Assignment 8
Antenna Types
- The dipole is the fundamental unit of antenna design. The impedance of a dipole at its center frequency is about 70 ohms. Impedance is the ratio of voltage to Amperage combined with the "springyness" of a circuit. in the mechanical sense it is like having a number for torque plus RPM combined with a measure for how much the metal twists in a vehicle power train. In the electrical world an impedance transformer (or any transformer) is exactly like a gear box in the mechanical world. By convention transmitters and commercial receivers are designed to operate with 50 ohm systems,
- while consumer receivers are designed to operate with 75 ohm systems
- Folded dipoles are dipoles with an extra wire. The extra wire changes the dipole impedance to about 300 ohms if the extra wire and dipole wire is the same size.
- Twin Lead Antennas are a form of folded dipole. This is a small antenna, but at a small price, so you actually get what you pay for. Not much in either case. This guy has the right ideas about how to make your own twin lead antenna.
- Rabbit Ears.Bunnie ears are still dipoles, and small, and slightly more expensive than twin leads, though they have the advantage that you can move them around the room or outdoors in hopes of finding a better spot to get a signal. Read how I used rabbit ears on this rabbit ear link.
- This dish clip on antenna is advertised a lot recently. Now, you should recognize this as a twisted folded dipole. Don't expect a lot of this form of dipole. It is, possibly, not better than rabbit ears, in that you cannot move it for best reception. It is outdoors, away from your TV, so maybe it could be better than an indoor antenna, if you are lucky. You will note the similarity to the twin lead type dipole, except you can't move this baby for best signal, or adjust for wavelength. If your reception is less that ideal, don't blame me! Uhg.
- Turnstile. This is simple. The dipole has a dead spot, so set up one crosswise to the other, feed both dipoles connecting from one to the other with 1/4 wave length delay line, and you can get an omnidirectional antenna. Great if all stations, being in differing directions have perfect signals at your location. This antenna is sold as an FM Band antenna, with dipoles, usually in the "folded" form cut for the FM band. It is OK for FM, if you never have multipath (ghosting) or interference problems. Got it? Omni is almost always a bad idea, especially for TV! How many times do I have to say that? So, you might have to look elsewhere for omni TV band antennas information. Gee, you may as well use a lamp cord for an antenna. OK?. If you are going to use more antenna than lampcord, use a directional antenna, please. Why pay for something that picks up garbage you don't want as well as the fragile signal you do want? (Oh, ya, this is the basic idea behind the gigantic Batwing antennas often used to transmit TV. This antenna is great for broadcasting in all directions. It is easy to make from tubing, so it makes a good Ham antenna. As with any antenna, you can scale it it for your particular wavelength.
- Other Omni. This antenna, sold for FM radio, is a bent folded dipole. It is advertised a an Omni, but only approximates omnidirectional reception.
- Yagi. The Yagi antenna is narrow band, designed to work on only one channel or FM. It has the best gain for its size, and a correspondingly narrow main lobe (beam). If you need the highest gain, or to discriminate against an interfering signal 20-40 degrees azimuth off the desired signal, use a Yagi.
- Log Periodic Logs have designed in broad bandwidth. The boom length being shared over a band of frequencies means lower gain than a Yagi, and a fatter main lobe, but far better rejection of signals off the side and rear. Logs are infrequently used except in professional settings. Usually they are expensive, large, heavy and rugged. If you have a lot of money to spend and want something that will last a very long while, check out Log periodics. These images take you to manufacturer sites.
- Typical TV-FM VHF Only Home Antenna. If you want to get VHF channels, this is probably the type of antenna for you. If you need FM band capability, you have to look for that as well, but be careful. FM stations can easily overpower your TV. See more on that in the interference section. If an antenna is supposed to work on a range of frequencies from 50 to over 200 MHz, and still not cost much, then compromise is in order. In real life, most consumer antennas are not pure
- Yagi, or log-periodic, but instead are a jumble of rods designed to get the most from the limited boom space over a wide range of channel
- Typical UHF Only Home Antenna. Here, in South Central Alaska, there is no place where you should use a UHF only antenna. But if where you live you have no VHF choices, or if you want to use one antenna for VHF and another for UHF, (which I don't think is a good idea for home use, but might be right for professional installations) here are views of UHF only home antennas. Years ago, the FCC studied UHF antennas, and concluded the panel style was best at lowest cost, with the corner reflector being second best. Panels are the only all band transmitting antenna used in Europe, so it must be pretty good with tens of thousands transmitting multiple channels. I have had luck with the yagi-corner reflector style and parabolic style, except on the lowest channels. On the highest channels, the parabolic may be
- Atypical Low Channel UHF. In the United States UHF TV channels 14-21 (470-518 MHz) are used for land mobile two way radio, making rugged, effective and inexpensive Yagi antennas commonly available in that band. You probably will need to drill holes in the boom to mount the antenna horizontally polarized. The image is my installation used to receive channel 14 for microwave relay to Kenai. The location is subject to windspeeds above 200 km/h. I used the Maxrad MYA-4903(N). The 50 ohm antenna uses an N connector. Mismatching is not critical in a broad band system like TV so long as the antenna is only used to receive a signal and the mismatch is at the antenna. Any reflected energy bounces back into space. No problem.
- Antenna gain over a dipole goes up as directivity increases. This can be seen in polar style gain charts. Thus far, I have not found any consumer TV antenna manufacturer offering charts except Winegard . Look at their antenna data sheets in PDF form, and you will find good information
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