Homemade Yagi antenna - design and model

In addition to the Gray-Hoverman antenna I designed and built, I wanted to construct a single-channel Yagi antenna to try and receive channel 19 - the one weak signal in my area.  As with the Gray-Hoverman design I used 4nec2 to design and model the antenna.


Here's the actual NEC file.  Feel free to download, use, modify, etc.  If you're interested in building this model, I suggest using this file and 4nec2 to get the exact dimensions of each element.

CM D--EVAL --num-cores=2 --uhf
CE
SY RAD=0.003175
SY FRAD=0.00215
SY DIPOLE_WIDTH=0.3
SY DIPOLE_HEIGHT=0.039
SY DIPOLE_A=DIPOLE_HEIGHT/3.41421356
SY DIPOLE_Z=DIPOLE_HEIGHT/2
SY DIPOLE_CENTER=0.013
SY REF_Y=0.296
SY REF_X=0.14
SY EL1_Y=0.261
SY EL1_X=0.07
SY EL2_Y_DIFF=0.008
SY EL2_X_DIFF=0.151
SY EL2_Y=EL1_Y-EL2_Y_DIFF
SY EL2_X=EL1_X+EL2_X_DIFF
SY EL3_Y_DIFF=0.008
SY EL3_X_DIFF=0.2
SY EL3_Y=EL2_Y-EL3_Y_DIFF
SY EL3_X=EL2_X+EL3_X_DIFF
SY EL4_Y_DIFF=0.002
SY EL4_X_DIFF=0.214
SY EL4_Y=EL3_Y-EL4_Y_DIFF
SY EL4_X=EL3_X+EL4_X_DIFF
SY EL5_Y_DIFF=0.013
SY EL5_X_DIFF=0.2
SY EL5_Y=EL4_Y-EL5_Y_DIFF
SY EL5_X=EL4_X+EL5_X_DIFF
SY EL6_Y_DIFF=0.007
SY EL6_X_DIFF=0.278
SY COND=2e+07
GW 1 1 0 -DIPOLE_CENTER/2 -DIPOLE_HEIGHT/2+DIPOLE_Z 0 DIPOLE_CENTER/2 -DIPOLE_HEIGHT/2+DIPOLE_Z FRAD
GW 2 9 0 -DIPOLE_WIDTH/2 DIPOLE_HEIGHT/2+DIPOLE_Z 0 DIPOLE_WIDTH/2 DIPOLE_HEIGHT/2+DIPOLE_Z RAD
GW 3 5 0 -DIPOLE_WIDTH/2 -DIPOLE_HEIGHT/2+DIPOLE_Z 0 -DIPOLE_CENTER/2 -DIPOLE_HEIGHT/2+DIPOLE_Z RAD
GW 4 5 0 DIPOLE_WIDTH/2 -DIPOLE_HEIGHT/2+DIPOLE_Z 0 DIPOLE_CENTER/2 -DIPOLE_HEIGHT/2+DIPOLE_Z RAD
GW 5 1 0 -DIPOLE_WIDTH/2 -DIPOLE_HEIGHT/2+DIPOLE_Z 0 -DIPOLE_WIDTH/2-DIPOLE_A -DIPOLE_HEIGHT/2+DIPOLE_A+DIPOLE_Z RAD
GW 6 1 0 -DIPOLE_WIDTH/2 DIPOLE_HEIGHT/2+DIPOLE_Z 0 -DIPOLE_WIDTH/2-DIPOLE_A DIPOLE_HEIGHT/2-DIPOLE_A+DIPOLE_Z RAD
GW 7 1 0 -DIPOLE_WIDTH/2-DIPOLE_A -DIPOLE_HEIGHT/2+DIPOLE_A+DIPOLE_Z 0 -DIPOLE_WIDTH/2-DIPOLE_A DIPOLE_HEIGHT/2-DIPOLE_A+DIPOLE_Z RAD
GW 8 1 0 DIPOLE_WIDTH/2 -DIPOLE_HEIGHT/2+DIPOLE_Z 0 DIPOLE_WIDTH/2+DIPOLE_A -DIPOLE_HEIGHT/2+DIPOLE_A+DIPOLE_Z RAD
GW 9 1 0 DIPOLE_WIDTH/2 DIPOLE_HEIGHT/2+DIPOLE_Z 0 DIPOLE_WIDTH/2+DIPOLE_A DIPOLE_HEIGHT/2-DIPOLE_A+DIPOLE_Z RAD
GW 10 1 0 DIPOLE_WIDTH/2+DIPOLE_A -DIPOLE_HEIGHT/2+DIPOLE_A+DIPOLE_Z 0 DIPOLE_WIDTH/2+DIPOLE_A DIPOLE_HEIGHT/2-DIPOLE_A+DIPOLE_Z RAD
GW 11 9 -REF_X -REF_Y/2 0 -REF_X REF_Y/2 0 RAD
GW 12 9 EL1_X -EL1_Y/2 0 EL1_X EL1_Y/2 0 RAD
GW 13 9 EL2_X -EL2_Y/2 0 EL2_X EL2_Y/2 0 RAD
GW 14 8 EL3_X -EL3_Y/2 0 EL3_X EL3_Y/2 0 RAD
GW 15 8 EL4_X -EL4_Y/2 0 EL4_X EL4_Y/2 0 RAD
GW 16 7 EL5_X -EL5_Y/2 0 EL5_X EL5_Y/2 0 RAD
GE    0
LD    5    0    0    0    COND
GN    -1
EK
EX 0 1 1 0 1 0 0
FR    0    7    0    0    500    1
RP    0    1    73    1510    90    0    1    5    0    0
EN

The big question is - after construction, how did it perform?  I compared the antenna to a Channel Master 4-bay antenna (CM4221), which is kind of an unfair comparison since the Channel Master is a wide-band antenna whereas the yagi is designed for one frequency.  Anyway, I mounted each antenna in the same location and used my TV to scan for all channels.  For each digital channel I wrote down the average signal strength.  My TV reports two signal numbers, the overall signal level in percentage, and the signal-to-noise ratio in decibels.  For both numbers, higher is better.

Channel	CM4221	Yagi (ch. 19)
15	98% (32db)	98% (31db)
16	89% (26db)	89% (27db)
19	56% (15db)	62% (20db)
24	95% (29db)	98% (30db)
31	84% (25db)	76% (22db)
34	89% (27db)	78% (23db)
41	87% (26db)	87% (26db)
50	98% (32db)	62% (17db)

As you can see, at the design frequency (ch. 19) the yagi outperforms the Channel Master.  For the most part, all other channels the Channel Master outperformed the yagi (as expected).  I was surprised to see channels 16 and 24 where better than the Channel Master, probably because those channels aren't far off of the designed channel.

Even with the antenna specifically designed for one channel, I was disappointed with the results.  62% is very low, I expected at least 80.  But I think the main problem was the mounting location which was 1 foot away from and behind my house from the broadcast antenna.  I could move the antenna and try again, but it's actually moot.  In the few weeks it took me to design, build, and test this antenna, channel 19 installed a low power digital repeater closer to my house.  So I no longer need this dedicated yagi antenna.  I guess you could say it was obsolete before it was completed.  Oh well, good thing it was relatively inexpensive to construct.  I estimate the cost of materials at $20.


Here is a youtube video I uploaded showing design and construction of the antenna.


Here are download links useful to antenna modeling:
4nec2 - http://www.qsl.net/4nec2/
nikiml's python scripts - http://clients.teksavvy.com/~nickm/scripts.html
Optimized versions of the NEC2 engine - http://users.otenet.gr/~jmsp/