Yagis for TV Channels 713

I noticed that the 174216 MHz VHF-TV band was almost exactly twice the frequency of the 88108 MHz FM broadcast band. Since the relative bandwidths were nearly equal, I scaled a small FM Yagi to the TV band and then reoptimized it. The resulting antenna has a 32″ boom, 32″ longest element, and direct 75Ω feed. Forward gain is 5.3–7.2 dBd and the worst backlobe is 21–23 dB down across the band. The clean pattern can suppress multipath and co-channel interference.

I designed the antenna using the AO 9.50 Antenna Optimizer. This image shows the antenna geometry. The red dot is the feedpoint. The bent driven element increases F/B at the low end of the band and the gain everywhere.

Modeling Results

Calculated performance is for 28 analysis segments per halfwave. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.

Channel  Frequency  Impedance    SWR   Mismatch  Conductor  Forward     F/R
            MHz        ohms             Loss dB   Loss dB   Gain dBd     dB 
    7       177    67.9 - j3.7   1.12     0.01      0.01      5.34     23.31
    8       183    87.4 - j3.6   1.17     0.03      0.01      5.35     22.47
    9       189    94.4 - j6.9   1.28     0.06      0.01      5.51     21.36
   10       195    91.5 - j7.0   1.24     0.05      0.01      5.87     21.44
   11       201    81.6 + j0.1   1.09     0.01      0.01      6.39     22.26
   12       207    72.1 + j16.1  1.25     0.05      0.01      6.85     22.59
   13       213    75.9 + j26.5  1.42     0.13      0.03      7.14     22.01

Antenna File

174-216 MHz Yagi
Free Space Symmetric
174 177 180 186 192 198 204 210 214 216 MHz
5 6063-T832 wires, inches
ang = 16.25619
r = 16.19217
de = 14.82229
d1 = 12.91589
d2 = 12.36376
d3 = 11.0753
dep = 7.896689
d1p = 10.70153
d2p = 17.40806
d3p = 31.29251
1     0  0  0     0   r  0   .375
1   d1p  0  0   d1p  d1  0   .375
1   d2p  0  0   d2p  d2  0   .375
1   d3p  0  0   d3p  d3  0   .375
shift x dep
rotate z -ang
1     0  0  0     0  de  0   .375
1 source
Wire 5, end1

Use ⅜″ aluminum tubing mounted through a nonconducing boom or supported by insulated mounting brackets. Symbols r, de, d1, d2, and d3 are element half-lengths (center to tip), dep, d1p, d2p, and d3p are element positions relative to the reflector (center to center), and ang is the driven-element angle. Split the driven element leaving a gap no larger than ″ and angle each half 16 so that the center of the tips is 3″ from the centerline of the reflector. Feed directly with 75Ω coax and use at least one ferrite choke at the feedpoint. Keep the stripped coax leads as short as possible.

Sensitivity Analysis

The following table shows the change in average performance over the channel centers in dB when altering a single dimension by ⅛″ (116″ for symbols that represent element half-length and 3 for ang).

Symbol    Gain    F/R
   ang    0.01   0.61
     r    0.02   0.12
    de    0.00   0.00
    d1    0.02   0.05
    d2    0.03   0.20
    d3    0.02   0.07
   dep    0.01   0.15
   d1p    0.00   0.16
   d2p    0.01   0.13
   d3p    0.00   0.06

Larger Antenna

I scaled another FM Yagi to the 174216 MHz TV band. It uses a horizontal folded dipole, three reflectors, and nine elements total. Suppression of co-channel signals matters less for digital TV than for analog FM so I biased the trade-offs toward forward gain and reoptimized the design. The boom lengthened to 82″. Gain is 7.610.2 dBd and F/R > 20 dB over the band. A halfwave coaxial balun will match the 300Ω feedpoint to 75Ω coax with negligible loss. Calculated performance is for 28 analysis segments per halfwave. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.

Channel  Frequency  Impedance    SWR   Mismatch  Conductor  Forward     F/R
            MHz        ohms             Loss dB   Loss dB   Gain dBd     dB 
    7       177     305 + j34    1.12     0.01      0.01      7.73     22.68
    8       183     348 + j5     1.16     0.02      0.01      7.97     21.22
    9       189     351 - j19    1.18     0.03      0.01      8.29     20.97
   10       195     337 - j20    1.14     0.02      0.01      8.76     21.71
   11       201     320 - j9     1.07     0.01      0.01      9.39     20.58
   12       207     278 + j19    1.11     0.01      0.01     10.03     22.12
   13       213     262 + j112   1.52     0.19      0.03     10.13     23.17

Antenna file:

9-Element VHF-TV Yagi
Free Space Symmetric
174 180 186 192 198 204 210 213 216 MHz
11 6063-T832 wires, inches
z = 19.92559
r0 = 17.29696
r1 = 17.41426
de = 14.5093
d1 = 12.30237
d2 = 12.21016
d3 = 12.01788
d4 = 11.78023
d5 = 11.33282
de1 = 9.342921
de2 = 12.39683
d1p = 18.30395
d2p = 32.08967
d3p = 46.24657
d4p = 65.10853
d5p = 81.64079
1      0  0  z      0  r1  z    .375
1      0  0  0      0  r0  0    .375
1      0  0 -z      0  r1 -z    .375
1    de1  0  0    de1  de  0    .375
1    de2  0  0    de2  de  0    .375
1    de1 de  0    de2  de  0    .375
1    d1p  0  0    d1p  d1  0    .375
1    d2p  0  0    d2p  d2  0    .375
1    d3p  0  0    d3p  d3  0    .375
1    d4p  0  0    d4p  d4  0    .375
1    d5p  0  0    d5p  d5  0    .375
1 source
Wire 5, end1

Sensitivity analysis:

Symbol    Gain    F/R
     z    0.00   0.10
    r0    0.00   0.10
    r1    0.01   0.01
    de    0.00   0.04
    d1    0.00   0.04
    d2    0.02   0.16
    d3    0.02   0.17
    d4    0.01   0.15
    d5    0.01   0.01
   de1    0.00   0.01
   de2    0.01   0.10
   d1p    0.00   0.08
   d2p    0.01   0.01
   d3p    0.00   0.06
   d4p    0.00   0.07
   d5p    0.00   0.02


May 7, 201488108 MHz