Here’s a great article on tuning in our little peanut whistles – consider sharing this with your potential listeners!”
Receiving Part 15 Transmitters
Glenn A. Elliott, 08 August 2004
(Revised Friday 27 August 2004)
“Picking up these ‘peanut whistles,’ especially on the fringe, is going to require some effort…
Here’s a great article on tuning in our little peanut whistles – consider sharing this with your potential listeners!”
Receiving Part 15 Transmitters
Glenn A. Elliott, 08 August 2004
(Revised Friday 27 August 2004)
“Picking up these ‘peanut whistles,’ especially on the fringe, is going to require some effort…
Summary:
A. Picking up these “peanut whistles,” especially on the fringe, is going to require some effort by you.
B. Not picking up noise and interference can be as important as picking up signal. Walk around your house with a pocket radio, tuned to a weak station or no station, and listen to the noise level. Put receivers and antennas away from noisy spots, and avoid, move, or dispose of noisy devices.
C. All receivers aren’t equal. Lots can pick up signals fairly well. The tough part is separating out what you want.
D. There’s no substitute for a good antenna. For FM, a “beam” mounted outdoors and high is the best. Next best is that beam mounted inside the attic of a wood house, and after that is the good old “T” antenna (stretched out and up high, in a window if your house isn’t wood, not crumpled up on the floor). On AM, try a simple manufactured “loop” first.
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Successful radio reception depends on factors at both the transmitter and at the receiver. Transmitter performance depends on both the transmitter power and the antenna. Part 15 transmitters are limited in this regard, either by overall signal strength limits or by specific restrictions on both power and antenna construction.
There are no FCC restrictions on the receiving setup, although zoning or restrictive covenants can limit receiving antenna construction. Still, much can be enhanced on the receiving end, and the nature of Part 15 requires this investment except at the closest distances.
This article is meant to be introductory, not exhaustive. Some technical terms and concepts may be mentioned that require further study. There is considerable material available on radio reception equipment and techniques.
1. Noise and interference can be problems at any frequency, although they generally become more evident on AM versus FM signals, and at lower frequencies.
a. Thunderstorms and even the atmosphere itself make substantial noise below 2 MHz (2000 kHz). For a given transmitter (running below 2MHz), receiving setup, and distance, fringe reception may be possible from late fall through early spring, when thunderstorms are absent and the atmosphere is “quieter,” but not during the rest of the year.
b. There are numerous noise sources in and near your home. Computers, fluorescent lights, electric motors and appliances, and any devices that use a microprocessor (to make them more functional or “smart”) are definitely candidates. Telephone, cable TV, and power lines can also act as huge antennas to bring noise into your home. Most AM broadcast receivers have antennas inside the receiver case, so moving an AM receiver around within your home and changing its orientation (lining it up north-south, east-west, or in between) will often affect how much noise (and signal) it picks up. In fact, you can walk around inside your house with a pocket AM (or AM/FM) radio, tuned to either a weak station or no station, to find noisy spots (to avoid) or devices (to avoid or move). The noise you hear on an AM or FM radio can also affect other receivers and indoor antennas.
c. Frequencies below 30 MHz (30,000 kHz) will display varying degrees of skywave or “skip” long-distance propagation. Frequencies below 2 MHz will skip in at night. The operator of a Part 15 transmitter in the AM broadcast band will virtually always account for this when selecting the operating frequency. However, if you notice that another station skips in on the same frequency at night (“co-channel interference”), the Part 15 operator would probably like to know. Reorienting the AM receiver (if it has an internal “rod” antenna) or using a loop antenna (see below) will also help in this situation unless the Part 15 and the skipping stations are on or near the same line of direction.
d. Other devices besides broadcast transmitters operate under Part 15. Basically, Part 15 devices are not protected from interference with each other. The broadcast transmitter may be able to change to another frequency if a number of people are affected. However, you may have to use a directional antenna or an “active noise canceller” to reduce interference from a local Part 15 device.
