Radio frequency Video Home
Radio frequency, or RF, is a frequency or rate of oscillation within the
range of about 3 Hz and 30 GHz. This range corresponds to frequency of
alternating current electrical signals used to produce and detect radio waves.
Since most of this range is beyond the vibration rate that most mechanical
systems can respond to, RF usually refers to oscillations in electrical
circuits.
Special Properties of RF Electrical Signals
Electrical signals that oscillate at RF have special properties that arise out
of electromagnetic forces that do not affect direct current signals. One
property arises out of electromagnetic forces that drive the RF current to the
surface of conductors, known as the skin effect. Another property is its ability
to appear to flow through paths that contain insulating material, like the
dielectric insulator of a capacitor. The degree of effect of these properties
depend on the frequency of the signals.
The skin effect is the tendency of an alternating electric current (AC) to
distribute itself within a conductor so that the current density near the
surface of the conductor is greater than that at its core. That is, the electric
current tends to flow at the "skin" of the conductor.
Introduction
The skin effect causes the effective resistance of the conductor to increase
with the frequency of the current. The effect was first described in a paper by
Horace Lamb in 1883 for the case of spherical conductors, and was generalized to
conductors of any shape by Oliver Heaviside in 1885. The skin effect has
practical consequences in the design of radio-frequency and microwave circuits
and to some extent in AC electrical power transmission and distribution systems.
Also, it is of considerable importance when designing discharge tube circuits.
The current density J in an infinitely thick plane conductor decreases
exponentially with depth δ from the surface, as follows:
where d is a constant called the skin depth. This is defined as the depth below
the surface of the conductor at which the current density decays to 1/e (about
0.37) of the current density at the surface (JS). It can be calculated as
follows:
where
ρ = resistivity of conductor
ω = angular frequency of current = 2π × frequency
μ = absolute magnetic permeability of conductor , where is the permeability of
free space (4π×10−7 N/A2) and is the relative permeability of the conductor.
The resistance of a flat slab (much thicker than d) to alternating current is
exactly equal to the resistance of a plate of thickness d to direct current. For
long, cylindrical conductors such as wires, with diameter D large compared to d,
the resistance is approximately that of a hollow tube with wall thickness d
carrying direct current. That is, the AC resistance is approximately:
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D >> d.
A convenient formula (attributed to F.E. Terman) for the diameter DW of a wire
of circular cross-section whose resistance will increase by 10% at frequency f
is:
The increase in ac resistance described above is accurate only for an isolated
wire. For a wire close to other wires, e.g. in a cable or a coil, the ac
resistance is also affected by proximity effect, which often causes a much more
severe increase in ac resistance.
Effect on impedance of round wires
For isolated round wires with radius on the order of or smaller than , the
assumption of exponential decrease of with depth is no longer valid. In this
case, must be found by solving
If we transform variables from to , this equation has the form of a zeroth-order
Bessel equation. Using the boundary condition and considering that must be
finite at for a solid wire, the solution to this equation is
where is the zeroth order Bessel function of the first kind, and and are Kelvin
functions.
The total current in the wire may be found by integrating from 0 to . It may
more easily be found by relating it to the derivative of the electric field at
the surface of the wire via its magnetic field. Ampere's Law at the wire surface
gives an azimuthal magnetic field
Maxwell's Equations in cylindrical coordinates gives
where the electric field points in the direction of the current. Equating these
two functions at gives
where the prime on the in the numerator indicates a first derivative, and we
have used . The impedance in the wire is given by
where and are the resistance and inductance per unit length of the wire.
Plugging in for and gives
where the fundamental resistance and unitless scaled "radius" are given by
and
Mitigation
A type of cable called litz wire (from the German litzendraht, braided wire) is
used to mitigate the skin effect for frequencies of a few kilohertz to about one
megahertz. It consists of a number of insulated wire strands woven together in a
carefully designed pattern, so that the overall magnetic field acts equally on
all the wires and causes the total current to be distributed equally among them.
Litz wire is often used in the windings of high-frequency transformers, to
increase their efficiency by mitigating both skin effect and, more importantly,
proximity effect.
Large power transformers are wound with conductors of similar construction to
litz wire, but of larger cross-section.
High-voltage, high-current overhead power transmission lines often use aluminum
cable with a steel reinforcing core, where the higher resistivity of the steel
core is largely immaterial.
In other applications, solid conductors are replaced by tubes, which have the
same resistance at high frequencies but lighter weight.
Solid or tubular conductors may also be silver-plated providing a better
conductor (the best possible conductor excepting only superconductors) than
copper on the 'skin' of the conductor. Silver-plating is most effective at VHF
and microwave frequencies, because the very thin skin depth (conduction layer)
at those frequencies means that the silver plating can economically be applied
at thicknesses greater than the skin depth.
