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Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff

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Is sci.physics now 100 % clutter ?


        any real physics at all being posted ?




At present, FCC restrictions require the use of an antenna with Part 15 
transmitters to be either 10 feet or less, or limit field strength to 
2400 / f in kHz. This applies in the the top of the AM broadcast band 
and certain other frequencies. Consult current FCC rules. However, as 
long as you are using 100 milliwatts and a 10 foot antenna, there is no 
specific limit on range. Typical antenna efficiencies at 1.6 MHz are 
about 0.5 to 1 percent. This limits effective radiated power to 1 
milliwatt or less. This sounds discouraging, but it is not really bad. 
If an antenna radiating 1 milliwatt is placed at the center of a sphere 
of 1 mile in diameter, all this power is still present at the surface, 
less any losses inside the sphere (very small in most cases can be 
considered zero as in free space). The following is a mathematical 
demonstration of this but you can skip it and take our word for it 
should you not care for the details.

The surface area of a 1 km (1000 meters) radius sphere is 4 x pi x r 
squared, where pi = 3.14159 and r squared = 1,000,000

Surface Area = 12.56 x 10 exp 6 (12.56 million square meters)

Dividing 1 milliwatt (1/1000 of a watt) by this figure, we get the power 
per square meter as (79 x 10 exp -12 watts)

The signal in volts per meter can be calculated by realizing that free 
space has an impedance of 377 ohms

The signal strength in volts/meter is equal to the square root of the 
power/sq meter x the impedance in ohms

multiplying power/sq meter by 377 ohms and taking the square root we get 
172.5 microvolts/meter at 1 kM (1000 meters or 0.62 miles)

At 1 mile this would be 107.15 microvolts/meter (uv/meter or uv/m)

Now, 107 uV/meter is a fairly good signal level. As signal levels of 50 
uV/m are still useable a range of over 2 miles should be obtained with a 
properly tuned antenna and 100 milliwatts. A decent AM receiver with a 
ferrite loop antenna can easily pick this up. A good AM receiver with a 
sensitivity of 10 uV per meter might hear this signal at several miles. 
However, ambient noise and interference will be the limiting factor. 
Using similar antennas (8 foot whip with loading coil), Amateur radio 
operators have operated transmitters from automobiles on the nearby (in 
frequency) 160 meter ham band (1800-2000 kHz) and with 10 watts, two way 
contacts of 25 to 50 miles are possible. This would correspond to 2.5 to 
5 miles range with 100 milliwatts. Typical AM receivers equipped with 
the usual ferrite rod or loop antennas have sensitivities ranging from 5 
uV/meter for excellent receivers with large loop antennas, to as poor as 
2000 uV/meter for cheap small AM portable sets. 50 to 300uV / meter is 
typical for average AM BC receivers. This depends on antenna size, 
receiver gain, and bandwidth. Note that this figure includes the loop 
antenna and is not the same as that would be measured at the 50 to 600 
ohm input terminals of the receiver (if so equipped). In this case, in 
order to get usable radiation, the antenna must take as much power from 
the transmitter as possible and radiate it with maximum efficiency. This 
requires a reasonable ground system and an antenna that is properly 
matched to the transmitter. A 10 foot vertical or whip antenna has an 
impedance of about 3000 ohms capacitive reactance in series with about 
0.1 ohm radiation resistance. The trick is to get as much current 
flowing in this 0.1 ohm resistance as possible. For 1 milliwatt radiated 
power this would be 100 milliamperes (0.1 ampere). To force this much 
current through the impedance of the antenna, an RF level of 300 volts 
is needed. This can be done by using a series inductance to cancel out 
the capacitive reactance of the antenna. A simple indoor 3 foot whip 
plugged into the transmitter, with no matching network, would look like 
about 0.007 ohm in series with 8000 ohms capacitive reactance. About 
3000 volts of RF signal would be needed to have this antenna radiate 1 
milliwatt. Since 100 milliwatts. A 100 milliwatt transmitter would 
supply about 2.3 volts of RF into 50 ohms. Open circuit or the level 
delivered into this antenna would be around 4.5 volts. The radiated 
power would be around 2 nanowatts. This would produce a very weak signal 
barely detectable at more than a few feet from the transmitter.

