DFW (Dallas-Fort Worth) Area
WFAA Channel 8
OTA TV Antenna
Page 2
Page 1 - Problem Definition, Background & Research
Just for future reference, I am including the basic frequency information for the UHF channels. These have a 6 MHz bandwidth, so it's easy to figure for an individual channel - for example, channel 14 uses 470-476 MHz.
| UHF Channel Frequencies | |
| 14–20 | 470–512 MHz |
| 21–51* | 512–698 MHz |
| channels 52–69** | 698–806 MHz |
*channel 34 used for radar, channel 37 used for radio astronomy
**to be auctioned for other uses once conversion to digital TV has been accomplished.
CALCULATIONS
Using the formula from the diagram, my basic calculation was to figure the wavelength for the mid-range frequency for channel 9 (WFAA is on channel 8, but their HD signal is on channel 9). Frequency range for channel 9 is 186 - 192 MHz - so I used 189.
I calculated the wavelength in meters as 0.93 * 300 / 189 = 1.476 meters
1.476 meters * 39.37 inches/meter = 58.12 inches for a full wavelength.
To make a folded dipole that is half the wavelength long, we need to use the full 58.12 inches, folded around a 2-3" pipe such that the ends are about 2" apart. We're shooting for the antenna to be about 29 inches (a half wave length) from end to end.
I rounded mine off at 58"- earlier, I used the middle of the frequency range, so the missing 0.12" should have little to no effect.
CONSTRUCTION
For material, I needed bare metal, something that would hold it's shape - I had some 00 copper grounding wire left over from another project, so I chose to use that. I cut off my 58" piece, and marked the center of the wire at 29". Then I marked each side at 14" out from the center mark in both directions.
Using a piece of 2" PVC pipe that I had laying around, I put the 14" mark on top of the pipe, grabbed the loose end and wrapped the wire around the pipe.
I then twisted some 14 gauge bare copper wire around the connection points, and used a propane torch, flux and lead free solder like a plumber uses to solder leads. Then I soldered on an outdoor 300-75 Ohm transformer that I had left over from who knows what.
Here's what I ended up with - like I said, crude, but simple and effective. The particular one pictured here is actually a half wave length folded (29") - it still works fine. But now I'm curious if a full wave length would do any better, so next I will build the full wave length model as outlined above. I will post a photo of that as well as any differences in my results.