RADIO ANTENNAS (This will focus on the AM band for commercial radio stations, but at the end there are a few tips for FM. And I am adding some general reception tips. http://www.keithsketchley.com/cellular.htm has some information on cellular phone antennas.) TOC Introduction Internal antenna Inside antenna Outside antenna Compact Special Antennas FM Tech Talk Introduction Reception of AM radio stations has become less important in recent decades, but is being revived by people wanting to receive talk shows and oldies music. It is easy to make a basic AM radio antenna, because it is not practical to match the antenna to the wavelength as is done for FM/VHF, and crude antennas will still increase signal significantly. I will be cautious about safety and use simple techniques, perhaps disappointing people who have deep technical knowledge. Do your testing at the time of day that the show will be on. The signals propagate differently at night. My companion article on radios and interference identifies good radio characteristics (selectivity and designed to pull in AM) and available good radios. You need a radio that can tune narrowly enough to distinguish between stations with adjacent frequencies. The article also advises on reducing interference, which can come from electrical devices inside your residence, or from nearby sources outside. INTERNAL ANTENNA The radio will have an antenna hidden inside the radio (usually a "ferrite core" type), and may have an external collapsible rod antenna. Note however, that the external antenna may be for FM - that varies with the model of radio. Some radios use the power cord as an antenna. You should try all of rotating the radio, re-orienting the power cord, and moving the external antenna (between vertical & horizontal, then rotating it if horizontal), to get best pickup of the station and least pickup of noise. You might try clipping a wire antenna to the external antenna (see later section on inside antennas). As well, you might try moving the radio to different locations in your residence, such as next to water pipes or power wiring. And note that your body may affect reception - try being near the radio or touching the extendable rod antenna. If that helps, you could sit there holding it for the whole show. (OK, I'll understand not holding it during commercial breaks. :-) EXTERNAL ANTENNA Your radio may have a connection for an external AM antenna (some have a connection only for an external FM antenna). An AM connection would typically have one terminal whereas an FM should always have two terminals, and some provide both. Look for markings on the radio or information in the manual identifying whether the antenna connection is for AM or FM, and what the "impedance" is if there are two adjacent terminals. You can make a simple AM wire antenna from a piece of wire more than a few feet long should work. For general safety use insulated wire and cover the loose end with tape - that also reduces chances of contacting something else that picks up noise or signal thus confuses your experiment. The wire is best oriented horizontally so that its length points to the station. If you have one of those T-shaped twin-lead FM antennas that are meant to tack to your wall, it is worth trying because it is easy - connect both terminals to your single connection for external AM antenna. There are retractable wire antennas sold for various uses. Radio Shack sells one that extends to 23 feet - they call it a portable shortwave antenna. (It's probably the same one as made by Sangean Electronics as model ANT-60 Portable Shortwave Antenna.) If your radio has two terminals that are specifically for an AM antenna, you need to connect the antenna to one and ground the other one to something that has a connection to the earth. Clipping a wire onto a water pipe should work in an old building (newer ones have plastic pipes that won't conduct). The safety ground of an electrical outlet should work (the screw(s) holding the decorative plate to the box) but be cautious of the possibility it is wired wrong unless it is wired to hospital quality standards. (You are using well insulated wire - right?) If your radio has two terminals for AM and specifies 300 ohm impedance, it will be better to use the old "twinlead" for TV. (Note that some stores sell two types of twinlead - 300 ohm and 75 ohm, distinguished by the width (distance between wires.) OUTDOOR ANTENNA For stringing a wire outdoors, consider the risk of lightning especially if you don't have taller buildings & trees close to the antenna - and even then if tied to a tall object it may get flashover. Such antennas need lightning arrestors. The C Crane Co. 1-800-522-8863 (TUNE), www.ccrane.com, sells a small lightning arrestor.) In some cases static electricity can build up on antennas (those familiar with electrical parts could hook a high-resistance between the antenna and ground to bleed off the charge - use a 10 to 20 Megohm resistor of 2 watt rating; Radio Shack sell a device for 75 ohm coax). However, I do not attempt to cover all safety requirements for outdoor antennas in this article - if you want to use one, do good research or hire an expert. A house attic is a good place for an antenna, but I would still use an arrestor and disconnect it when not in use or if lightning is expected. All attachments, stand-offs and feed-thrus should be well insulated to avoid chafing the insulation or otherwise shorting out the radio signal. Keep the antenna away from power wires. Again I'll note that the antenna will work best for AM if horizontal with its length pointed toward the radio station transmitting antenna. SPECIAL/AMPLIFIED ANTENNAS There are special compact (loop) small antennas sold in Radio Shack and on the C Crane website. Some contain an amplifier. As well, there are signal amplifiers sold, including by Radio Shack for car antennas. However, before purchasing one try it - you want to ensure that it handles AM frequencies. Note that amplifiers will boost both the desired signal and the in-band noise. A good amplifier design will be optimized for a frequency range for greater efficiency and to reject out of band noise. If the noise you are hearing is 60 Hz powerline noise I think an amplifier will help, because the noise is coming from inside the radio due to volume being high - either due to the volume control or the operation of automatic volume circuits. The C Crane Co. 1-800-522-8863 (TUNE), www.ccrane.com, sells compact (loop) antennas about which they make great claims. They range from $60. to $360 (yes, I know that is twice as much as their fancy radio sells for). As well, Radio Shack sells at least one. (A friend recommends you find a store person who is knowledgeable and straight, and ask them which is best - he did that with compact TV antennas and found that performance did not corelate to price.) Fancier models of the Select-A-Tenna compact loop, one of those C Crane Co sells, can be used three ways: - without connection - with connection - remotely located. While they are twice the price of the basic unconnectable model, you may want that flexibility in the long term. The unconnected antennas must be placed as close as practical to the back of the radio to couple into the external antenna. Loops are directional so must be pointed perpendicular to the direction to the station (i.e. the face of the loop pointing to the station, not the edge). These two companies sell serious loop antennas: KIWA Electronics, Yakima WA, www.kiwa.com Wellbrook Communications, in the UK, www.wellbrook.uk.com and charge serious prices for their very capable products. CARS Car radios may perform better than radios in your house. Special car antennas are available from C Crane company, or see Tech Talk later in this article. KIWA Electronics sells a portable loop to fit the car's antenna connection. Radio Shack sells an amplifier for car radio antennas. You could set up a car radio in a building by getting a 12 volt DC power supply, and setting up a car antenna on a ground plane, but that's work. FM BAND Read information in the preceding sections on the twinlead antennas and the C Crane company who also sell FM antennas. Since FM is much higher frequency than AM, tuned length antennas are the norm (e.g. the length of your car radio antenna is probably optimized for the FM band). FM signals propagate in a straight line, so are affected by hills, buildings and the earth's curvature - which will limit range as the signals from distant stations go above you. So locating the antenna higher in your building is better to reduce blockage. The FM band is quite close to the VHF television band (channels 2 through 6) and aircraft "localizer" & VHF communication bands, so simple antennas made for those uses may be adequate for FM radio. However, if they have filters they may not be usable outside their intended range. An external antenna connection on your radio would usually be 300 ohm or 75 ohm impedance. Your antenna feed needs to match that. The old wide "twinlead" cable used for TV is 300 ohm, whereas the newer coaxial cable and the special FM antennas of narrow twinlead are 75 ohm. For a dipole antenna such as the T-shaped twin lead antenna orient it so that the two legs are horizontal and perpendicular to your line of sight to the station's antenna. And you might look for the station on your local cable TV service as some radio stations are. FM radios with antenna connections can easily be connected to the cable feed, using the twinlead (300 ohm; two wires kept at constant spacing by the plastic) or the coax (75 ohm; round concentric, usually screw-on) connection as marked or physically obvious from comparing with your TV. A splitter will be needed to isolate the radio from the TV. TECH TALK AM radio stations can be received well by a more practical horizontal wire antenna. AM wire antennas practical for home and city use are called "travelling wave" rather than "resonant" - the latter matching a wavelength fraction. (A modern car antenna is resonant for the much higher FM frequencies, since resonant length is inversely related to frequency. A resonant antenna for AM would be several hundred feet long. The term "long-wire" correctly applies only to antennas that are several wavelengths long. If you know anyone who is a "ham" (Amateur Radio Enthusiast), they may be familiar with antennas - operation at "160 meters" (1800 Khz) will be close to the stations we want. The ARRL Antenna Handbook provides some designs for fixed wire and mobile whip antennas for "160 meters", with enough explanation that one could re-calculate for a different frequency (while simple antennas designed for 1800 KHZ would work OK at 1400 Khz, the complex designs are highly tuned which narrows the frequency range they can receive well, thus they need to be adjusted or modified to match the frequency. The ARRL books provide designs using windings to get length of the fixed wire antenna "down" to 123 feet, which could be accomodated in a large house lot. Also, some people "DX" AM radio stations to get the music they want ("oldies" music fans are prominent among them). They have techniques for antennas, including ways of cancelling an interfering station on the same frequency. AM frequencies are of course usually given as Khz (from about 550 to 1700) whereas higher frequencies are given in MHz which is 1,000. times higher - so 1400 Khz is 1.4MHz for example. AM radio signals are somewhat affected by terrain, but are not strictly line of site like FM is. An overwater path is usually good, though the position of the sun may affect it. Signal propagation is better at night (at long distances this is in part due to reflection off layers in the atmosphere, which is highly variable). However, stations not on a clear frequency must reduce power at night. "Loaded" whip antennas are sometimes used on cars. They use an inductor (coil) to function as though they were longer. They are common at higher frequencies but here we need a 20-fold increase in effective length from a practical straight whip length. The technique causes losses which multiply with effective length, and narrows the frequency range over which the antenna is most effective - hence many are adjustable. Nevertheless the ARRL Antenna Handbook provides a design for 1800 Khz, using bulky loading coils. Loop antennas put the length of wire into a loop of several turns. At AM frequencies the total length is probably much less than a half wavelength. (If so, it needs to be less than one-tenth of a wavelength.) They may have a tuning adjustment and may have an amplifier. Loop diameter is not critical at AM frequencies, though