Introduction To Waveguide Bench And Measurement Of Source Frequency And Wavelength

Abstract:

In Our Experiment we will measure two important parameters using a number of basic microwave components. Frequency will be measured using a cavity wavemeter and guide wavelength using waveguide slotted line.

Introduction:

- General Background:

A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave. The original and most common meaning is a hollow metal pipe used for this purpose.

Waveguides differ in their geometry which can confine energy in one dimension such as in slab waveguides or two dimensions as in fiber or channel waveguides. In addition, different waveguides are needed to guide different frequencies: an optical fiber guiding light (high frequency) will not guide microwaves (which have a much lower frequency). As a rule of thumb, the width of a waveguide needs to be of the same order of magnitude as the wavelength of the guided wave.

Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance: i.e., at a distance R from the source the power is the source power divided by R2. The waveguide confines the wave to propagation in one dimension, so that it doesn't lose (in ideal conditions) power while propagating.

Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. This description is exact for electromagnetic waves in a rectangular or circular hollow metal tube.

- Objectives:

To be familiar with some microwave waveguide components and know their useTo know how to measure frequency using cavity wavemeterTo know how guide wave length λg is measured using a slotted lineTo understand the meaning of cut-off wavelength and frequency To Use the general relationship for waveguides of: 1/λg²=1/λ² – 1/λc² to calculate the guider wavelength, cut-off wavelength and free space wavelength and frequency

Theory:

- Measuring Source frequency using cavity meter:

The principle of cavity wavemeter is based on the fact that very high Q-resonances can be obtained in metal waveguide cavities. Such cavities are usually of uniform circular or rectangular cross-section and resonate when their axial length equals an integral number of half guide wavelength.

L=0.5nλg Where L = axial length of cavity n = 1, 2, 3, …., the order of resonance λg = guide wave length of resonating mode

The cavity length L may be varied by altering the position of the short circuit plunger. Off resonance the cavity absorbs little or no power from the main waveguide transmission system. The type of resonant mode and the order of resonance enables the exciting frequency, the source frequency f, to be calculated. From theory:

f=c/l= 3*10^8√[(n/2L) ²+(1/lc²)]

Where c = The velocity of electromagnetic waves in free space lc = cutoff wavelength of mode resonant in the cavity n = order of resonance

- Guide Wavelength And Its Measurements :

Free space wavelength "l" is the distance traveled by the wave front of the electromagnetic wave in free space in the duration of one cycle, and it is related to frequency f by:

λ=c/f

When the waves are guided by a wave guide they travel in the form of distinctive wave patterns known as modes and the guide length of the guided transmission is known as wavelength λg. For rectangular and circular waveguides, λg is related to "l" by the formulae:λg = (λ. λc/√λc² - λ0²)

Where λc = The cutoff wavelength f the propagating mode

For rectangular waveguides, transmission is limited to a single mode operation in its dominant H10 mode. The cutoff wavelength for H10 mode is:

λ=2a Where a= internal broadside dimension of the waveguide

Experimental Method :

- The three consecutive nulls:

X1 = X2 = X3 =

- The guide wavelength:

λg = 2(x2-x1) = 2(x3-x2) = 2(x3-x1) =

- The cutoff wavelength for the dominant mod H10:

It is standard for our microwave trainer WG16, The guide wavelength:

λc = 2a=2 * 22.86 mm(broad dimension) = 45.72 mm for WG 16

- The guide wavelength at the source frequency 10.7 GHz:

References:

Wikipedia: http://en.wikipedia.org/wiki/Waveguide

Conclusions & Recommendations:

- waveguides are structures which guides electromagnetic waves and usually used in high frequencies and has too many applications in microwave devices and optical fiber communication systems

- Frequency was measured by using the recorded micrometer

reading by cavity wavemeter at resonance using the E011 mode calibration curve

- The guide wavelength was measured recording the positions of electric field nulls using waveguide slotted line