5488 MHz

A dipole, intentionally mismatched at resonance, makes a simple broadband receiving antenna for TV channels 26.

This compares ⅜″ dipoles in free space fed with 75Ω and 300Ω. I optimized the length of each antenna to maximize worst-case gain from 54 to 88 MHz. The optimal length for 75Ω feed is 87″. For 300Ω it is 110″. Both curves include ohmic and mismatch losses. The 300Ω curve includes a balun loss of 0.5 dB, the value I measure at 98 MHz for small cylindrical baluns with twin-lead and spade lugs. Worst-case gain of the mismatched dipole is 2.4 dB greater than that of the conventionally matched antenna.

The following table gives the optimum length for various conductor diameters.

 Conductor    Diameter    Optimum    Worst-Case
                           Length     Gain dBd
#18 copper     0.0403″      97.4″      -3.73
#16 copper     0.0508       97.9       -3.57     
#14 copper     0.0641       98.6       -3.42
#12 copper     0.0808       99.4       -3.26
 6063-T832     0.375       109.7       -2.14
 6063-T832     0.5         113.4       -1.91
 6063-T832     0.75        120.0       -1.58
 6063-T832     1.0         124.3       -1.38

The free-space gain figures include ohmic, mismatch, and balun losses. Split the dipole at its center and feed with 75Ω coax through a 75:300Ω balun.

I optimized all designs with the AO 8.50 Antenna Optimizer using 30 segments/halfwave.

Low-VHF TV Dipole
Free Space Symmetric
54 88 MHz
1 6063-T832 wire, inches	; 6063-T832 is an aluminum alloy
a = 54.87098			; commonly used for antenna elements
1   0 -a 0   0 a 0   .375
1 source
Wire 1, center

Small Yagi for 174216 MHz

I noticed that the high-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 31″ 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 improves the pattern at the low end of the band and forward 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 tip axis is 3″ from the reflector axis. 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 largest performance degradation over the channel centers in dB when altering a symbol value by Tol.

Symbol      Tol   Gain    F/R
   ang   1.0000   0.03   0.63
     r   0.0197   0.01   0.14
    de   0.0197   0.01   0.01
    d1   0.0197   0.01   0.16
    d2   0.0197   0.01   0.12
    d3   0.0197   0.01   0.07
   dep   0.0394   0.01   0.33
   d1p   0.0394   0.02   0.41
   d2p   0.0394   0.01   0.26
   d3p   0.0394   0.00   0.14

Large Yagi for 174216 MHz

This Yagi uses parts available at Home Depot. It has 14 elements made of aluminum angle on a ten-foot boom made of 1″ ABS pipe. The 0.5″ 0.5″ right-angle element shape is electrically equivalent to a 0.4″ round conductor. I optimized the design with the AO 9.61 Antenna Optimizer. The red dot marks the 75Ω feedpoint.

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.

Frequency  Impedance    SWR   Mismatch  Conductor   Forward     F/R 
   MHz        ohms             Loss dB   Loss dB   Gain dBd      dB 
   177    72.9 + j2.3   1.04     0.00      0.01      9.12      28.91
   183    91.4 - j1.0   1.22     0.04      0.01      9.63      25.79
   189    95.7 - j8.6   1.30     0.08      0.01     10.26      25.02
   195    88.5 - j4.1   1.19     0.03      0.02     10.92      25.24
   201    93.6 - j1.7   1.25     0.05      0.02     11.21      25.62
   207    72.8 + j7.6   1.11     0.01      0.04     10.93      25.10
   213    67.1 + j16.6  1.29     0.07      0.10     10.75      25.54

Construction

Use four eight-foot lengths of aluminum angle to make the elements. The boom is a ten-foot piece of 1″ ABS pipe (1.9″ OD). Use PVC conduit straps to mount the boom to a flat plate attached to the mast. Use the same straps to mount the elements on the boom with the horizontal part facing forward. Attach each element to the strap with two sheet metal screws and secure the strap to the boom with another. Angle the driven element halves 12 so that the vertical edge of the element tip is 31116″ from the vertical edge of the reflector. Position the inner ends as close together as possible and stabilize them using any convenient method. Connect 75Ω coax close to the right-angle bend. Keep the stripped coax leads as short as possible. Install at least one ferrite choke at the feedpoint. You may need boom guys to prevent sag. In windy areas use three sets of nonconductive guys arrayed at 120 intervals around the boom.