Examples
In copper, the skin depth at various frequencies is shown below.
frequency d
60 Hz 8.57 mm
10 kHz 0.66 mm
100 kHz 0.21 mm
1 MHz 66 µm
10 MHz 21 µm
In Engineering Electromagnetics, Hayt points out that in a power station a bus
bar for alternating current at 60 Hz with a radius larger than 1/3rd of an inch
(8 mm) is a waste of copper, and in practice bus bars for heavy AC current are
rarely more than 1/2 inch (12 mm) thick except for mechanical reasons. A
possible solution to this problem consists of using cables with multiple
insulated conductors. A thin film of silver deposited on glass is an excellent
conductor at microwave frequencies.
RF Video
RF Video was started by Rob Feinstein, along with his friends Doug Gentry and
Gabe Sapolsky, in the early 90s and began as an outlet for hand recorded "fan
cam" videos of wrestling events from smaller companies throughout the United
States. They would tour with Extreme Championship Wrestling (ECW), setting up
their own booth at events to sell their videos and other memorabilia and give
out "updates" listing other pieces for sale through their mail order business.
Eventually RF Video became large enough to open full time kiosks in shopping
malls stocked with some of the more popular videos and the option to order from
the catalog through the kiosk instead of mail, phone, or internet.RF Video now
has over 60 commercial outlets in the United States alone,and is pioneering a
new type of DVD shoot interview.The GF Shoot Series(GF standing for Gay-Fag)one
of the various nicknames given to Feinstein over the years that really stuck.The
GF Shoots will feature Rob interviewing a well known Gay Wrestler. Both will sit
naked on a purple leather couch sipping Robs favorite type of wine. Top
interviews for the GF Series include Ken Shamrock,Greg Valentine,Ian Rotten,and
Phineas Godwin.
Performer
While touring with ECW, Rob began appearing on ECW TV with the Blue World Order
as a parody of Syxx of the new World order. As part of the parody he carried a
hand held camcorder everywhere (as Syxx had begun doing) and began calling
himself by different "number" names, first going with several variations of pi
before settling on 7-Eleven. He also wrestled as a Jobber for ECW under the name
of "The Predator" in late 1997 before retiring from wrestling. However,Rob has
since returned to the ring,this time as a masked figure known as the FuttBucker.[1]
Promoter
RF Video opened up their own wrestling promotion, Ring of Honor, in 2002. ROH
had the bulk of their original promotion and publicity through RF selling tapes
and eventually grew into a respected "indy fed".
In March 2004, following his legal problems, Feinstein publicly resigned from RF
Video and ROH before selling his share of Ring of Honor to partner Cary Silkin.
In June, a controversy arose when it became known that Rob was still actively
involved in the operations of ROH there was a "formal split" between ROH and RF
Video. The revelation also resulted in the cancellation of an upcoming show in
Maryland less than a week before the scheduled date; When Maryland promoter Dick
Caricofe, who was allowing ROH to use his wrestling license, heard that
Feinstein was still involved, he pulled out of the show, which forced it to be
moved to a Philadelphia suburb and ROH has not returned to Maryland since. There
was also temporary heat between Ring of Honor and Total Nonstop Action Wrestling
which resulted in some of ROH's top talents leaving TNA until 2005 when things
cooled off.
RF Video's own Doug Gentry was the head booker for Pro Wrestling Unplugged.
Contrary to rumors, RF Video did not purchase the company in 2005.
Perverted-Justice incident
In March 2004, Rob Feinstein was caught in an internet sting designed to catch
sexual predators (set up by Perverted-Justice.com). Feinstein, who believed he
was talking to a 14 year old boy, claimed he was a wrestler with Pro Wrestling
Guerrilla and eventually asked the boy if he could meet him at his home later,
having implied in the chat that he was considering the boy's offer for sex.[2]
When Feinstein arrived at the home, he was met by a news crew. Although no
formal charges were ever brought against him by police, the incident proved to
be disastrous publicity for both Ring of Honor and Feinstein. It caused a
temporary split with Total Nonstop Action Wrestling, cancellation of an upcoming
show in Glen Burnie, Maryland, and eventually resulted in Feinstein selling his
share of Ring of Honor.In a 2007 interview with pro wrASSlin ilLUSTnotSTRAIGHTed
Feinstein commented on the incident saying "I was just going to get something to
eat...he said there was going to be brownies." Feinstein has also commented on
the boys looks saying "he looked like a plant about to bloom....skin as soft as
a georgia peach...I could just gobble him up.."
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With the rising popularity of
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Ultima Online was one of the first
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