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 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
