[Elecraft] Hum Coupling

[A Walker]

Hi Michael,

Thanks for your comments. I think you are on to something.

The only hope for lower magnetic field susceptibility for the K2 would be if 
either the VFO circuit were changed so as not to use an inductor for 
adjusting the quiescent resonance point of the VFO, or good magnetic 
shielding were used. One possibility would be to use a gapped pot core with 
slug tuning for L30, but it might be difficult to get sufficient adjustment 
range. Or, perhaps one could use some kind of shield for L30 alone. The 
standard sort of thing for low-level or precise RF circuits is to enclose 
them all in steel shielding, with steel partitions between circuit sections 
or stages (it's called "egg crating"). It's something Elecraft could keep in 
mind for a "K3."

If I had a K2 in front of me, I think I would try to put some kind of 
shielding around L30. Here is the web site of one vendor of shielding 
materials:

http://www.lessemf.com/mag-shld.html

By the way, I would guess that the Q of that Toko T1005 coil is determined 
by not only DC resistive loss in the wire, but also skin effect loss and 
core loss (and circulating currents from distributed capacitance, etc., 
etc.). I suppose we will never know for sure, as RF work at some point gets 
very empirical -- we just have to try it out, and if it works we're on to 
solving the next problem.

Best of luck,
73, Allen Walker

>From: "Masleid, Michael A." <Michael.Masleid at ispat.com>
>To: A Walker <walker490 at hotmail.com>
>CC: Elecraft at mailman.qth.net
>Subject: RE: [Elecraft] Hum Coupling
>Date: Wed, 03 Nov 2004 13:01:47 -0600
>
>Hello Allen,
>
> >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.
>
>The magnetic field is picked up by L30.
>
> >To work best, the magnetic shield should completely surround the 
>susceptible parts.
>
>Putting the shield on only one side seems to make things worse.
>
> >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!]
>
>Of course, keep it away from the latching relays.
>
>I used a tape head demagnetizer.  Placing the probe near the base of L30 
>causes
>a vast huge amount of FM.  Don't try this unless you cut the current way 
>down.
>
>Looking on the scope, FM modulation is at 60 Hz, not 120 Hz, so we're not 
>modulating
>the permeability, we seem to be inducing a 60 Hz voltage onto L30/T5 which 
>is
>modulating the varacters.
>
>What I don't understand is this:  I figure L30 has perhaps 14 turns, and 
>2.3 ohms
>winding resistance.  I figure that the effective aperture is 1 cm squared.  
>I figure
>that the winding resistance of T5 is 0.05 ohms, so a 1 gauss vertical 60 Hz 
>field will
>induce 37 micro volts on T5.  That should cause 0.3 Hz FM modulation on 80 
>meters.
>No one should notice that?  BTW, I calculated 2.3 ohms from the Q, 
>sensitivity at
>T5 goes way up if L30 has less resistance.
>
>I know that the cup and core used in L30 will focus any external field 
>through the
>winding, but I don't know how to calculate how much.
>
>It would be nice if someone could put an L30 (TOKO T1005Z 4.7 uH) into a 
>Helholtz
>coil and get the numbers on it (uV/gauss at 60Hz, winding resistance).
>
>73, Michael

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