Antennas
How to make a 1/2 Wave Dipole
Needed:
- Wire (speaker wire will work just fine)
- Solder gun & solder
- Measuring tape
- Coaxial cable (ideally ladder line, but you're more likely to have spare coax lying around. For amateur radio usage you'll most likely use 50ohm coax into your radio)
First, you'll have to decide for which frequency you're making your antenna. For amateur radio usage, you'll usually pick a frequency in the middle of the band you're using (i.e. 146MHz for the 144-148MHz 2M band). In this example however, we'll be making our dipole for broadcast FM. Chances are, the antenna for your home stereo radio was just made for the middle of the broadcast FM (88-108MHz FM in North America). If my favourite station is near the end of that range, the reception will be less than optimal. I'll use 106.9MHz as my example favourite radio station.
A dipole is cut to length according to the formula l=468/f(Mhz). Where l is the length in feet and f is the centre frequency. In metric that is l=143/f(Mhz) where l is the length in meters. We'll use Imperial, as the majority of the wiki readers seem to be American.
With that in mind:
468 / 106.9 = 4.38
As this is a 1/2 wave dipole, each side will be 1/4 of the length:
4.38 / 4 = 1.1
So each side of the dipole will be 1.1 feet long. Cut two pieces of wire a little longer than 1.1 feet, as you'll be soldering one end, and attaching the other end to a loop as well. With one end of your coax, you'll want the centre of the coax to be soldered to one wire, and the shielding of the coax soldered to the other matching piece of wire. You will want to make sure these two never touch, they need to be going in a 180-degree direction from eachother. The easiest way to do this is often to place an insulator between the two. In my case, I used a 2" scrap piece of plastic. Once you've verified the two sides are not touching, you can hook the other end of the coax up to your radio. Keep in mind the radiating pattern is 180-degree perpendicular to the wires, so it is directional. Once you've got your dipole pointed towards the tower of the broadcasting station (getting its location, and then using a compass is a simple way), make sure it's well secured. You've now built an antenna optimized for your favourite radio station.
This same process can also be used for television, which is useful for those without cable TV or satellite who are currently using "bunny ears." If you have a favourite TV station, you can optmize your antenna for your favourite channel. Keep in mind that there is a frequency for both audio and video. North American Televsion Frequencies
If you're building this for transmit as well as receive (amateur radio), do NOT transmit until you've tested the SWR.