Antenna File

14-Element VHF-TV Yagi
Free Space Symmetric
174 180 186 192 198 204 210 214 215 216 MHz
14 6063-T832 wires, inches
ang = 12.13631		; driven element angle
dep = 6.866247		; element positions
d1p = 8.814632
d2p = 11.29366
d3p = 15.89009
d4p = 21.83114
d5p = 29.16191
d6p = 37.02126
d7p = 46.46181
d8p = 58.55393
d9p = 69.7012
d10p = 84.74487
d11p = 102.4394
d12p = 119
r = 16.30631		; element half-lengths
de = 15.19612
d1 = 13.17046
d2 = 13.03342
d3 = 12.85786
d4 = 12.74795
d5 = 12.52534
d6 = 12.0414
d7 = 12.00256
d8 = 11.75232
d9 = 11.58127
d10 = 11.69231
d11 = 11.79454
d12 = 11.18977
1    0   0  0       0   r  0    0.4
rotate end1 z -ang
1  dep   0  0     dep  de  0    0.4
rotate end
1  d1p   0  0     d1p  d1  0    0.4
1  d2p   0  0     d2p  d2  0    0.4
1  d3p   0  0     d3p  d3  0    0.4
1  d4p   0  0     d4p  d4  0    0.4
1  d5p   0  0     d5p  d5  0    0.4
1  d6p   0  0     d6p  d6  0    0.4
1  d7p   0  0     d7p  d7  0    0.4
1  d8p   0  0     d8p  d8  0    0.4
1  d9p   0  0     d9p  d9  0    0.4
1 d10p   0  0    d10p d10  0    0.4
1 d11p   0  0    d11p d11  0    0.4
1 d12p   0  0    d12p d12  0    0.4
1 source
Wire 2, end1

Sensitivity Analysis

The following table shows the largest performance degradation over the channel centers in dB when altering a symbol value by Tol.

Symbol      Tol   Gain    F/R
   ang   1.0000   0.02   0.47
   dep   0.0394   0.02   0.32
   d1p   0.0394   0.03   0.31
   d2p   0.0394   0.02   0.05
   d3p   0.0394   0.02   0.03
   d4p   0.0394   0.01   0.04
   d5p   0.0394   0.01   0.03
   d6p   0.0394   0.01   0.02
   d7p   0.0394   0.01   0.06
   d8p   0.0394   0.00   0.10
   d9p   0.0394   0.00   0.13
  d10p   0.0394   0.00   0.10
  d11p   0.0394   0.00   0.21
  d12p   0.0394   0.00   0.23
     r   0.0197   0.01   0.13
    de   0.0197   0.00   0.02
    d1   0.0197   0.01   0.24
    d2   0.0197   0.02   0.78
    d3   0.0197   0.01   0.73
    d4   0.0197   0.02   0.71
    d5   0.0197   0.02   0.25
    d6   0.0197   0.01   0.03
    d7   0.0197   0.01   0.18
    d8   0.0197   0.00   0.23
    d9   0.0197   0.00   0.14
   d10   0.0197   0.01   0.51
   d11   0.0197   0.01   0.57
   d12   0.0197   0.00   0.24

Combining Stacked UHF-TV Arrays

You can combine the output of two 75Ω antennas with 75Ω coaxial cables of equal length and a hybrid power splitter. At 584 MHz I measured negligible differential amplitude and phase error for five splitters from my junk box. I measured losses of 0.9, 1.0, 1.5, 1.9, and 2.9 dB. I think either of the two lowest figures is a reasonable trade-off for the convenience these wideband devices provide.

For lower loss, connect the 75Ω cables in parallel with a junction splitter. The resulting 37.5Ω impedance yields an SWR of 2 and a mismatch loss of 0.5 dB, which is lower than the loss of a hybrid splitter. To further reduce it, raise the 37.5Ω to 75Ω with 31116″ of 54Ω Belden 8219 RG-58A/U. SWR is 1.24 maximum and total loss 0.1 dB maximum over the 470698 MHz UHF-TV band.

You can combine 300Ω antennas with two 300:75Ω baluns and one of the methods described above. For lower loss, use equal lengths of 300Ω line and join them to obtain 150Ω. Match this to 75Ω with a 108Ω quarterwave transformer comprised of the center conductors of side-by-side 31116″ lengths of 54Ω Belden 8219 RG-58A/U. Connect but float the shields. Use a ferrite choke on the 75Ω feedline. SWR is 1.24 maximum and total loss 0.1 dB maximum over 470698 MHz. You can dispense with the quarterwave transformer if you're willing to accept 0.5 dB of mismatch loss from the resulting SWR of 2.

For a 300Ω feedpoint, use 300Ω line from each antenna to 424Ω quarterwave transformers made of #12 wires 5″ long spaced 1⅜″. Parallel the resulting 600Ω impedances to obtain 300Ω.

Loss for twin-lead increases substantially when wet. Cover it with sealed PVC tubes to improve wet-weather performance. Alternatively, you can construct weather-resistant 300Ω line with #12 wires spaced ″.


March 23, 201788108 MHz