Re: TEKTRONIX 491 measuring wifi signals

Chuck Harris

What works as shielding at 30MHz, looks like a sieve at 2.4GHz.

The principle leak is around the door... traditionally.

The ovens use what is called a choke seal around the door, which
physically is a dead ended slot around the door. Both the gap
between the door and the door frame, and the slot in the door
form a high Q, inductance to the 2450MHz signal, and stop most
of it from leaking into the room.

"Most" is still about a watt.

I just noticed that my microwave is one labeled Montgomery Wards,
a retailer that has been gone for two decades. They sent it to
my in-laws from their repair facility, where it was tagged both
unrepairable and repaired. It worked fine. They put the wrong
tag on it. Still going strong!

The Tektronix 491's mixer is pretty old fashioned, using discrete
glass diodes. But, I would still be careful when using it around
a 1500W RF source that leaks several watts.

I remember when my MIL tested a brand new microwave oven leakage
tester by putting it in the oven... God I miss her!

-Chuck Harris

Miguel Work wrote:

My Pansonic microwave oven is working since late 80s, is one of the first micro controlled, the keyboard has no signs of wear 30 years after, has survaived my four brothers family!
Using it every day and still working. It has a double shield.

It costs about 800$


-----Mensaje original-----
De: [] En nombre de Chuck Harris
Enviado el: martes, 24 de marzo de 2020 13:44
Asunto: Re: [TekScopes] TEKTRONIX 491 measuring wifi signals

When the first commercial microwave ovens were made, they had simple mechanical timers, and no "heat" settings. The next generation had simple mechanical timers, and thermal timers similar to turn signal "clickers" to adjust the on vs off time. Every enhancement since has been simply refinements of the same basic formula: Turning mechanical timers into electronic timers, into microprocessor timers.
Turning mechanical bells into electronic bells. Turning linear transformers into switching power supplies...

But the basic on/off, "bang-bang" method of controlling the heat has remained intact: how many seconds on, out of 10 seconds, determines the "heat" from the oven.

Some researchers wanted to take the microwave oven into the realm of laboratory heating, and found the "bang-bang" style of heat adjustment way too brutal. For instance, if you wanted to heat a saline solution in a watch glass, it would explode around the edges where the water was so very thin, even though the "heat" was on low... even one second at full power followed by 9 seconds of being off was too brutal.

So, a colleague of mine developed a precision microwave that took the duty cycle modulation of the magnetron from seconds on out of 10 seconds, to power line cycles on out of 10 power line cycles. And, even balanced the distribution of on cycles so that if you were at 50%, it was every other cycle of the 60Hz power, (that may have been my suggestion, I don't remember anymore) ... Saline in a watch glass was his standard test.

Heading back to 491's, and wifi signals:

Modern microwave ovens have replaced the heavy, and copper laden, power transformers with switching power supplies...
which is why they are now available for less than $100.

There is a big cost, however. High voltage capacitors are still expensive, so the new supplies aren't at all well filtered, and end up modulating the 2450MHz nominal magnetron output at the switching supply rate, and the switching supply rate is not at all stabilized, so it changes with load. This wreaks havoc with wifi routers which occupy the same ISM band as do the ovens.... Only the ovens leak way more power than the routers put out.

A friend once asked me why his "smart" house, and his cordless phones, stopped working every so often... I asked him if he had a microwave oven. He said yes. I asked him to notice if his wife was using the microwave oven when his "smart"
house stopped being so "smart".

You should be able to see this wandering RF noise fest on your
491 whenever the microwave oven is running.

-Chuck Harris
on off timer that im Ford wrote:
I stand corrected (not all that unusual).  Thanks, Chuck.  My guess
would be that microwave oven manufacturers either don't know about or
don't want to deal with the subtleties of magnetron anode and filament
voltages and currents.JimSent from my Verizon, Samsung Galaxy

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