David Lesher wrote:
> B J Foster
yahoo.com> writes:
> >>>> The VOR band is 108MHz to 118Mhz (VHF). The lowest mobile band (UHF) is
>>>> 900Mhz. If the VOR system has such a poor filter then it is going to be
>>>> affected by *every* *other* system between VOR and GSM.
>>> You don't seem to understand RF very well. Lots of weird things happen,
>>> when you mix multiple emitters and Murphy.
>
>> Sure. Such as?
>
> Well, when you mix 2 signals, you get 4: the two originals, the sum and
> the difference. So, for example, mix a 800 and 915 mhz signal, and guess
> what, you get 800, 915, 1715 and... 115 Mhz crap. ("Hetrodyning")
Okay. I accept that it can happen in theory. The VOR receiver should be
rejecting out-of-band frequencies, so apart from poorly shielded
pre-1970s VOR equipment the only way it can happen is if a static
non-linear resonating object exists somewhere in the path, e.g. a metal
object with a shape such that half wave lengths occur in the 800-915
region, i.e. approx. 15-18cm dimensions. I concede that the existence of
metal objects of these rough dimensions in a passenger plane,
reflecting, refracting and resonating in unpredicatable ways is a
distinct possibility.
>
>>> First of all, GSM phones emit subcarriers, specifically the harmonics
>>> of its switching speed. They tends to get into LOTS of things such as
>>> hospital telemetry.
>
>> Harmonics are at higher frequencies, e.g. 800Mhz GSM has harmonics at
>> (say) 3x = 2.4GHz. Now this might interfere with 802.11 but it is even
>> *less* likely to interfere with VHF.
>
> No, the GSM *switching* frequency; TDMA, which GSM is a implementation
> of, time-slices between mobiles at a rate that's ~~in the KiloHertz range.
> That's what gets you the infamous "GSM buzz" that gets into many audio
> systems.
No. TDMA is baseband. It affects other mobiles only because they are
operating in the same band. If the cellphone isn't adequately shielded
however, then the baseband may leak out. This is in the category of "CPU
leakage" which I mentioned was more likely.
>
>> Now VHF harmonics might interfere with GSM, but the problem isn't the
>> "safety of the cellphone" is it?
>
> No, it's the mixing of everything: switching and RF harmonics, leakage
> from local oscillators, you name it... and the unpredictable results.
Okay, I accept that there can be poorly shielded devices and passive
resonators which act unpredicatably. Conversely, shielding from 800/915
MHz is easy with a 7-8cm gap (quarter wave) between panels.
>
>>> Second, the front end of a the VOR is 100 Mhz; but the IF's are often
>>> poorly shielded; put an emitter near by them and....
>
>> ...what?
>
> The receiver hetrodynes down that 108 Mhz to {often} 10,7 Mhz. Another
> 10.7 source nearby can sneak it and ...
>
True.
>
>>> Years back, a friend was in a FAA test aircraft, ISTM it was the old
>>> 727 they had, and while testing a new Mode-S transponder, all was
>>> well...until they came in to land, the gear cycled and the GPWS went off
>>> when the radar altimeter went bonkers. It had been tested BUT oops when
>>> the gear doors were open in transit, it reflected enough RF back that...
>
>> What were the frequencies of the altimeter and the transponder? TCAS?
>
> No idea now.
>
> The basic problem is an aircraft, as delivered, is a known set of
> issues. They've tested that COM1 does not FUBAR NAV2, and the
> satphone won't step on the FADEC's etc.
>
> Add a few thousand cell phones per day, untested since new & now in
> unknown condition, and you have a lot of unknowns...way too many.
Okay, accepted.
>
> Even testing is not the end-all. Back in the 1960's, an amateur radio
> friend was commuting to/from Cape Kennedy twice weekly. He somehow
> cajoled Eastern to test his 146 mhz radio for interference, and they
> OK'ed its use in writing... BUT on that one particular 727, no other.
>
