FM Dipole

FM band lies between 88MHz and 108MHz. Most often used antennas in this band are dipole or some variant of it, and higher gain is achieved by vertical stacking. Polarization used is vertical or circular. We will be describing vertically polarized dipoles only.

Dipole

Indoor wire dipole antenna 72 Ω

For receiving only, we will choose 100MHz as our target frequency as it will cover all the band.

Materials:

1. Any twin-lead cable,
2. RF connector.

Construction

Construction details and Measurements are for Registred Users only

Indoor wire folded dipole antenna 300Ω

It is a variant of the previous antenna, with slightly better bandwidth. The choice should be based on the impedance of the FM receiver.

Materials:

1. Any twin-lead cable,
2. RF connector.

Construction

Construction details and Measurements are for Registered Users only.

Wire dipole 72 Ω

This is the simplest outdoor FM dipole. It is very simple to make, but the pipe version is recommended because it has greater bandwidth. This version will not work well at the edges of the band.

Materials:

1. PVC pipe with 22mm diameter,
2. Square metal piece,
3. Two U shape screws,
4. PVC pipe holders,
5. Wire clamps,
6. Steel or brass wire,
7. RG-6 coaxial cable.

Construction

Construction details and Measurements are for Registered Users only.

Dipole from aluminum pipes

This is the best choice for outdoor dipole if the FM receiver has impedance 75Ω.

Materials:

1. Two aluminum pipes with diameter 6mm,
2. PVC pipe with 22mm diameter,
3. Square metal piece,
4. Two U shape screws,
5. PVC pipe holders,
6. T-shape PVC pipe connector,
7. Two 5mm screws and small metal strips,
8. RG-6 coaxial cable.

Construction

1. Do steps 1-3 from the previous antenna,
2. Make holders for pipes from screws ⌀5mm (lower-right photo),
3. Put RG-6 cable through the plastic pipe and connect hot-lead and ground to each screw,
4. Isolate them with hot-glue,
5. Cut the ⌀6mm aluminum pipes to the measure,
6. Cut the ⌀8mm aluminum pipe to the length of a few centimeters (we need two of them),
7. Put the ⌀8mm pipe pieces over the ends of the ⌀6mm pipes (lower-right photo),
8. Drill through the axes inside ⌀6 pipes with ⌀4, we want it a bit wider),
9. Make ⌀5mm threads inside each pipe,
10. Close the other ends of the pipe with hot-glue,
11. Assemble – screw pipes onto the holder.

Note: This antenna has been tested to withstand strong winds for years, but it is not guarantee how strong storms can it withstand. Folded dipole is much stronger.

Folded dipole from aluminum pipes

This antenna is recommended when FM receiver has 300Ω impedance. It is the best receiving dipole, because it has the greatest bandwidth.

The instructions for this antenna are the same as for the next antenna, but we can use thinner pipes.

Transmitting folded dipole antenna

The construction is evident from the image. It is important that folded dipole is isolated from the metal boom. PVC boom can be used as well or the plastic or silicone standoffs on metal boom.

The thicker the pipe (the larger cross-section area), the more power it can handle. The diameter of the pipe is proportional to the bandwidth.

High-Power transmission

Special care should be taken so that the connectors, cable and BalUn can handle the power. RG-58 is often not suitable for higher power signals. PL and SO239 are good choice of connectors for this application. If BalUn is made from a coaxial cable, not transformer, it should be made with at least RG-213 cable.