From: bcoleman@hayes.com (Bill Coleman)
Newsgroups: rec.radio.amateur.misc
Subject: Re: CRT radiation into ham rcvrs
Date: 23 Jul 92 19:06:54 GMT
Organization: Hayes Microcomputer Products, Norcross, GA

In article <1992Jul22.124413.27165@ke4zv.uucp>, gary@ke4zv.uucp (Gary Coffman) writes:
> In article <5798.2a6b0ad5@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>In article <9g=m-lm.nagle@netcom.com>, nagle@netcom.com (John Nagle) writes:
>>>        The Mac 13" color monitor emits 60Hz magnetic fields strongly
>>> enough to interfere with a telephone handset at 3'.  

BTW, I've received confirmation from employees at Apple that this report
of emissions of strong magnetic fields interfering with telephone handsets
is entirely BOGUS.

>>VLF emissions aren't likely to cause trouble for Hams. What usually causes
>>trouble is RF emissions from the microprocessor clocks and other digital
>>signals radiating to the outside.
> 
> While clocks and other digital hash are a problem with computers, the
> primary culprits are usually the very strong VLF deflection *pulses* that
> the monitor uses to scan the screen. These several *watt* pulses radiate
> harmonics across a broad spectrum. 

Yes, in poorly sheilded monitors, that's true. However, it appears that
the Apple 13" monitors don't suffer from this problem.

> Thus the new low field monitors *do*
> make a marked difference in interference levels since the way they reduce
> the fields is to increase the overall effectiveness of the shielding.

Agreed.

>>Fortunately, Macs are best-sheilded and filtered computers available. Perfect
>>for the ham radio user. 
> 
> The etch-a-sketch, er Mac, is fairly well shielded, but is not anywhere as
> effectively shielded as a Sun or AT&T 386WGS. The latter has a double metal
> case and finger stock at all the joins. The Sun 386i is also double shielded.

Different macs use different sheilding. Various versions of the Macintosh II
family use double sheilding as well. My Mac IIx at work is fully metal
sheilding, with finger stock in the top, plus a conductive/RFI metallized 
coating on the inside of the casting. 

> I keep my scanner sitting on the Sun with no hint of interference. The Mac
> depends on coated plastic and a couple of flimsy shield plates. 

Regardless of the materials, the models that use this technique are quite
RF-tight.

> It *is* a
> tight box though, as many people who have had them overheat will attest.
> The aftermarket in add on fans is doing well. 

Gary, this type of comment really shows how dated you are. Apple hasn't
sold any desktop Macs without fans in almost two years. (The powerbooks
don't have fans, but they don't need them) The Mac Plus was the last desktop
unit that was convection cooled and it hasn't sold since October 90. 
Although the compact format has always be a hot seller for Apple, most
sales previous to October 90 were in the form of the fan-cooled Mac SE.

> Mac IIs are better in this
> respect, but their separate monitors do tend to radiate a lot of hash. 

In the original referenced posting, it is exactly the monitor we are 
speaking of. My experience with this monitor shows that it radiates
little or no "hash." It is just as well sheilded and filtered as the
rest of the Macintosh family. 

Indeed, the whole lot is FCC Class B certified. I doubt you could say
the same for Suns or ATT workstations.

-- 
Bill Coleman, AA4LR                ! CIS: 76067,2327    AppleLink: D1958
Principal Software Engineer        ! Packet Radio: AA4LR @ W4QO
Hayes Microcomputer Products, Inc. ! UUCP: uunet!hayes!bcoleman
POB 105203 Atlanta, GA 30348 USA   ! Internet: bcoleman%hayes@uunet.uu.net
Disclaimer: "My employer doesn't pay me to have opinions."
Quote: "The same light shines on vineyards that makes deserts." -Steve Hackett.

Newsgroups: rec.radio.amateur.misc