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Re: [RFI] Microwave RFI - radiated (?)

To: "Donald Kerns" <dkerns@cruzio.com>, <dgsvetan@rockwellcollins.com>
Subject: Re: [RFI] Microwave RFI - radiated (?)
From: "Jim P" <jvpoll@dallas.net>
Date: Thu, 8 May 2008 13:28:04 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>

My old analog-timer "MINUTEMASTER" by Litton (yes,
Litton; the manufacturer of magnetrons, etc) produces a lot
of RFI that affects 80 Meters for instance, and it is a straight
transformer design (no high frequency switching transistors
in the DC - DC power source; there is a schematic inside
the chassis as a matter of fact!)

It is a transformer design, and the 'noise' produced is kind
of a rythmic on-and-off pulsing at 1/2 Hz  or so ... never
entered my mind as to why (internal tube arcing or
whatever) but next 'service' opportunity (e.g. when the
analog timer contacts stick again requiring a sharp rap
from a wrench to the body of the timer) I will do some
'testing'. As a side note, I don't notice any of the usual
signs that arcing is taken place (smells; no ozone, no odors
of any kind really.)

History tidbit; Litton beginnings w/Charles Litton and WWII:


Thanks for the info, BTW; I always took the noise to the
be the norm, based on this 'case sample of one' (always
a dangerous think to do: base wide-ranging assumptions on
case sample of unit!)

73,  JimP  // WB5WPA //

----- Original Message -----
From: <dgsvetan@rockwellcollins.com>
To: "Donald Kerns" <dkerns@cruzio.com>
Cc: <rfi@contesting.com>
Sent: Thursday, May 08, 2008 12:09 PM
Subject: Re: [RFI] Microwave RFI - radiated (?)

> Don,
> I am trying to figure out WHY your microwave produces RFI.  Unless there
> have been some major changes in microwave design very recently, the
> commodity status of this device has relegated it to a very cost-conscious
> design using mature technology.  For the most part, microwaves have used
> conventional iron core transformers (with minimal amounts of iron and
> copper, to be sure) for the HV supply.  The other prime components are
> usually the HV rectifier, perhaps a low value HV filter cap, and the
> magnetron tube itself.  (In some brands of oven, the recitifer and
> magnetron are integrated into one assembly.)
> Now, when you mention the rather strong RFI, two possible ideas come to
> mind:  either your microwave is using a high power, high voltage switching
> power supply (which tends to be a costly device for several reasons) or
> you have a malfunction in the HV circuit (arcing) or controller.  A high
> power switching power supply that has little or no RFI suppression could
> and would be a terrible nosie maker.  (From previous postings about
> variable speed laundry machines, we know that appliances seem to be exempt
> from FCC emissions requirements.)  Can you supply the make and model of
> your microwave?  It would be very interesting to know the nature of its HV
> power supply.  Of course, the low voltage control electronics could have a
> small switcher powering them, and perhaps that's the problem.
> Finally, there could be arcing in the HV unit or the magnetron itself. You
> may wish to do a little RFI sniffing as a precautionary measure.  I've
> seldom heard of microwave ovens causing interference except in the region
> of 2.45 GHz, which is their operating RF frequency range.
> 73, Dale

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