[mythtv-users] Antennas

Daryl McDonald darylangela at gmail.com
Sat Nov 16 04:03:42 UTC 2013


On Fri, Nov 15, 2013 at 9:41 PM, Joseph Fry <joe at thefrys.com> 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!
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I find myself in your debt once again Mr. Fry, and I appreciate you
taking the time to elaborate. Since I can argue with none of your
logic, I'm compelled to implement changes to my Neanderthal setup.
Fortunately, we have not experienced any Franklinesque weather since
my antenna went outside. My home will be much safer because of your
compassion.
To be clear then either of the two leads from the antenna can be
grounded or must it be the one going to the shielded side of the coax?
Further, my system ground is on the other side of the house, but there
is a water faucet near the antenna. If I clamp on there then the
system will be one?

Daryl


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