larger may pick up more signal. Loop antennas are usually more noise-resistant than wire antennas as they are magnetic not electro- static so will no pick up noise from lightning or arcing, and directional. The unconnected loop antennas are resonant antennas thus narrow band, and must be adjusted to station frequency using the knob. Ferrite core antennas, usually used inside radios, are really loop antennas using the ferrite core to concentrate the radio signals (electro magnetic field) to allow smaller diameter. Signal reception will probably be best with the radio's long dimension perpendicular to the direction to the station, as the ferrite core is probably mounted along that dimension thus the loop of wire around the core will be pointed correctly. A good amplifier design will be optimized for a frequency range for greater efficiency and to reject out of band noise. That leaves in-band noise being amplified - if you have strong local inteference at AM frequencies an amplifier won't help. However, if you are hearing noise simply due to the signal being very weak, an amplifier may help the radio cope with the noise. One phenomenon I have observed is 60 Hz buzz from power wiring. I think that tends to come from general pickup by the radio circuits, and from the power cord, not from the antenna - since antenna circuits should not pass that low frequency into the radio. What I have observed seems to be due to the radio being at maximum volume when the signal is very very weak (as well, the radio may have automatic internal circuits to boost volume when the signal is weak). FM antennas work best when oriented horizontally, perpendicular to line of sight to the station, to match the primary polarization of the signal. (Yes, car antennas are vertical which is not optimum. Many FM stations transmit two polarizations of their signal in order to catch car antennas, and to penetrate between hills or buildings (in which case you can orient the antenna vertically.) If you have an estate big enough for a tuned antenna, there are two common types: - center fed, with each leg 1/4 wavelength long and connected to opposite sides of 75 ohm coax to connect it to the radio (like the typical home FM strip antenna of T-shape). - 1/2 wavelength with each end turning back to the center where it is connected to twinlead to connect it to the radio. (IE total antenna wire length is a full wavelength.) The spacing between the legs should be TBD to get the right impedance (the 300 or 75 ohm wanted by the radio). A simple wire antenna needs to be one-half wavelength to be resonant. At 1400 KHZ, one wavelength is 703 feet. You can can do the math for fractions, and for other frequencies (simple ratio, shorter wire for higher frequencies - e.g. 1406 feet for 700 KHz). Reduce length about 5 % from theoretical to compensate for the resistance of the wire, and another 4 % for insulated wire. Note that length needed for vertical antennas is reduced by half if there is an effective ground plane (conductive surface). That is called a reflected dipole). Your car antenna for example uses the metal body as the other half of the effective length. Wire diameter is not critical - large enough for good mechanical strength should be adequate electrically (fine wire would have too much resistance). Note that "gage" numbers are higher for smaller diameter. AM transmitting antennas are usually located on moist soil, on a site a few acres in size as they are quite large and require a large ground plane (usually one or more towers with wires attached). FM antennas will be on towers, preferably on high ground or a tall building - they are fairly small. You may find advantage in orienting an FM antenna to the specific location even if close, whereas for AM you should have no difficulty getting the station if its transmitter is close to you and if it's far away just pointing toward the city is adequate. Sophisticated techniques are used to pull in distant stations while minimizing reception of near/stronger stations on the same frequency. Making serious attempts to receive distant stations is called "DXing" - if you find people or books/articles on that topic you may get good information. Highly directional antennas are a good start - loops are quite directional but are bi- directional (strong reception both to and away from the station). A loop antenna combined with a wire antenna higher than the loop may be very directional - point the loop center axis to the station in that case. Another technique is to combine antennas in a way that electrically cancels the pickup from the unwanted station. (I intend to determine characteristic impedance of a simple wire antenna (several hundred ohm I think, depends on height above ground). GENERAL RECEPTION TIPS Reception of distant stations may be better in darkness, because reflection off the ionosphere is more likely. However, stations may reduce power at night to avoid interfering with other stations on the same frequency. In some cases you can receive a station that is on the same frequency as another because their actual transmission frequencies are slightly different. (Perhaps due to transmitter adjustment.) You'll need a radio that has fine analog tuning with narrow bandwidth. The transmitter or your receiver may drift in frequency, so a receiver that follows the signal may help, though that could hurt if the weaker signal varies in strength (the receiver may jump to the stronger signal when the weaker one fades). Another case where analog tuning (rather than fixed digital steps) may be useful is when the transmitter may be off the frequency. That might happen when the equipment is of poor capability or is poorly maintained. A receiver that you tune in fixed digital steps would need wide bandwidth in that case, but is not likely to as narrow bandwidth is needed to separate stations. DISCLAIMER This is not professional advice. I assume no responsibility for how you use it or the consequences of your use. -------------------- Info to be included: http://www.lifechangingradio.com/loopant.htm shows how to wind a loop for AM reception. Bulky. ------------------------------------------------------------------------- Copyright Keith Sketchley 2011.07.05 Legalities detailed on http://www.keithsketchley.com/ apply. 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