Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: resistance v. impedence  (Read 353 times)
W7ARE
Member

Posts: 16




Ignore
« on: April 17, 2008, 04:05:33 PM »

What is the difference between resistance and impededence?
Logged
DIPOLE
Member

Posts: 176




Ignore
« Reply #1 on: April 17, 2008, 04:24:07 PM »

Technically, resistance is the opposition to a DC current, while impedance is opposition to an AC current.  In many ways you can think of them as the same, but impedance is not constant and is dependent on the frequency of the AC current.  So for any given component like a capacitor or inductor, the impedance of that component is not fixed like the resistance of a resistor, but rather it's dependent on the frequency of the AC current going through it.
Logged
VK1OD
Member

Posts: 1697




Ignore
« Reply #2 on: April 17, 2008, 04:57:57 PM »


Resistance is the ratio of voltage to current for a resistor.

Impedance is the ratio of voltage to current for an impedor which may contain both resistance and reactance.

Impedance is a complex quantity, containing a real part being the resistance and an imaginary part being the reactance. Being a complex number, it has magnitude and phase angle (in polar form).

If you don't understand the last paragraph, the magnitude of impedance |Z|=(R^2+X^2)^0.5 and the phase angle is arctan(X/R) where X is the equivalent series reactance and R is the equivalent series resistance.

At DC, the reactance of an inductor is zero, and of a capacitor is infinite which simplifies circuits somewhat.

The key difference between resistance and reactance is that a real resistance dissipates energy whereas reactance stores energy.

Owen

Logged
AA4PB
Member

Posts: 12667




Ignore
« Reply #3 on: April 17, 2008, 05:09:23 PM »

Impedance is a combination of resistance, inductive reactance, and capacitive reactance in a circuit. The reactances are very dependent upon the frequency. In a DC circuit (frequency = 0) there is no reactance so we only consider its resistance. In a AC circuit (frequency > 0), like an antenna, there is reactance and resistance so we describe the combination called impedance. Because of the voltage vs current phase difference in a reactance, you cannot add the reactance and resistance using simple math. You must use vector analysis that takes the phase relationships into consideration.
Logged
W7ETA
Member

Posts: 2528




Ignore
« Reply #4 on: April 17, 2008, 08:01:23 PM »

Plus.
 
While the impedance will vary with frequency, the DC resistance remains the same.

I pass that along because after being a ham for a while, because I had a DUH! moment for the resistance not changing if I load an antenna for a band it wasn't designed for.

73
Bob
Logged
VK1OD
Member

Posts: 1697




Ignore
« Reply #5 on: April 17, 2008, 08:36:53 PM »


"I pass that along because after being a ham for a while, because I had a DUH! moment for the resistance not changing if I load an antenna for a band it wasn't designed for. "

Bob, the antenna feed point impedance can be thought of as having components of resistance and reactance. The resistance includes the effects of conductor loss resistance (the resistance at radio frequency of the conductors, resistance which converts electrical energy to heat and referred to the feed point) and radiation resistance (the virtual resistance that represents the radiated power at a given feed point current).

Both of these are potentially frequency dependent.

For example, the radiation resistance of a centre fed half wave dipole above ground is something around 60 ohms, whereas for a full wave dipole it is more like 4000 ohms.

Owen
Logged
AA4PB
Member

Posts: 12667




Ignore
« Reply #6 on: April 18, 2008, 04:52:55 AM »

Even loss resistance can change somewhat with frequency due to "skin effect". Skin effect causes the RF current to flow near the surface of a conductor rather than through the center thereby increasing the resistance at higher frequencies. It takes a good deal of frequency change to notice the skin effect however.

Baically though, resistance has the same effect on AC as it does on DC so you cannot correctly apply resistance to DC and impedance to AC. Resistance is more closely related to reactance and impedance is the combination of the reactance and resistance in a circuit.
Logged
W5DXP
Member

Posts: 3540


WWW

Ignore
« Reply #7 on: April 18, 2008, 10:52:24 AM »

Resistance is represented by the letter 'R'. Impedance is represented by the letter 'Z'. Reactance is represented by the letter 'X'.

The + or - sign of the reactance tells us whether the reactance is inductive or capacitive.

Z = R +/- jX   where R can be zero and/or X can be zero and where 'j' is a mathematical operator indicating the square root of -1.

The short answer to you question is that resistance is a component of impedance. The other component of impedance is reactance. If the reactance is zero, the impedance is purely resistive, i.e. Z = R. But Z = R + j0 is a special case of impedance and is usually not the case.
--
73, Cecil, w5dxp.com
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WA7NCL
Member

Posts: 625




Ignore
« Reply #8 on: April 18, 2008, 11:08:38 AM »

Interesting answers.

Both are the ratio of voltage to current across/into the terminals of a device with a source applied (commonly a time varying source).

A resistance has the voltage and current in phase.  A resistance dissapates energy and does not store it, assuming the resistance is positive.  If you have a negative resistance, the element is potentially an energy source.

Impedance is more general.  The current and voltage are not necessarily in phase due to the presence of an energy storeage element.  The storeage element is usually something like an inductor or capacitor. In the case of sinusoidal currents or voltages the energy storage elements are called reactances.

In general then an impedance can consist of both a resistive element (usually dissapative) and a storage element.  This is what hams usually think of when somebody says impedance.
Logged
W4VR
Member

Posts: 1190


WWW

Ignore
« Reply #9 on: April 19, 2008, 01:23:51 PM »

Surely you must be kidding!  How could you have passed the amateur radio exam?
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!