How to Make an FM Antenna
Determine the frequency of the station desired to be tuned., Calculate the length of the antenna., Improve the antenna design., Cut the feed line., Prepare the antenna for connection to the feed line., Prepare the feed line for connection to the...
Step-by-Step Guide
-
Step 1: 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. , The formula for a 5/8 wave antenna using typical 300 ohm "twin lead" antenna lead-in line 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.
These instructions do not apply to round 50 and 75 ohm coaxial cables.
An antenna built for middle of the 88Mhz
- 108Mhz FM band (98 Mhz) would be:
0.9566 meters (3.1 ft) 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.
This is a dimension for the top portion of the "T". , 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 original
37.66 inch antenna will be outperformed by a
75.32 inch version; and it will be out performed by a
112.98 inch version
- and so on.
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). , 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 and in this wiki are the same type of cable, 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 AND 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. , 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). , 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., 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 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 complete, the antenna will look like a large "T" When completed correctly, a path can be traced that starts "up" on the right side wire in the feed line cable at the bottom (or vertical) of the "T" that continues via connection made to the wire in the bottom right half of the horizontal half of the "T" and continues to the twisted wire connection at the right-most end on the horizontal "T".
The path continues across the entire top of the "T" of the antenna to the twisted wire connection at the left-most end.
Here, the path continues on the wire in the bottom left horizontal half of the "T" to the twisted wire connection to left side wire in the vertical feed line of the "T".
The path continues "down" the vertical and finishes at the end of the left side wire
- next to where the path started. -
Step 2: Calculate the length of the antenna.
-
Step 3: Improve the antenna design.
-
Step 4: Cut the feed line.
-
Step 5: Prepare the antenna for connection to the feed line.
-
Step 6: Prepare the feed line for connection to the antenna.
-
Step 7: Tin the exposed twin lead wires with solder.
-
Step 8: Solder the antenna and feed line together.
Detailed Guide
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. , The formula for a 5/8 wave antenna using typical 300 ohm "twin lead" antenna lead-in line 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.
These instructions do not apply to round 50 and 75 ohm coaxial cables.
An antenna built for middle of the 88Mhz
- 108Mhz FM band (98 Mhz) would be:
0.9566 meters (3.1 ft) 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.
This is a dimension for the top portion of the "T". , 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 original
37.66 inch antenna will be outperformed by a
75.32 inch version; and it will be out performed by a
112.98 inch version
- and so on.
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). , 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 and in this wiki are the same type of cable, 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 AND 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. , 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). , 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., 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 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 complete, the antenna will look like a large "T" When completed correctly, a path can be traced that starts "up" on the right side wire in the feed line cable at the bottom (or vertical) of the "T" that continues via connection made to the wire in the bottom right half of the horizontal half of the "T" and continues to the twisted wire connection at the right-most end on the horizontal "T".
The path continues across the entire top of the "T" of the antenna to the twisted wire connection at the left-most end.
Here, the path continues on the wire in the bottom left horizontal half of the "T" to the twisted wire connection to left side wire in the vertical feed line of the "T".
The path continues "down" the vertical and finishes at the end of the left side wire
- next to where the path started.
About the Author
Kimberly Sanchez
Kimberly Sanchez is an experienced writer with over 11 years of expertise in education and learning. Passionate about sharing practical knowledge, Kimberly creates easy-to-follow guides that help readers achieve their goals.
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