have much lower external
fields, but they are not zero. Enclosing the transformer in a steel box can
help, provided that the transformer can run cool enough. [Whatever you do,
never run a bolt through the core of a toroidal transformer if there is any
possibility that the two ends can be connected together (such as by both
touching the same metal box). It would be a shorted turn!] Of course, if the
whole equipment using the big power transformer is in a steel box, that
helps.
Here's a possibility for experimenting with K2 susceptibility: Try using one
of those bulk tape erasers near it. They put out huge 60Hz fields. [Just
don't get it near your credit/bank cards!]
Since I don't have a K2, I can't try this myself, but I would be happy to
correspond with anyone trying to solve these problems. I am considering
getting a K2, and joined this reflector to learn more about it.
73,
Allen Walker
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