View Single Post
Old 18-Jul-2013, 5:49 PM   #41
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
Long Yagi for VHF (Ch 8-11)

Time: Summer/Fall 2010
Location: KY

Yagis (channel-cut for 10, Wade 10y10s) were used as raw material and a starting point in the design. Channel 10 was selected because it seemed a good compromise for the driven element between Ch. 8 and Ch 11. Wade Yagis used a 3-bar folded dipole for the driven element. Such a folded dipole is probably broadband enough to cover channels 8 through channel 11 with good impedance match. Also, since channel 10 antennas generally have longer elements than channel 11 antennas, this allowed flexibility in trimming elements (longer than desired elements can be trimmed).

The front three elements from a YA-1713 were added to the channel 10 antenna. Repeated testing and 4NEC2 analysis were performed. Eventually, an extra length of tubing was added containing two more elements, repeat analysis and testing. Then, all elements were analyzed in 4NEC2 for length and location. Gradually the element positions and lengths were changed toward the 4NEC2 specified parameters, with testing in between small steps to confirm the direction of the change.

When it was thought the antenna was near optimized for channel 11, then searches were performed in 4NEC2 to determine if channel 8 could be improved without adversely affecting channel 11 performance. Few changes were available. A change was made, lengthening the reflector improved gain by more than one dB on channel 8 and reduced gain for channel 11 by maybe 0.1 dB. Measurements confirmed these differences.

Final element progression illustrated in images below:

- 13 elements
- 14 elements
- 15 elements







The antennas were made from parts of:

1. Wade Channel-cut Yagi (trimmed directors, less reflector)
2. Winegard YA-1713 (reflector)
3. Winegard YA-1713 (front three elements trimmed)
4. Portion of Channel Master boom (portion extending out front)
5. Channel Master elements (front directors, trimmed)
6. Portion of Antennacraft boom (support boom)
7. Homemade support boom brackets

The final 15 element Yagi had total length ~170 inches.

Two identical antennas were constructed.

Later, a pair of these long-Yagis were stacked and performance compared to stack of Winegard YA-1713 Yagis.

.
Attached Images
File Type: jpg 13 element Yagi.jpg (57.0 KB, 3989 views)
File Type: jpg 14 element Yagi.jpg (87.2 KB, 3801 views)
File Type: jpg 15 Element Yagi.jpg (74.5 KB, 3806 views)
tripelo is offline   Reply With Quote