2. All receivers are definitely not the same, either in performance or features.
a. Many if not most Part 15 AM broadcasters operate between 1600 kHz and 1700 kHz (in the “extended” AM band that the FCC authorized in 1991). AM broadcast receivers produced before 1990 (some even later) generally cannot receive above 1600 kHz. The same appears to be true for some manufactured AM active antennas (including active loops) and passive loops. The McKay-Dymek DA5 tuned active ferrite-rod loop is one of these.
b. Sensitivity is the ability of the receiver to produce useable output from weak signals. It is usually measured in microvolts (uV), and a lower number means better performance. In stereo FM broadcast receivers, sensitivity is especially important regarding the ability to hear a station in stereo instead of mono (better stereo sensitivity will almost always mean better mono sensitivity as well). Receiver sensitivity varies but many if not most receivers have adequate sensitivity.
c. Selectivity is the ability of a receiver to reject signals that are close to the frequency of the desired station, especially when the adjacent signals are stronger than the desired signal. Receivers vary in selectivity more than sensitivity.
d. Bandwidth refers to the “width” of frequencies that will be picked up around the desired frequency. Cheaper receivers, especially FM and AM broadcast receivers, will only have a single bandwidth filter. Better receivers have two or more bandwidth filters to offer the option of picking up less of the signal (lower fidelity) in exchange for less noise or less interference from stations on adjacent frequencies.
e. Better AM broadcast and shortwave receivers will also have an “RF gain” control, which adjusts how much the incoming radio signal is amplified. This is important, because the receiver’s RF (radio frequency) amplifier will also amplify any noise within the receiver bandwidth, and will also tend to pick up adjacent signals more at higher “gain” (amplification) levels. Maximum RF gain is not always desirable.
f. Part 15 transmitters are also allowed to operate in the frequency bands 160-190 kHz, 13.553-13.567 MHz, 26.96-27.28 MHz, and 49.82-49.90 MHz (and others not covered here). Shortwave receivers can usually pick up the middle two bands.
(1) A good “general coverage” receiver can probably pick up the 160-190 kHz band, although you may get better reception by using an “LF [low frequency] converter” which receives the LF signal and moves it up into a higher shortwave band where the receiver has better performance. The converter output is connected by a cable to the receiver antenna input (or one of them).
(2) Operation above 30 MHz is often FM, and a good scanner should receive an FM transmitter in the 49.82-49.90 MHz band.
(3) Part 15 transmitters in any of these bands may use single sideband (SSB, a derivative of AM). Good general coverage or shortwave receivers should be able to receive SSB on the lower three bands, but receivers that can pick up AM or SSB above 30 MHz are harder to find (although definitely not impossible).
g. Two good AM broadcast band receivers are the GE SupeRadio (AM/FM; 4 models – original, II, & Plus all discontinued and only go to 1600, III currently made; see this FAQ) and the C. Crane Company CCRadio (discontinued, AM only) or CCRadio plus (currently made, by Sangean for CCC; adds FM, TV 2-13 sound, and WeatherRadio). Both radios are portable and mainly built for good AM performance. The audio section in the CCRadio and CCRadio plus is optimized for speech, so music might not sound as good as on some other AM receivers. Some reviews have preferred the SupeRadio III over the CCRadio plus (the SR3 is about $50-60 new, the CCR+ over double that), and state that the FM/TV/WxR performance of the CCRadio plus is only average.
h. A good FM broadcast band receiver is the Boston Acoustics Recepter. This is an AC-powered clock/alarm radio that also receives AM fairly well, and has connectors for both AM and FM external antennas.
