[mythtv-users] Antennas

[Stephen P. Villano]

On 11/15/13 9:41 PM, Joseph Fry wrote:
>>>> 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!
> _______________________________________________
>
Again, worth it.
Yes, one has to generalize or not go into detailed facts at times when
explaining to the lay person. Electronics *is* a part of physics. :)