These high RF voltages appear on the antenna as a result of using a 
loading coil to get a series resonant circuit together with the self 
capacitance of the antenna. This capacitance depends on the antenna 
dimensions and will be around 30 pf for a 10 foot vertical, depending on 
its configuration and diameter. This capacitance can be calculated from 
electromagnetic theory or inferred from an antenna analysis program such 
as ELNEC. For frequencies in the 1.6 MHz region, around 350 to 400 uHy 
will be needed. At 530 kHz around 3000 uHy will be needed.

This inductance can be reduced by using a "capacity hat" on the top of 
the antenna. This is made up of 4 or 6 lengths of wire (12 inches) 
arranged horizontally like the spokes of a wheel, with one end connected 
to the top of the antenna (see fig). The calculated capacitance of this 
short vertical antenna is 36.3 pF and is fairly constant over the entire 
AM broadcast band. This allows a coil of 220 to 2400 microhenries to 
make this antenna resonant from 1700 to 530 kHz. The exact inductance 
will be affected by antenna location, height above ground, length of 
ground lead and size of ground plane, proximity to other objects, and 
other environmental factors. Therefore it is best to start off with a 
coil of somewhat higher inductance than actually needed, and adjust the 
number of turns experimentally. Best radiation efficiency is at the 
higher frequencies.

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Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Serg Io <invliad@invalid.com> - 2017-02-12 21:12 -0600
  Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff noTthaTguY <abu.kuanysh05@gmail.com> - 2017-02-12 19:31 -0800
  Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff The Starmaker <starmaker@ix.netcom.com> - 2017-02-13 09:46 -0800
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-14 07:32 -0800
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff noTthaTguY <abu.kuanysh05@gmail.com> - 2017-02-14 10:19 -0800
  Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff moroney@world.std.spaamtrap.com (Michael Moroney) - 2017-02-13 22:07 +0000
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff The Starmaker <starmaker@ix.netcom.com> - 2017-02-13 18:45 -0800
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff moroney@world.std.spaamtrap.com (Michael Moroney) - 2017-02-14 12:49 +0000
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-14 07:38 -0800
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Poutnik <poutnik4nntp@gmail.com> - 2017-02-14 06:27 +0100
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff john <johnsefton288@gmail.com> - 2017-02-14 02:31 -0800
        Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Serg Io <invliad@invalid.com> - 2017-02-14 07:57 -0600
          Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff john <johnsefton288@gmail.com> - 2017-02-14 07:28 -0800
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Mahipal <mahipal7638@gmail.com> - 2017-02-14 05:23 -0800
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Serg Io <invliad@invalid.com> - 2017-02-14 08:22 -0600
        Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff john <johnsefton288@gmail.com> - 2017-02-14 07:18 -0800
          Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Serg Io <invliad@invalid.com> - 2017-02-14 09:49 -0600
            Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-14 08:04 -0800
        Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Mahipal <mahipal7638@gmail.com> - 2017-02-14 07:29 -0800
        Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-14 07:41 -0800
          Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff edprochak@gmail.com - 2017-02-14 11:48 -0800
            Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-15 09:03 -0800
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff benj <benj@nobody.net> - 2017-02-14 18:52 -0500
        Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff Mahipal <mahipal7638@gmail.com> - 2017-02-14 21:14 -0800
    Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff James McGinn <jimmcginn9@gmail.com> - 2017-02-14 07:36 -0800
      Re: Is sci.physics now 100 % scrap ? and AM transmitter antenna stuff noTthaTguY <abu.kuanysh05@gmail.com> - 2017-02-14 13:26 -0800
    Michael Moroney can't/won't read me properly. Jeff-Relf.Me <@.> - 2017-02-14 18:18 -0800
      The inmates always hate their description moroney@world.std.spaamtrap.com (Michael Moroney) - 2017-02-15 04:28 +0000

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