3. The receiving antenna is as important as the receiver itself, but is often neglected. An adequate (or even poor) receiver with a good antenna can often outperform a good receiver with a poor-to-adequate antenna. Getting “wire in the air” can make all the difference.
a. Telephone, cable TV, and power lines can act as antennas to bring noise into your home, but they may bring in lots of signal. Try putting your AM radio next to a wall outlet, near the phone or cable TV line, or next to or on top of an appliance like a microwave. You might be surprised by the results. Even FM broadcast reception may benefit.
b. In the AM broadcast band (and more so at 160-190 kHz), the radio wavelengths are extremely long. However, “long-wire” or “random-wire” antennas can perform acceptably to well even if the length is not significant compared to the wavelength of the received frequency. Using an antenna tuner or “preselector” (or both) can enhance the performance of a wire antenna.
c. A strange antenna that has worked in differing applications:
(1) Wrap several turns of a piece of insulated wire (#18-24) around the telescoping antenna of an FM or SW receiver or the internal or external ferrite rod antenna of an AM receiver.
(2) Attach an alligator clip or clamp onto the other end of the wire, then try attaching it to:
(a) A metal window frame or screen.
(b) A water pipe or heating water pipe (not a gas pipe).
(c) A metal or jacketed-wire clothesline.
(d) The finger hook of an old dial phone (if it’s connected to the phone line, that is).
(e) A metal bed frame (or any other metal frame).
(f) Whatever you can think of that is metal and in reach.
(3) Using an antenna tuner along with this may help.
d. In many cases, receiving enough signal is not so much the problem as not receiving noise and interference. A directional antenna is a commonly-used solution.
(1) Especially at frequencies at or below 2 MHz, a tuned vertical loop antenna can really help. A loop consists of multiple turns of wire around a frame (or around a ferrite rod). A tuned loop uses a variable capacitor with the loop (which is a coil) to tune the loop to pick up a particular frequency and reject others. The loop also picks up maximum signal in the plane of the loop and minimum signal when the loop is across the direction of the desired signal (“broadside”). If the direction of the desired signal is removed enough from the direction of a noise or interference source, the loop can be turned to receive maximum signal and minimum noise. Because of their construction, loops also tend to reduce static pickup. A loop that is designed to work with a matched (and tuned) RF amplifier is “active,” versus a “passive” loop. Active loops can achieve better performance but at greater expense than passive loops.
(2) Above 10-12 MHz, “half-wave” dipoles and “beams” become increasingly more useful.
(a) A “half-wave dipole” is simply two conductors (or “arms,” usually wires or tubes) lying along the same line from a common center, each arm being one-quarter of the wavelength of the desired reception frequency (or the center of a narrow band of frequencies). One wire of a two-conductor cable or “feedline” is connected to one arm at the center of the dipole, and the other conductor is connected to the other arm. The most common indoor FM antenna is a “folded dipole” (or “T”), which is a dipole in which each of the arms is folded back upon itself, but the folded parts are spaced away from each other and the free ends are NOT joined at the center. A dipole is slightly directional, with maximum pickup when placed across the direction to the desired source.
(b) A “beam” (or “Yagi beam”) builds upon the dipole by mounting it across a central shaft, and then placing slightly and progressively shorter “director” dipoles in front of it, and slightly and progressively longer “reflector” dipoles behind it. The directors and reflectors are not connected to any feedline; only the dipole that is the “driven element” is connected to the feedline. A beam concentrates signal pickup towards the directors (the front) and minimizes signal pickup from the sides and rear. The feedline should come away from a dipole or beam at a right angle. A number of TV beam antennas incorporate an FM beam section. A beam antenna is particularly useful in “multipath” situations where the signal from an FM broadcast station is being received over one or more paths which are interfering with each other. This usually involves the direct path and one or more signals reflected from hills or mountains or large structures. An FM beam can be pointed along the desired signal path and will then reject the other signals if the paths are sufficiently separated.
e. “Active” antennas (here not including active loops) consist of an indoor or outdoor antenna (usually a single element and called a “probe” if it is fairly short) and a tuned RF amplifier. They can produce good results in some situations but not so good in others. Active antennas usually cover 30 MHz down to 2-3 MHz, but there are some for FM and TV reception.
