Saturday, September 12, 2009

AM Band Antennas

wpe1A.jpg (148146 bytes)

A good AM Band antenna can be a simple long-wire strung between two trees or across the top of the roof. Even a modest length wire will give your receiver greatly improved reception with less static because the signal pickup is occurring some distance from the interference generating appliances in the house. An insulator mounted high in a tree so that the wire has a large vertical rise will give great results. Mount the antenna as high and as far from the house as practical. Use a good quality ceramic insulator for holding the wire and add a commercial lightning arrestor where the antenna meets the house (Fig.1). Ceramic insulators are available with built-in wood screws and can be screwed into a tree or the wood parts of the house by hand. The wire may be tied to the far insulator as shown but the wire will stretch with time and require adjustment. A "trick" is to pass the wire through the insulator and fasten a fishing weight to the end so that the weight hangs a few inches below the insulator. As the tree sways in the breeze, the weight will move up and down and the wire will remain straight! (I must confess that my long wire antenna is made with insulated wire thrown over a branch with a rock tied to the end... Who has time to do it right? I do have an arrestor, however!) A good place for the arrestor is directly above the point where the water line enters the house. Run a heavy gauge ground wire straight down to the water pipe and attach it with a brass grounding clamp (assuming a copper water pipe - always use compatible materials or corrosion will result.). This connection also makes an excellent ground for the receiver. A shielded cable lead-in wire can give improved results when the residence has unusually noisy appliances. If you do not have an AM radio with a coaxial antenna jack then consider using an auto radio. Auto radios are well shielded to prevent ignition noise from interfering with reception and all that is needed to make a superior receiver for the home is a 12 volt power supply and a speaker. Inexpensive AM car radios are quite common, being discarded for fancy stereo upgrades and even the cheapest car receiver will outperform most home radios.

Fig. 2 shows how to add a loading inductor to the antenna in the event that a little more signal strength is desired. Most long wire antennas will be considerably less than 1/4 wavelength at AM band frequencies and behave as though a small capacitor is connected in series. The inductor resonates with this capacity and will increase the signal strength significantly. The required inductance range is from about 200 microhenry at the high end of the band to about 2 millihenry at the bottom end of the band for a 20 foot antenna.

wpe1B.jpg (158147 bytes)

Fig. 3 shows how to build a programmable loading coil using a 4.5 inch PVC coupling (found in the plumbing supply area of the local home improvement store) and 22 gauge insulated wire. The coil is wound with 100 turns with taps brought out every 10 turns by twisting a little loop in the wire The total inductance of this inductor is about 1 millihenry so short antennas may need more turns for the lower frequencies.

.wpe22.jpg (217154 bytes)



Post a Comment