From: SFPhysics@aol.com
Date: Mon Jul 26 2004 - 03:11:36 PDT
From: SFPhysics@aol.com Message-ID: <bb.42d19fc3.2e3632d8@aol.com> Date: Mon, 26 Jul 2004 06:11:36 EDT Subject: Antenna Question...
>>I have one of those questions about common phenomena that you think you
understand, until...
Often when I walk past the TV (about 3 feet away from the antenna) the
picture disintegrates. Then when I move away it comes back to normal. We see
this also with the direction you point the antenna on a radio. What causes
this? Is this the result of the interaction of my electromagnetic field with
that of the antenna? Where does my emg field come from? (I've just sort of
assumed we all had one, but I have never discussed this). Why do different
people influence the antennas differently?
Thanks for any clairty.
>Steve Miller
<<
Hello Steve:
I am assuming you are talking about a monopole, bow tie, or "rabbit ears" and
the "loop" VHF (Very High Frequency) & UHF (Ultra High Frequency) antennas.
The human being has very little in the way of an electromagnetic field. In
order to measure the very low electric currents in the human body you need to
attach electrodes and have sensitive amplifiers. To date I have found no
articles stating that humans can radiate anything more than body heat in the
infrared region. The cheap antennas like the rabbit ears "dipole" and the UHF loop
are somewhat bidirectional and have a two lobe sensitivity pattern 180 degrees
apart. Your body is a conductor, mostly water with some salt in it, and any
conductor brought near a dipole antenna acts as a reflector or absorber of the
high frequency TV electromagnetic waves. If you stand between the dipole and
the source you block the signal. Standing behind the dipole your body can act
like a reflector to cause destructive interference of the waves when the
distances are multiples of the wavelengths. The human body near an antenna also
acts as one side of a capacitor thus effecting reception by causing the antenna
to become mismatched with the input circuits of the TV tuner.
To your question of why different people influence TV antennas differently,
they don't really. Humans are all about the same but what you are wearing is
likely quite different. A pocket full of change, a metal watch, a metal
necklace, zippers on clothes, fillings in your teeth, metal framed eyeglasses, or
any other metallic object on your person will alter your reflection pattern and
your capacitance with the antenna.
The simple antennas also suffer from frequency related problems. To be most
efficient the antenna needs to be adjusted for each channel to a distance of
about 0.5 wavelength tip to tip on the dipole ends. Thus the monopole and
dipoles for VHF do not do well for the lower channels because they aren't long
enough. Channel 2 which transmits a 6 MHz bandwidth signal starting at 54 MHz
would need an antenna a little over 2.5 meters (8 feet) long. That is why you
see some people add tinfoil extensions to the rabbit ears. By the time you get
to the highest transmitted signal, channel 69, you are at 801.25 MHz with a
wavelength around 36 cm. This is why in lesser signal areas people have been
known to find the odd positions of the antenna elements through much
experimentation and have them all memorized.
I hope that my commentary has been helpful.
Best wishes to all on the list,
Al Sefl
Who had trouble with his first rabbit ears antenna because the rabbit
wouldn't stay still...
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