[mythtv-users] Antennas

Daryl McDonald darylangela at gmail.com
Sun Nov 17 00:01:47 UTC 2013


On Sat, Nov 16, 2013 at 4:56 PM, Stephen P. Villano
<stephen.p.villano at gmail.com> wrote:
>
> 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. :)
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Thanks for all the comments. I do have some electrical experience, and
basic understanding. the house is only 9 years old, so the water pipe
is probably not connected to the electrical ground, so I will get a
ground block and fish a copper lead to the panel.


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