f. FM broadcast reception is almost always best with an unobstructed line-of-sight (LOS) path between the transmitting and receiving antennas. However, a large metal structure like a water tower that has LOS between itself and the transmitting and receiving antennas can reflect enough signal to provide a better path than a somewhat obstructed direct line.
g. One case study found that:
(1) The most effective FM broadcast receiving antenna was a beam mounted on a tower or mast, as high as possible. If mounted on a mast above a roof, the beam should be at least 4-6 feet above the roof.
(2) The most effective indoor FM antennas were:
(a) A beam mounted in the attic of a house or apartment building that is not made of reinforced concrete and does not have a metal framework, siding, or roof (effects of brick may vary depending on the metal or metal ore particle content of the clay). A small FM beam (3 or 4 elements) should fit in any such attic.
(b) The traditional FM folded dipole, subject to the same building constraints, although the most effective mounting will vary. Generally, keep it stretched out and up high, and in a window if your house isn’t wood.
h. Another directional antenna for FM broadcast use is the “Flying V” or “Flying Vee.” This antenna is a dipole in which the arms are bent towards the desired direction of reception until the angle between them is between 60 and 120 degrees. Because of interaction between the two arms, each is shortened by 3-5 percent. The directionality and signal pickup (which is also called “gain”) seem to increase with the element diameter (in other words, aluminum or copper tubing works better than thick wire, which works better than thin wire).
(1) A half-wave Flying Vee for 88 MHz with a 90 degree center angle and assuming 5% shorter arms will have an arm length of just under 30-5/16 inches, a “width” between the free ends of just over 42-27/32 inches, and a “depth” (from the center to the width line) of just under 21-7/16 inches.
(2) More gain can be obtained with a “5/4-wave” Flying Vee, which starts from a dipole that has a total length of 1.25 (5/4) times the wavelength of the desired frequency (each arm is 5/8 of the wavelength). A 5/4-wave Flying Vee for 88 MHz with a 90 degree center angle and assuming 5% shorter arms will have an arm length of 75-25/32 inches (just under 6-1/3 feet), a width of just over 107-5/32 inches (just under 9 feet), and a depth of just under 53-19/32 inches (just under 4.5 feet). This antenna could easily be attic-mounted, or even ceiling-mounted. A wire version could be floor-mounted under a carpet in a room above the room with the FM receiver, with the feedline “fished” down through the wall.
i. Some ready-made AM broadcast tunable passive loops are the Radio Shack 15-1853 (discontinued), the Terk Advantage AM-1000, and the Select-A-Tenna. The Radio Shack and Terk units and the “M” model of the Select-A-Tenna have short connecting cords to connect to the external antenna terminals of an AM receiver that has them. They can also “inductively couple” to the internal ferrite rod antenna of a portable AM receiver if the receiver and loop are placed close together (and usually at right angles to each other). Please note that the Select-A-Tenna has been produced for over 30 years, so there are older versions (made sometime before 1991) that will only tune up to 1600 kHz. The Select-A-Tenna is also the largest of these three loops (about a foot in diameter) and all are for indoor use (subject to the limits for using an FM beam or dipole indoors). You can put one of these loops on a plastic “Lazy Susan” turntable to position it more easily. If you are using a small AM receiver inductively coupled to the loop, you may be able to find a Lazy Susan large enough for both, so the receiver is always positioned for maximum pickup from the loop.
j. One of the newer AM broadcast active loops is the C. Crane Company “Twin Coil Ferrite.” This is a recently patented design, which may help to keep its price up for a number of years yet, but the antenna has received some good reviews. The “loop” part can be mounted outside a building, connected with a cable to the amplifier inside.
Hopefully this article has given you some ideas on how to improve your radio reception. Don’t be afraid to experiment – you might be pleasantly surprised by the results.