Make an FM Antenna
Improving the reception of commercial FM radio (88Mhz - 108Mhz) can be done by you at home, simply by replacing the antenna with a 5/8 wave folded dipole antenna. Many radios and most home stereo receivers include antenna terminals for the connection of an external antenna. Usually the supplied antenna is a minimalist design (sometimes only an "internal" type, or as a telescopic rod or a short length of wire). This can be greatly improved for very little money outlay. All of the items needed are available from a local electronics store or hardware stores.
Steps
- Determine the frequency of the station desired to be tuned. The antenna is tuned to a certain length, based on the frequency of the connected radio. Regardless of the particular frequency, the entire FM broadcast band (88 - 108 Mhz) of the radio's tuner will receive stronger signals from the antenna, with the largest increase seen at the frequency selected in this step, and slightly less as the radio is tuned away from the design frequency.
- Calculate the length of the antenna. The formula for a 5/8 wave antenna using typical 300 ohm "twin lead" is L = 300/f x 5/8 x 1/2; where "L" is the length in meters of the antenna and "f" is the frequency in Mhz of the station to be tuned. This can be simplified to L = 93.75/f.
- An antenna built for middle of the 88Mhz - 108Mhz FM band (98 Mhz) would be: {{safesubst:#invoke:convert|convert}} or 95.66 cm (centimeters) long. For those more familiar with imperial units than metric, the formula to convert centimeters to inches is: cm X 0.3937. This means 95.66cm X 0.3937 = 37.66 inches long.
- Improve the antenna design. The antenna improvement in this article is a design for a simple 5/8 wave "folded dipole" or "T" shaped antenna. This design will outperform any internal or telescopic rod antenna that may have been supplied with the receiver. It is also similar to those provided with some more expensive home stereo receivers.
- To improve upon this simple design, simply double, triple, quadruple, etc. this value as such: 37.66" x 2 = 75.32", or 37.66" x 3 = 112.98", and so forth.
- The 112.98 inch antenna will outperform the 75.32 inch antenna, which outperforms the 37.66 inch antenna.
- Of course, there is a "point of no return" when the multiple is so great that the signal at the ends of the antenna cannot travel the entire length due to the electrical resistance of the wire. This limit is around 100 meters (a little more than the length of a football field).
- Cut the feed line. As described above, this antenna looks like a "T". Thus far, the calculation has been for the top horizontal part (top of the T) of the antenna. A vertical (bottom of the T) needs to be connected to the top horizontal to facilitate connection of the antenna to the receiver's antenna terminals. While both the horizontal and vertical parts function as an antenna, the vertical portion is referred to as the feed line.
- Cut a length of twin lead that is equal to or a multiple of the length calculated above. That will be long enough to extend between the receiver's antenna terminals and the horizontal portion when installed.
- 600 Ohm Ladder Line and 450 Ohm line are physically larger than 300 Ohm Twin Lead and is rated at 600 and 450 ohms respectively; as opposed to 300 ohms for twin lead. These cables can be used, but require the use of a different formula to design. Standard 300 Ohm twin lead cable was selected because it enjoys wide availability.
- Prepare the antenna for connection to the feed line. Locate and mark the halfway point on the length of the horizontal portion of the antenna.
- Use a razor knife to cut a 1 inch (2.5cm) long slit (centered on the halfway mark), parallel to and between the two wires in the antenna's twin lead cable.
- Cut through one of the twin lead wires at the halfway mark.
- Strip the insulation from the cut ends of the wire at the halfway point and the ends of the entire horizontal length (this should be about a 1/2 inch (1.27cm) from each side).
- Prepare the feed line for connection to the antenna. Use a razor knife to cut a slit between the wires of the twin lead about an inch (2.5cm) in length at both ends. Carefully remove a half inch (1.27cm) of insulation from each of the wires on both ends.
- Tin the exposed twin lead wires with solder. Re-twist the individual strands of all wires to keep them tightly grouped together. If soldering is not an option, skip to the next step after stripping insulation from the wires indicated at the end of this step.
- Apply a small amount of electrical grade soldering flux (do not use plumber's flux as it contains acid). A small 20 to 50 watt soldering gun or iron to heat the wires is sufficient.
- Shortly after the flux melts, apply electrical grade solder to the wire near the soldering gun's / iron's tip (use of straight solder or flux core solder is fine - but do not use acid core solder).
- Apply just enough solder to the heated wire so that melted solder flows back into the insulation, then remove the solder and heat from the wire. Do this for both wires at (1) both ends of the feed line, (2) both wires at both ends of the horizontal antenna piece and (3) both wires at the cut made at the center of the horizontal antenna piece.
- Solder the antenna and feed line together. Solder the two wires at one end of the horizontal piece together and repeat for the other end (if not soldering, create a solid electro-mechanical connection by tightly twisting the wires together instead of soldering).
- Lay an end of the feed line at the center of the horizontal antenna piece so that the soldered leads of are close to each other. The left wire of the feed line should be soldered to the left wire of the antenna, and the right wire of the feed line should be soldered to the right wire of the antenna.
- When completed correctly, a path can be traced that starts on one wire in the feed line that continues to one wire in the bottom of the antenna and continues to one end of the antenna. It continues across the top wire of the antenna to the opposite end. It begins the return on the other bottom wire of the antenna to the other wire in the feed line and finally terminates at the end of the feed line.
Tips
- A "300 to 75" ohm balun will be needed if the receiver offers only a 75 ohm (coaxial cable)antenna connection. These are devices that allow for the connection of 300 ohm twin lead that converts to and provides a screw-on 75 ohm connector.
- The antenna constructed here is "balanced" and will be inconvenient to connect to the typical telescoping antenna, which is "unbalanced". If there is no external antenna connector on your radio, you might do just as well by attaching a length (any length, but the longer the better) of wire (any kind) to the existing antenna and routing the wire up (the higher the better) and toward the transmitter you want to receive.
Warnings
- Antennas that are placed outdoors should have lightning protection on the feed line.
Things You'll Need
- 300 ohm twin lead antenna wire
- 20 - 50 watt soldering iron / gun
- Solder w/ rosin flux core (not plumber's solder)
- Flux (not plumber's flux) (if not integral to solder wire)
- 300 / 75 Ohm balun (if needed)
- Wire stripper
- Wire cutter
