[SATLUG] OT: Built my own DTV antenna

Geoff geofff at w5omr.shacknet.nu
Sun Apr 26 12:30:09 CDT 2009


Jeremy Mann wrote:
> Can somebody explain in laymans terms "extraneous harmonics"? I'm just
> curious because I'm NOT using an amplified antenna so I shouldn't be
> affected by this problem.

>From a technical standpoint (and calling on my RF background from my
close associations from my fathers  Broadcast engineering career, and
our ham radio careers) "extraneous harmonics" directly relates to
"inferior product design".

I have noticed that my old Microwave oven, when operated in the kitchen
and I used my laptop with wifi on the kitchen table, would kill my wifi
connection.  I suspect a leaky microwave cabinet.  That doesn't excuse
the manufacturer of the wifi device in the laptop of not better
shielding and preventing frequencies other than 2.4 ~ 5GHz from getting
into the wifi signal path.

The fact that your 'blips' are affecting both your wifi -and- your DTV
signals suggest that something -very local- is interfering with the
entire RF spectrum, or at least the part above about 400MHz. 

Your neighbors microwave, a nearby 800MHz police transmitter perhaps,
microwave relay points from downtown studios to a near-by radio
broadcast station (KRDY aka Radio Disney comes to mind) or the broadcast
station itself might not be spectrally pure.  It could even possibly be
a CB or Ham Radio transmitter that's close enough to effect. 

Bottom line is, there's all kinds of RF signals out there and the people
who build things like powered tv antennas (with an RF pre-amp built-in)
or wifi antennas in laptop or wireless routers, don't necessarily pay
close enough attention in the design to use their devices -only- on the
frequencies they were designed for.

As for Microwaves, Wikipedia coughs up this description, which is
germane to the discussion:

====================================
http://en.wikipedia.org/wiki/Microwave_oven
"A microwave oven works by passing non-ionizing microwave radiation,
usually at a frequency of *2.45 GHz* (a wavelength of 12.24 centimetres
(4.82 in), through the food. Microwave radiation is between common radio
and infrared frequencies. Water, fat, and other substances in the food
absorb energy from the microwaves in a process called dielectric
heating. Many molecules (such as those of water) are electric dipoles,
meaning that they have a positive charge at one end and a negative
charge at the other, and therefore rotate as they try to align
themselves with the alternating electric field of the microwaves. This
molecular movement creates heat as the rotating molecules hit other
molecules and put them into motion.

Microwave heating is more efficient on liquid water than on fats and
sugars (which have a smaller molecular dipole moment), and also more
efficient than on frozen water (where the molecules are not free to
rotate).[5] Microwave heating is sometimes explained as a resonance of
water molecules, but this is incorrect: such resonance only occurs in
water vapor at much higher frequencies, at about 20 GHz.  Moreover,
large industrial/commercial microwave ovens operating at the common
large industrial-oven microwave heating frequency of 915 MHz (0.915
GHz), also heat water and food perfectly well.
=====================================

This is not to say that someone operating a Microwave oven is your
culprit, but just one of -many- possibilities.

Even still, if you had a 'sniffer' of RF that was able to receive
signals at/around Microwave frequencies and could prove a correlation
between your wifi going down, and your TV acting screwy, it would
probably you -and- your neighbor to help find the offending device (if
that's the cause) and have them replace the faulty/defective/leaky
microwave oven and possibly save them from some form of cancer from
being exposed so closely to RF radiation at those frequencies.


73 = Best Regards,
-Geoff/W5OMR (/5 Baja Spring, TX)



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