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The antenna is magic doorway that RF energy uses to enter or leave a circuit. It is nothing more than a simple piece of wire that only has one connection. The reason that it can do what it does has always been a mystery to most people. It has become the flag that an electronics engineer flies over his house.
This project started out to be a demonstration of how to build a transmitter that has only the antenna as its main component. The antenna has a length which must be exactly ½ of the wavelength that it is to be used for. It also has a width. That width changes the effective length that an antenna will offer to the radio waves.
Even though there are as many different antenna designs as there are grains of sand on the beach, the most powerful antenna is still the simple dipole. The dipole is two 1/4 wave antennas working together. It is commonly known as a ½ wave antenna. A single 1/4 wave antenna can do the job all by itself but not as well as when two of them work together. RF energy favors balanced symmetrical designs.
For transmitting the best width is the thinnest one. It seems hard to believe that a skinny piece of wire all by itself is the best form of antenna, but it is. The reason is because a skinny wire makes a stronger magnetic field that a thick one. Several antennas may be combined to make a directional antenna system. Even though that system may be more directional it is still less powerful. It does not matter if the metal used is magnetic or not.
The problem with a skinny wire is that it cannot support itself. Any support materials added to the antenna cause capacitive losses to occur. Therefore, a thicker wire or tube gets used instead because it can support itself. When a thicker wire is used the antenna has to be shortened. When taken to the extreme limits the antenna has to be shortened as much as 28%.
This transmitter project is intended to demonstrate that a short wide antenna will have the same frequency as a long skinny antenna. The antenna is the main controlling factor of the frequency in this design because there are no transistors or crystals or anything else to force it to be different than what the antenna chooses. By changing the antenna you change the frequency.
The target frequency is the lower end of the UHF band. This is chosen so that the experimenter may use a common TV set as test equipment. The object is to use antennas of differing widths and then to shorten them until they all work at the same frequency. The widths to be tried range from a piece of fine piano wire to an aluminum soda can.
The figure out how much to shorten the antenna there is a graph that gives a number called the K factor. You know in advance what the wavelength is and how wide the antenna element is. The K factor that you get from the graph is a percentage of the length that an infinantly skinny wire would have at that frequency.
To use the graph you start by dividing ½ of the wavelength to be used by the diameter of the antenna element. That number is called the Length to Diameter Ratio. When you put that number into the graph you get the K factor. You then trim the antenna to that percentage to get the proper length.
The graph also gives you the impedance of the antenna. A shorter antenna will have a lower impedance than a longer one. The impedance of the antenna should match the impedance of the transmitter for highest efficiency. For this experiment the impedances of the shorter antennas will not quite match the impedance of the transmitter. They will have a lower efficiency but they will still show that the frequency that results is affected by the width of the element.
To save the reader the trouble of doing all of those calculations and graph readings the following table is presented to show what the results are at some of the common frequencies in use today.
Table of Antenna Lengths (tip to tip in inches)
Frequency
| Channel | 3 | FM (6-7) | 13 | 14 | 67 | 83 | Phone |
| MHZ | 61 | 100 | 211 | 471 | 789 | 885 | 900 |
| ½ Wave | 96.80 | 59.05 | 27.99 | 12.54 | 7.48 | 6.67 | 6.56 |
| .015 wire | 94.57 | 57.66 | 27.28 | 12.19 | 7.26 | 6.47 | 6.37 |
| .020 wire | 94.52 | 57.60 | 27.26 | 12.18 | 7.25 | 6.47 | 6.36 |
| .025 wire | 94.48 | 57.57 | 27.25 | 12.17 | 7.25 | 6.46 | 6.35 |
| .032 wire | 94.43 | 57.54 | 27.23 | 12.16 | 7.24 | 6.45 | 6.35 |
| .047 wire | 94.35 | 57.48 | 27.19 | 12.14 | 7.22 | 6.43 | 6.32 |
| .094 hanger | 94.19 | 57.37 | 27.11 | 12.09 | 7.19 | 6.40 | 6.29 |
| .025 tube | 93.87 | 57.16 | 26.97 | 12.00 | 7.11 | 6.33 | 6.22 |
| .375 TV ant | 93.75 | 57.01 | 26.87 | 11.94 | 7.06 | 6.28 | 6.17 |
| 1.00 pipe | 93.17 | 56.63 | 26.59 | 11.69 | 6.84 | 6.04 | 5.94 |
| 2.6 soda can | 92.35 | 55.89 | 25.97 | 11.06 | 5.98 | 5.15 | 5.07 |
In the left column are abbreviations for the type of material used for the antenna element. 'Wire' is for piano wire. 'Hanger is for coat hanger. 'Tube' and 'pipe' are for copper tube and pipe. 'TV ant' is for common TV antenna elements. 'Soda can' is for common soda can. The right column is for cordless telephones.
If you look at a common TV antenna it usually has a UHF dipole at the front end with reflector elements that extend over and under the antenna. If you measure the width of the elements and the tip to tip length of the dipole you will find that it is ideal for a frequency of about 374 MHZ. That frequency is between VHF and UHF bands. A table of the electromagnetic frequency spectrum says that frequency is for government or military use.