Part 15 AM BC applications

For most short range applications a 4 foot whip antenna in parallel with a 56 ohm resistor may be connected to J2. A maximum field strength of 24000/f(kHz) microvolts per meter at 30 meters is allowed between 510 and 1600 kHz. This is 15 uv/m at 1600 kHz 100 feet from the transmitter. Below 490 kHz, 2400/f (kHz) at 300 meters is allowed (This is 12 uv/m at 1000 feet). These levels are that of a weak AM broadcast band station. Alternatively, 100 mw of RF max into a 10 foot antenna is allowed in the AM broadcast band, and 1 watt of RF is allowed max into a 50 foot antenna between 160 and 190 kHz. These figures actually assume 100 percent power amplifier efficiency and the power is defined as input to the final RF amplifier stage. This is from the vacuum tube days when RF power was not easily measured with simple equipment. However, nowadays solid state RF amplifiers are specified in terms of RF output. An efficiency of 60-80% is typical of transistor power amplifiers, although 90% or more may be reached in some instances. Therefore, to comply with part 15 of the FCC rules, either make sure the RF output is kept at 1.6 to 2V rms or less into a 50 ohm load, or use the unmatched 4 foot whip antenna. DO NOT use the 100 mw input into a 10 foot antenna unless you can measure and verify the RF output power. Although you may operate anywhere in the AM band, the high end (1600 kHz) is to be preferred as antenna efficiency is better.

Analysis of a common 8.5 foot steel whip of the kind used for CB radio setups in autos and trucks shows a typical impedance of 0.12 - j4700 ohms at 1.65 MHz. This was done with a computer program called ELNEC, widely available from many shareware sources or bulletin boards, or FTP sites.This was done assuming average ground and with eight ground radials under the antenna. An inductance of about 450 to 500 microhenries would be needed to cancel out the capacitive reactance. Assuming a slug tuned coil with a Q factor of 100 (typically that found in an AM radio tuned circuit), the antenna would have a feed impedance close to 50 ohms and a gain of -26 dBi. This is an efficiency of 0.25 percent, typical for loaded 8 foot radiators in this frequency range. With a very large coil having a Q factor of 300-500, this could be improved a few dB but the inevitable ground loss resistance is a factor limiting this figure, being 15 to 40 ohms typically. Theoretically a figure of 173 millivolts per meter is the field strength at one km per kilowatt of radiated power. One milliwatt radiated power would give therefore 173 microvolts per meter at one kilometer (107 microvolts per meter at 1 mile). Assuming an effective radiated power of a quarter of a milliwatt using the AM88 with an 8.5 foot (102 inch) whip antenna that is properly matched, and allowing another 6 dB loss, signal strengths of 15 to 20 microvolts per meter could be produced at one kilometer with this setup. A good AM receiver using a loop antenna (sensitivity 5-10 microvolts per meter for 10 dB signal to noise ratio) will easily be able to hear this signal at one kilometer.

A center loaded 10 foot radiator may also be used. This involves splitting the radiator into two sections and adding an inductance to bring the radiator to resonance at the desired frequency. Approximately enough inductance is needed to resonate with the self capacitance of the top whip antenna. For 1600 kHz this will be on the order of 400 to 1000 microhenries, depending on whip length and diameter, as well as exact frequency. A good RF ground system is required, and antenna bandwidth of 10 kHz is typical. Radiation resistances of 0.1 to 0.3 ohms are typical, and the radiation efficiency of a system such as this will be a few percent at best, assuming ideal grounding, and 0.5 % for the typical home experimenter setup. However, experience at 1880 kHz with 160 meter Amateur mobile operation, using 10 watts AM and a center loaded 8 foot whip mounted on an automobile, shows that 2 way contacts at 50 to 100 km (30 to 60 miles) are possible and fairly common. Extrapolating this data based on theory, with 100 mw, (20 db below 10 Watts) therefore, ranges of 5 to 10 km. (3 to 6 miles) would seem possible without violating FCC rules. However, noise and interference will be the main limitation. Its all in the location and antenna system, and how well everything is tuned and matched.

However, the rules specifically state a ten foot antenna and ground length. The wire used in the loading coil could count if it radiates. A helical whip using 200-300 ft of wire wound on a pole 8 to 10 feet long would physically appear to be a legal antenna especially if it has a plastic jacket However, these helical antennas commonly used for HF mobile SSB communication would likely be illegal unless the wire was wound in a specific configuration so as not to have more than 10 feet radiating. This is a continuously loaded antenna and you cannot easily separate the inductor from the antenna, so some kind of legal interpretation and clarification may be needed. Use of these antennas with their legality question is at the discretion of the experimenter. Operating in the 160 meter ham band (General Class License needed - A Code Free Technician class WILL NOT DO) sidesteps these problems of legality as long as the transmitter is not used for commercial purposes of any kind.

Part 15 160-190 kHz (1750 meters)

In this band, no license is required (although a number of experimenters using this band are also licensed hams). CW (morse code) is predominant owing to the weak signal narrow band capability of this mode. Some experimenters have set up CW radiobeacons for others to try to copy and send them reception reports. Distances of 15 - 50 km (10 to 30 miles) are routine, with a few reports of over 150 km (100 miles), and one instance, 1300 km (800 miles). This latter feat requires a very low noise rural receiving location away from power lines and other sources of man made noise, very well tuned and matched antennas, and a very sensitive (<.01 microvolt) narrow band (200-400 Hz wide) receiver. This is a feat that requires knowledge, dedication, and patience, but it shows what can be done. A receiver of 200 Hz bandwidth and 1 dB noise figure has a -157 dBm noise floor ( 0.003 microvolts into 50 ohms approximately ) and a good CW operator can copy a very weak Morse code signal equal to the noise level. Considering the efficiency of a 50 foot vertical antenna at 180 kHz is probably less than a fraction of a percent ( 0.3 - 0.5% would be good ), this is not bad DX for 3 to 5 milliwatts effective radiated power. More details of this mode of operation can be obtained from the Longwave Club of America, 45 Wildflower Rd, Levittown, Pa. 19057 (215) 945-0543. If you can locate a copy of it, an excellent source of information is the "Low and Medium Frequency Scrapbook", by the late Ken Cornell, W2IMB, published by Communications Technology Inc, 1977. This is a classic in this field and still is an excellent reference. The circuits are somewhat dated (some vacuum tube) but naturally, the principles are still the same today and the ideas still apply. The antenna information and experimental data are a gold mine of information in this field. A number of experimenters publications (QST, CQ magazine, Ham Radio (defunct), and 73 magazine, as well as Electronics Now (formerly Radio Electronics) and Popular Electronics have run articles in the past on the subject. Check on the internet for these and other sites.


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