To improve the reception of the commercial FM band (88Mhz - 108Mhz) in a traditional way you could try to replace the antenna you use with a folded dipole antenna at 5/8 wave. Most home stereos and most radios are equipped with special terminals that allow you to connect an external antenna. Generally, the one supplied with these devices is very minimal (it can be built-in, a telescopic antenna or simply a piece of wire). It is possible to make a better antenna for very little money. Everything you need can be purchased at a hardware store or electronics store.
Steps
Step 1. Determine the frequency you want to tune to
The antenna must be of a certain length, based on the frequency the radio is tuned to. Regardless of the frequency, the whole FM band (88 - 108Mhz) will have a more powerful reception thanks to the antenna, with a greater increase around the frequency on which it will be calibrated, and a little less as the one on which the radio moves away from it.
Step 2. Calculate the length of the antenna
The formula used to calculate the length of a 300 ohm dual wire 5/8 wave antenna is L = 300 / f x 5/8 x1 / 2; where “L” is the length of the antenna in meters and “f” is the frequency (in MHz) you want to receive. This formula can be simplified to L = 93.75 / f.
For example, an antenna calibrated to a frequency of 98 MHz approximately halfway in the FM band (88 Mhz - 108 Mhz) will be 0, 9566 meters or 95, 66 cm long. If for some reason the measurements in inches are convenient for you, you can convert the measurements from cm to inches using this formula: cm X 0, 3937. So 95, 66cm X 0, 3937 = 39, 66 inch
Step 3. Upgrade an existing antenna
In this article we will see how to enhance a simple 5/8 wave "folded dipole" antenna, also known as a "T antenna". This design allows it to outperform any standard internal or telescopic antenna. It is similar to the antennas mounted on more expensive, high-end stereo tuners.
- To further improve reception, double, triple or quadruple the measurement you obtained using the above formula. For example: 95, 7cm x 2 = 191, 4, or 95, 7 x 3 = 287, 1 and so on.
- Thus, a 287cm long antenna will receive better than a 191.4cm long antenna, which in turn will offer better quality than the 95.7cm long antenna.
- However, there is a “point of no return”, where the multiple is so high that the signal at the end of the antenna cannot travel along the entire length of the antenna due to the electrical resistance of the cable. This limit is around 100 meters (about the length of a football field).
Step 4. Cut the cable
As described above, this type of antenna resembles a "T". The formulas explained so far are used to calculate the horizontal length of the antenna. To this horizontal part, it is necessary to add a vertical one that serves to facilitate the connection of the antenna to the appropriate terminal of the receiver. Although both parts are part of the antenna, the vertical one has a specific name: “feed line”.
- Cut the double antenna cable of length equal to a multiple of the previously calculated value. The cable must be long enough to go from the receiver terminal to the horizontal part of the antenna above.
- The 600 Ohm and 450 Ohm ladder lines are physically larger than the 300 Ohm dual antenna cable, with values of 600 and 450 ohms respectively, as opposed to the 300 ohms of the dual cable. You can use these cables if you want, but you will need to use a different formula to calculate their length. In this guide we will use the normal 300 ohm cable due to its easy availability.
Step 5. Prepare to connect the antenna to the feed line
Find and mark the exact half of the horizontal part of the antenna.
- Use a utility knife to cut a 2.5 cm (1 inch) section in the exact center between the two horizontal antenna cables.
- Cut one of the wires of the double cable that are on the mark in the center of the horizontal part of the antenna.
- Remove the sheathing insulation at the beginning of the cables and also in the middle, as in the photo. You should take off about 1.27 cm (1/2 inch) on each side.
Step 6. Prepare the feedline
Use a utility knife to cut the feedline cable in the center, to divide the two wires and create a gap of about 2.5cm, and strip the beginning of the cables (about 1.27cm) as you did in the previous step.
Step 7. Solder the exposed wires together
Twist the loose threads together so that they remain firmly in place. If you can't pay, go straight to the next step.
- Apply a small amount of electronic solder flux (do not use what plumbers use because it contains acids). A small 20-50 watt soldering iron will be enough to heat the wires.
- Immediately after melting the flux, solder the wires using electronic tin by placing it on the wires and bringing the soldering iron tip closer (use flux tin as well, but do not use metal solder alloys that contain acids).
- Use just enough tin to flow slightly into the cable insulation. Repeat the process for both cables (1) at the end of the feed line, (2) both cables at the end of the horizontal part of the antenna, and (3) both cables you cut in the center of the horizontal piece.
Step 8. Solder the antenna and feedline together
Solder the two wires at one end of the horizontal part and repeat for the other part (if you don't have a soldering iron available, electromechanically connect the wires by twisting the wires tightly instead of soldering them).
- Bring the end of the feedline closer to the center of the horizontal part of the antenna so that they stay close together. The left feedline wire should be soldered to the left antenna wire while the right feedline wire should be soldered to the right antenna wire.
- If you have followed all the steps correctly, it will be possible to detect the continuity of the line along the various connections from one end to the other for each of the two poles.
Advice
- If your receiver has only a 75 ohm coaxial cable antenna connection, you will need a 300 - 75 balun. These devices convert the dual cable's 300 ohm signal into a 75 ohm signal.
- The antenna covered in this guide is a "balanced" antenna, not recommended for connections with telescopic antennas, which are "unbalanced". If your radio does not have an external antenna socket, you could simply connect a piece of electrical wire of any length (the longer it is, the better the reception) to the existing antenna, placing it as high as possible in the direction of the transmitting station you wish to receive.
