A dipole, intentionally mismatched at resonance, makes a simple broadband receiving antenna for TV channels 2–6. The graph 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 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
I noticed that the high-VHF TV band was almost exactly twice the frequency of the 88–108 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, 32½″ 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.
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
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 ¼″ and angle each half 16¼° so that the tip axis is 3¾″ from the reflector axis. Feed directly with 75Ω coax with one or more ferrite cores at the feedpoint. Keep the stripped coax leads as short as possible.
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
This Yagi uses parts available at Home Depot. It has 14 elements made of aluminum angle on a ten-foot boom made of 1½″ 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.
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
Use four eight-foot lengths of aluminum angle to make the elements. The boom is a ten-foot piece of 1½″ 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 each element on the boom with the vertical edge to the rear. Attach the 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 311⁄16″ 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 where the horizontal and vertical edges meet. Keep the stripped coax leads as short as possible. Install one or more ferrite cores 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.
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
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
The TV channel repack provides an opportunity to improve UHF-TV antenna performance over the newly restricted bandwidth. I optimized this Hoverman for maximum forward gain over 470–608 MHz without regard for the pattern.
Calculated performance is for 28 analysis segments per halfwave with the AO 9.67 Antenna Optimizer. 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 473 256+j165 1.84 0.39 0.01 11.96 14.44 479 286+j143 1.62 0.25 0.01 12.21 16.64 485 299+j118 1.48 0.17 0.01 12.34 17.20 491 299+j99 1.39 0.12 0.01 12.40 15.97 497 295+j90 1.35 0.10 0.01 12.41 14.25 503 292+j88 1.34 0.09 0.01 12.38 12.66 509 291+j90 1.36 0.10 0.01 12.34 11.29 515 294+j94 1.37 0.11 0.01 12.29 10.18 521 301+j98 1.38 0.11 0.01 12.25 9.26 527 312+j101 1.39 0.12 0.01 12.20 8.46 533 325+j101 1.40 0.12 0.01 12.18 7.80 539 340+j97 1.39 0.12 0.01 12.16 7.24 545 356+j87 1.37 0.11 0.01 12.16 6.75 551 369+j71 1.35 0.10 0.01 12.17 6.33 557 378+j49 1.31 0.08 0.01 12.19 5.96 563 380+j23 1.28 0.07 0.01 12.21 5.63 569 372-j4 1.24 0.05 0.01 12.23 5.36 575 353-j29 1.21 0.04 0.01 12.25 5.11 581 327-j47 1.19 0.03 0.01 12.25 4.90 587 296-j55 1.20 0.04 0.01 12.23 4.71 593 264-j54 1.26 0.06 0.01 12.17 4.55 599 234-j44 1.35 0.10 0.01 12.05 4.41 605 208-j28 1.47 0.16 0.01 11.86 4.29
Use #10 bare copper wire for the zigzag. You can fabricate all of the reflectors from one eight-foot length of aluminum angle. The 0.5″ × 0.5″ angle is electrically equivalent to a 0.4″ round conductor. Any orientation should work well, but for best accuracy position the corner closest to the zigzag so that the sides are equidistant from the intended Z coordinate. Space the corner 313⁄16″ from the zigzag to properly locate the current centerline. Do not support any conductor at its end where it is susceptible to detuning by a dielectric.
For 300Ω feed, split 300Ω twin-lead and route the leads in a straight line across the 1¾″ feedpoint gap.
For 75Ω feed, a halfwave coaxial balun provides very low loss. This one, constructed for a different antenna, is made of 93Ω coax. For the repack Hoverman, cluster the three cables in a parallel triangle and solder the shields together. Solder the feedline center conductor to one balun center conductor using the shortest possible lead (shorter than shown above). Bend the balun center conductors at a right angle and route them straight across the feedpoint gap. The antenna model accounts for this feedpoint jumper.
