[mythtv-users] Antennas

Joseph Fry joe at thefrys.com
Sat Nov 16 02:41:53 UTC 2013


>>> And bus bar ground systems.
>>
>> Talk to hams about their bus bar ground systems.....

>
> OK, so if the lamp wire acts as an antenna why wouldn't a ground wire
> also act as an antenna?

It does, however the tuner is not reading that wire, so it doesn't
matter.  Essentially, because the ground wire is connected to earth
ground, any rf signal will be quickly conducted into the ground...
essentially, the purpose of the ground wire is to create a short
circut to ground.

Whenever you measure a voltage, it must be measured across some
resistance.... if its shorted, then the resistance is 0 (or near
enough) and therefore there is no voltage.  For example.. grab a
battery meter and measure the voltage across a battery... then connect
a wire from the + to the - on the battery... the voltage on the meter
drops to 0 because there is no resistance on that wire, and therefore
the meter sees no current since all the current is running on the path
of least resistance.

An typical antenna has two elements (some have many but each element
is tuned to a different wavelength).  When the radio wave hits those
elements, one registers a + while the other -... as the wave continues
past, they switch, then switch again, and again, and again.... all at
the frequency of the radio wave (thousands of times per second).  The
greater the difference between the + & -, the stronger the signal is.

One element is connected to one wire, the other element to the other
wire.... which is why you have 2 wires coming from your antenna.  If
you were to measure the voltage at any given instant you would see
your battery meter register something since there is a difference of
potential between the two elements of the antenna and current wants to
flow through your meter.  In reality, because they are flip flopping
back and forth, you need an oscilloscope to actually measure the
voltages because your battery meter would bounce back and forth so
fast that it would average out to 0 and you wouldn't actually see the
meter move.

Your tuner is actually reading the difference in potential between the
two elements on your antenna... and data can be encoded within those
differences through a process called modulation (FM, AM, QAM, etc)...
it's really pretty cool.

The problem with using speaker wire, or any unshielded wire, is that
it will also act as an antenna in addition to carrying your signal.
But because it is typically not in perfect line with your antenna as
it relates to the broadcasting tower, its going to receive the same
radio wave a tiny bit later, or sooner, than the actual antenna....
this will create an second, weaker, fluxuation in the wire at the same
frequency as the desired channel, but just slightly out of phase with
the signal from the antenna.  When your tuner receives the signal it
will be distorted.

Back in the analog days, using unshielded wire wasn't so bad... you
would simply get a ghost of the image an inch or so to the right or
left... often out of phase wave was so much weaker that the the ghost
was so much lighter you wouldn't notice it much, but it was there.
However digital isn't as forgiving... those ghosts can sometimes be
detected as part of the actual signal and thus instead of 1010 you
might get 1111.... and the TV picture breaks up.

An additional issue with speaker wire comes from the fact that when
you run current through a wire it creates a magnetic field that can
induce a current in another wire.  if one wire is + and the other -,
the currents are running 180 degrees of one another... meaning that
they are pushing against the each other.  If the wire were long
enough, they would cancel eachother out and you would have no signal
strength left at the other end.  If you look at typical 300 Ohm twin
lead wire, they are spaced out about 1/4 inch... they make twin lead
with wider gaps that have a higher "Ohm" value.  Ohms are a measure of
imepedance, which, for the purpose of this discussion, can be thought
of as resistance to induced currents.  Using a speaker wire or lamp
wire, your running the two wires so close together that they have less
impedance, about 200 ohms for typical lamp cord.... therefore you get
some extra signal loss as a result.

Your probably thinking you can just separate the wires and run them
further apart, maybe an inch or two?  Well the problem with that is
that the further apart they are, the more likely that some sort of
external interference (power lines, microwave, etc) could induce a
current in one wire, while not inducing the same current in the other
wire.  Remember the tuner measures the difference between the two
wires, if one wire has an induced current in it, you want it induced
in both equally so that the difference between them remains the same.
So you want them as close together as possible, but not so close that
they fight each other.

This is why they created coaxial wire.  There is a center conductor
surrounded by an outer conductor.  The outer conductor is grounded so
that any interference it receives is shorted to the earth ground.  The
center conductor is then protected from interference by the grounded
shield.  The actually makes the signal weaker, as one element of the
antenna is essentially fixed to ground and thus the potential
difference between the two doesn't fluxuate as much because your
basically ignoring half the received signal... however the shielding
from interference makes for a MUCH cleaner signal.  Thus a tuner can
be much more sensitive, as it doesn't need to worry about being
overloaded by the low level noise that a twin lead carries.

Wow... another novel.

Anyone reading this should know that a lot of what I said here is
generalized and thus not entirely factual.  The topic is far more
complicated than can easily be communicated to a lay person, so I took
some liberties to make sure they cold understand the principals.  If
you feel you can offer a better explanation, feel free!


More information about the mythtv-users mailing list