[Elecraft] Hum Coupling

[A Walker]

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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