How to Build an Antenna

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"Thank you so much for visiting our class on Friday! The kids loved it...they thought it was pretty cool to meet a "real" Astronomer! Thanks again, Janine"

Updated November 19, 2011

There are several things you need to do to "catch" the radio signal reflected from a meteor ionization trail. The antenna is the instrument that does this. To put up an antenna you will need to consider the following points:

  1. Can you put the antenna on the roof of your school? See the page on mounting your antenna to help decide if this is possible. If so, we recommend using a 3 element Yagi. This design has been tested in over five schools and found to work well and is simple to make. A design for larger antennas, such as a 6 element Yagi style antenna, are also provided but not recommended simply because of the additional time involved in construction. If you can't mount it on the roof of your school then you will have to decide whether you have room to mount the Yagi in your classroom.
  2. Decide what frequency you will be observing at.
  3. Read the instructions on this page on how to design your antenna, followed by instructions on how to assemble it.
  4. Follow the instructions on how to build a mount.
  5. Read the page on the cable you will need to attach your antenna to your receiver.
  6. If you are using an outdoor antenna, you can improve your ability to detect meteors by adjusting the elevation angle of the antenna and the direction it points (azimuth).

What is an Antenna?

An antenna is an electromagnetic device that collects or emits radio waves. It consists of material that conducts electricity arranged in such a way that it is in tune with the frequency of a radio signal. Like a tuning fork in the presence of a sound of the same pitch, an antenna tuned to a particular frequency will resonate to a radio signal of the same frequency. When properly tuned an antenna will collect this energy and make it available to drive the amplifiers in a radio receiver making it possible for the human ear to hear the signal. 

Radio signals are at a frequency that cannot be heard by the human ear and they are so weak they cannot be detected without an antenna and radio receiver. Also, radio signals travel in the form of waves of photons - the same thing as light - but these photons are at a much longer wavelength than light and can't be detected by the human eye, which are "tuned" to see light waves. Sound travels in waves through molecules of air, water, or other material. Radios convert waves of photons into waves of sound.

For the detection of meteors, we need to construct and attach an antenna to the radio. This section describes how to do this.

Yagi Antennas 

Yagi is the name of the Japanese man who designed this type of antenna. It is very easy for the amateur to build using materials such as clothes hangers or copper wire used for house wiring.

If you can run a wire from your classroom outside to the roof of your school or other suitable location to set up your antenna, a three element Yagi antenna is recommended. It will efficiently collect the radiated energy of a distant FM radio station whose waves have been reflected by a meteor trail. We have tested this antenna and it has been used successfully in our test schools.

This image of a home made antenna currently being used on a school gives an idea of what a yagi antenna looks like - several pieces of metal spaced apart from one another with a wire attached to one of them. You may recognize it as being like a TV antenna on someone's house.

If you wish to learn more about antenna design, continue reading, otherwise, proceed to your next topic of interest.

Design

The Yagi antenna consists of 2 parts:

the antenna elements
the antenna boom

There are three types of elements:

the Reflector (REFL) 
the Driven Element (DE)
the Directors (DIR)

The Reflector is at the back of the antenna furthest away from the transmitting station. In other words the boom of the antenna is pointed towards the radio station over the horizon with the Reflector furthest away from the station.

The Driven Element is where the signal is intercepted by the receiving equipment and has the cable attached that takes the received signal to the receiver.

See the table at this link to see the length of the antenna elements and the spacing between them for given frequencies. Lengths and spacings are given for the FM radio band. Or you can use the formulas below to calculate the dimensions for your antenna.

Formulas

There are formulas that you can use to decide both the length of the pieces and the spacing between them. The dimensions of the elements is frequency-dependent.  Here are the general rules for length:

Reflector length    =    0.495 x wavelength

Dipole radiator    =    0.473 x wavelength

Director D1          =    0.440 x wavelength

Director D2          =    0.435 x wavelength

Director D3          =    0.430 x wavelength

The wavelength is calculated using this formula: Wavelength (in metres) = 300 / frequency (MHz). What this formula embodies is related to the speed of light. Another way of putting it is that the length of the waves you will be detecting with your antenna is calculated by dividing the speed of light (300,000,000 meters per second) by the frequency you'll be observing at, let's say 92.1 MHz (Megahertz). A Hertz is one cycle per second. That means one vibration per second. A Megahertz is one million cycles per second. So 92.1 MHz equals 92,100,000 cycles per second (92.1 times 1 million). See the article on The Electromagnetic Spectrum in the Library part of this website for more information on radio waves.

To determine the wavelength of a radio station with a frequency of 92.1 MHz, simply divide the speed of light (300,000,000 meters per second) by 92,100,000 cycles per second. The seconds cancel out in the formula with the wavelength ending up at 3.26 meters. In other words the waves passing you by right now from a radio station transmitting at 92.1 MHz are 3.26 meters long.

Once you have determined the wavelength of the radio channel you will be tuned to you can calculate the length and spacing of the antenna elements using the formulae

If you want to build an antenna with more elements, succeeding directors  should be a factor of 0.005 shorter successively (eg. Director 4 is 0.425 x wavelength).

The final (end) director  is a factor of 0.007 less than the second last director.

Here is an example:

At 435MHz (a frequency too high for detecting meteors and used for example only): 

REFL        =    13.5ins.

DE      =    12.875 ins.

D1     =      12.0 ins.

D2    =       11.875 ins.

D3     =      11.75 ins.

Next    =    11.5 ins   etc.

Last    =    0.375 ins. less

Getting the right length is part of the tuning. Spacing between elements is the other part: 

R-DR         =    0.125 x wavelength    =    8.6 cm at 435MHz.

DR - D1     =    0.125 x wavelength    =    8.6 cm at 435MHz.

D1 - D2     =    0.250 x wavelength    =     17.1 cm at 435MHz.

D2 - D3 etc =  0.250 x wavelength    =     17.1 cm at 435 MHz.

Ref:    Radio Data Reference Book,  G. R. Jessop, G6JP, RSGB.

These formulas are courtesy of Mel Chappel, G0GQX.

Materials

Welding rod material or aluminum tubing are the best materials to use as antenna elements. However, these materials are not the easiest to get and the welding rod you can buy is frequently too short for the FM band.

Therefore, we are recommending copper house wire, the type used for wiring wall outlets. Get single strand 10, 12, or 14 gauge insulated copper wire.

More details on how to construct an antenna for your school can be found by following this link.

Copyright 1999-2012 by Sky Scan, Edmonton, Alberta, Canada. 

We gratefully acknowledge the financial support of the 

Edmonton Centre of the Royal Astronomical Society of Canada, Department of Physics (University of Alberta)

and the

Natural Sciences and Engineering Research Council of Canada

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