Red is the loss for 75Ω RG-6 (Belden 1530) and blue is for 93Ω RG-62 (Belden 8254). These curves are for a propagation delay of 0.93 ns, which is about 8½″ of coax. Use the specified velocity factor of your coax to determine the exact length. Place one or more ferrite cores on the feedline as close as possible to the feedpoint to suppress any residual shield current.
Repack Hoverman Free Space Symmetric 470 504 539 574 608 MHz 13 copper wires, inches y0 = .8707846 ; zigzag Y coordinates y1 = 9.171193 y2 = 3.29843 y3 = 8.049558 y4 = 13.74166 z1 = 8.102206 ; zigzag Z coordinates z2 = 14.49329 z3 = 19.44896 p = -3.971499 ; reflector spacing from zigzag r1 = 11.49479 ; inner reflector length h1 = 8.087689 ; inner reflector height s1 = 1.829235 ; inner reflector inner tip to center r2 = 12.25223 ; outer reflector length h2 = 21.55919 ; outer reflector height s2 = 2.259527 ; outer reflector inner tip to center 1 0 0 0 0 y0 0 #18 ; feedpoint jumper 1 0 y0 0 0 y1 z1 #10 ; zigzag 1 0 y1 z1 0 y2 z2 #10 1 0 y2 z2 0 y3 z3 #10 1 0 y3 z3 0 y4 z3 #10 1 0 y0 0 0 y1 -z1 #10 1 0 y1 -z1 0 y2 -z2 #10 1 0 y2 -z2 0 y3 -z3 #10 1 0 y3 -z3 0 y4 -z3 #10 shift y s1 1 p 0 h1 p r1 h1 .4 6063-T832 ; reflectors 1 p 0 -h1 p r1 -h1 .4 6063-T832 shift y s2 1 p 0 h2 p r2 h2 .4 6063-T832 1 p 0 -h2 p r2 -h2 .4 6063-T832 1 source Wire 1, end1
The following table shows the largest performance degradation over the optimization frequencies in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
y0 0.0394 0.02 0.01
y1 0.0394 0.03 0.02
y2 0.0394 0.04 0.02
y3 0.0394 0.01 0.01
y4 0.0394 0.04 0.03
z1 0.0394 0.00 0.02
z2 0.0394 0.01 0.01
z3 0.0394 0.02 0.03
p 0.0394 0.02 0.07
r1 0.0394 0.04 0.42
h1 0.0394 0.01 0.03
s1 0.0394 0.01 0.06
r2 0.0394 0.01 0.13
h2 0.0394 0.01 0.08
s2 0.0394 0.00 0.01
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 in excess of 3 dB of 0.9, 1.0, 1.5, 1.9, and 2.9 dB.
For lower loss, connect the 75Ω cables in parallel with a junction splitter. The resulting 37.5Ω impedance yields SWR of 2 and 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 4⅛″ of 54Ω Belden 8219 RG-58. SWR is 1.16 maximum and excess loss 0.1 dB maximum over 470–608 MHz.
You can combine 300Ω antennas with two 300Ω baluns and one of the methods described above. Another method joins equal lengths of 300Ω line to obtain 150Ω. A 108Ω quarterwave transformer comprised of the center conductors of side-by-side 4⅛″ lengths of 54Ω Belden 8219 RG-58 matches 150Ω to 75Ω. Connect but float the shields. Place one or more ferrite cores on the 75Ω feedline as close as possible to the transformer. SWR is 1.16 maximum and excess loss 0.1 dB maximum over 470–608 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 5⅜″ long spaced 1⅜″. Parallel the resulting 600Ω impedances to obtain 300Ω. Do not use ¾-wave transformers. Their bandwidth is much narrower.
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 ½″.
A Laird 28A0807-0A2 snap-on, split-ferrite core, stocked by Arrow, Mouser, and Digi-Key, has an impedance of 450Ω at 200 MHz and 700Ω at 500 MHz. A nonsplit Fair-Rite 2643625202 from Arrow, Mouser, and Digi-Key has an impedance of 400Ω at 200 MHz and 460Ω at 500 MHz. For higher impedance, use more than one core.
88–108 MHz
