Emergency Antennas. Presented by Ham Hilliard W4GMM

Transcription

1 Emergency Antennas Presented by Ham Hilliard W4GMM

2 Dipole antenna Vertical antenna Random wire antenna

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5 Dipole antenna

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7 The half wave dipole antenna consists of a conductive wire or rod that is half the length of the maximum wavelength the antenna is designed to operate at end to end. This wire or rod is split in the middle, and the two sections are separated by an insulator at the center. Each wire or rod is connected electrically usually to a 50 ohm coaxial cable at the ends of the center insulator closest to the middle of the antenna. This gives 2 identical lengths of the conductor on each side of the center insulator. Radio frequency voltages at the design frequency are applied to dipole antennas at the center, between the two conductors. They are used alone as antennas, and as the driven elements in other types of antennas that are sometimes slightly modified from an exact half wavelength. The word Dipole means "two poles." The dipole antenna is made of a wire broken in the center and where broken, each half of the wire connects to an insulator that divides the wire in two equal sections as stated above. Two wires from the voltage source, which is the transmitter, are connected across the insulator. On one side of the dipole, the current in the form of moving electrons flows first from the voltage source (the transmitter) toward one end of the dipole. At the end, it reflects back down the coax toward the voltage source. The same thing occurs on the other half of the wire on the other half cycle of alternating current.

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9 Calculating the Length of a Half-Wave Resonant Dipole The approximate length in feet of a half-wave resonant dipole is found by dividing 468 by the frequency in MHz. 468 / Frequency = total dipole length in feet. You may (should) remember that formula from your study to pass your exam. Some examples of using the standard dipole formula: 468 / 3.5 MHz = feet total 468 / 7.0 MHz = feet total The actual physical length of a dipole antenna will be determined by several factors. Using larger diameter wire will make the dipole resonate lower in frequency. Therefore, to make it resonant at the higher desired frequency, it must be shortened. Raising a dipole higher above ground will make it resonate higher in frequency. An insulated wire will make the dipole resonate lower in frequency than a bare wire. Using the above formula, cut the antenna a little longer than the calculations say. If the SWR is best at a lower frequency than you desire, the antenna will have to be made shorter by pulling the excess wire through the end insulators, folding the ends of the extra wire back on itself. Then wrap the ends of the overlapped wire on itself so it won't come loose. This causes the excess wire to "short" itself to the rest of the antenna. If you are using insulated wire, you will need to cut off the excess wire. The reverse is true if the antenna resonates too high in frequency. The extra wire can be let out to make it resonate on a lower frequency. This is why you originally cut the wire a little longer.

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11 Vertical antenna

12 Vertical antenna are one half of a dipole. One half of the dipole element is straight up in the air and the other half consists of ¼ wave radial wires laid out at the bottom of the vertical component. These radials can number as few as 4 to as many as 120. These radials make up the ground plane of the vertical antenna. Verticals can be made of ridged tubing, fiberglass poles with a wire run down the middle, which is the antenna radiator, or just a wire pulled up in a tree. Vertical antenna are Omni directional with a moderate angle of radiation. They work well at all distances and they are more efficent with more radials. Your VHF/UHF antenna on your car is a vertical antenna.

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15 Random wire antenna

16 The random wire antenna is probably one of the least expensive, easiest and cheapest HF antennas to use if you have a tuner and you want to get the "most" out of a length of "random" wire without having to pull out that calculator, doing the math, getting the center insulator built or bought, running the feedline, and all the rest that goes with putting up a more elaborate antenna. All you need for a random wire antenna is some wire, your tuner, one or more supports up as high as you can get them to string the wire from the supports to the tuner, at least one or two insulators and a little time. One single wire, no solder connections, very simple... all the way from the tuner to the end support. That's it in a nutshell...or is it? Many hams have tried till they are blue in the face to install the random wire antenna that works on most; if not all of the HF bands with terrible results. Swr usually is all over the place and the tuner will just not do it's job. You can get good loading and low swr on sometimes 2 or 3 bands, but one or more of the bands that you want, just will not cooperate with an swr that can be adjusted with the "tuner". So after much frustration..down it comes and you go on to a totally different type of antenna...all that time just wasted in your opinion...until now! We recently found some good information about random wire lengths that you should and should not use.

17 So those are the numbers above that we have to stay as far away from as possible when building a long-wire antenna. Here they are in order: REVISED: Here are the final numbers (in my opinion) in green below that would be good for a long-wire antenna: (You may want to make a note of them) REVISED:

18 We have looked at three different antennas that are good for emergency work. They are in order of choice, dipole antenna is my first choice, vertical antenna is my second choice, and the random wire is my last choice. Depending on your location and surroundings will dictate what type of antenna you may have to use. A little common sense is needed when choosing an antenna. All of these antennas can be stored in the trunk of your vehicle