Tuesday, December 23, 2008

Impedance matching in RF circuits

Impedance matching is one of the most important things of RF circuits.It is used for the purpose of transffering maximum power into a load circuit.Let's make it clear:

Suppose,you have a RF amplifier and it has an output impedance of 50 ohm.You also have an antenna that has an impedance of 75 ohm.Here the amplifier is the source and the antenna is the load.If you want to transfer the maximum possible power into the load (antenna) the source impedance must be matched to the load impedance.

Methods of Impedance Matching


There are several techniques used for impedance matching in RF circuits.They can be catagorised into two main types- transformer matching and LC type matching.At first we will discuss some LC type matching networks and show how to design them.
There are a number of LC type impedance matching networks such as the L network,T network,Pi network,split-capacitpr network and some other derivations.


Before going to discuss the networks we have to talk about notations.In all the circuits of this writing R1 is source resistance or impedance and R2 is the load resistance or  impedance .Inductors and capacitors  are noted with L and C.Inductive and capacitive reactances are noted with XL and Xc.

The L ntework                       
                                                             
                                                        
The L network is one of the simplest networks for impedence matching.It consists of a coil and a capacitor.
See figure 1 for network details and formulas for calculating every component value.The network of figure 1 calls for R1 to be greater than R2.This network can be used when R2 is greater than R1 but the capacitor should be used across the load.The L network is normally used for circuits of semi-widebandwidth.


The Inverse L Network    

                                                     

                                                                                                                                                        This is a variation of the L network.It is used when R2>R1.See figure 2 for this network.Everything is like the previous L network.You can calculate component valus from the formulas given in figure 2.

The Pi Network 

                                                 

The network of figure 3 is called the Pi matching network because it looks like the Greek letter 'Pi'.
It is widely used for impedance matching purposes.It can be used both in R1>R2 and R2>R1 requirments.But here we give the formula for when R1>R2.The pi network is bi-lateral so you can use any port for source or load.For calculating the component values first we have to define a Q value.You can get the minimum Q value needed from the formula given in figure 3.You can use a greater value but rember that high Q makes the bandwidth lower.After defining the Q use other formulas to calculate component values.

The Split-capacitor Network

                                                            
 
This network is like the Pi matching network and chiefly used when  R2 > R1.Following figure 4 you can calculate the component values easily.

Rember: All the above networks and formulas are given considering R1 and R2 are pure resistive impedances and no reactive impedance is present.But they are applicable in many real world circuits where the reactive part is negligible.


Impedance matching is a vast thing to discuss.We wll complete our discussion gradually.If this 
writing can help readers any way I will be happy.Please leave your comments.

For downloading a good document on Impedance matching click Here

Sunday, December 7, 2008

DIPOLE ANTENNA CALCULATION AND CONSTRUCTION

Dipole antenna is the antenna that a new ham or an experimenter erect first.This is becausethe dipole is cheap and easy to put up.It works well with a little effort.Here we will discusshow to calculate and costruct a dipole.
Calculation: The name 'dipole' implies that it has two poles.This antenna is generally calculated for half wavelength.The feed point impedence is normally 73 ohm above a certain height but it varies from 0 to 120 ohm at other heights.We will,however,use 72 ohm as it is used normally.75 ohm co-axial cable is used as the feed line.One can use a 1:1 balun between the antenna and feed line.The 'BALUN' will be discussed someday.Anyway,the length of a dipoleis given below.

Total length: L= 143/F
Where,
L is the total length of the dipole in meters
F is the frequency of operation (e.g F of transmitter or receiver) in MHz
Length of each part: = 71.5/F

Total Length in feet: L(ft)= 468/F(MHz)
You might have to adjust the length a bit for rosonance.
(To be continued..)
NB:For free ebooks on antenna just mail me at galib.ehobby@gmail.com

Friday, December 5, 2008

How to Calculate Air Core Coils

Hobbysists and radio amateurs sometimes face problem in finding standard value inductors for RF projects.Besides,if someone wants to wind their own inductor they might face problem.Here we will discuss how to calculate and wind air core (or sometimes called coreless) inductors.This types of inductors are generally used in HF and VHF circuits.We generally use the equation below for getting the number of turns needed in our inductor.

N=rot[L(9a+10b)/a] ........ (1)

Where:
N is the number of turns
L is the inductance in micro henry(uH)
a is the coil radious in inches
b is the coil length in inches
rot= squire root of
How to wind:
If you need a small value inductor then just take a pencil or 6mm drill bit or what you get at hand.Then measure its radious in inches.Take a piece of copper wire and make sure that it will pass the current without problem.Then assume a logical length of the coil.At last if you know the inductance just put all data in equation (1) and you will get the number of turns required.It is very easy,isn't it?