I remembered this thread and when I was looking at the slashdot article
on RF shielding paint the other day, remembered that I had problems with
my K2 and a flourescent "Ott" light, presumably from its transformer. I
ordered some "Metglas" tape from these folks, and tried a few layers of
it around the transformer, and near the K2, but there was no effect. I
opened up the bottom of the K2 and noticed the greatest disturbance from
the lamp transformer was around L31. That area also seemed to be the
most sensitive to detuning when I put on and took off the cover (and my
hand). So I put a few layers of the tape (insulated inside a poly) on
top of L31 and there seemed to be a slight effect, but not enough to
make me want to try to put it there. I may open the top and try L30.
In better news, though, I found that moving the light from the left of
the K2 to the right eliminated the modulation, at least on RX.
Leigh.
A Walker wrote:
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." <[EMAIL PROTECTED]>
To: A Walker <[EMAIL PROTECTED]>
CC: Elecraft@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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