A dipole, intentionally mismatched at resonance, makes a simple broadband receiving antenna for TV channels
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, leaded baluns. Worst-case gain of the mismatched dipole is 2.4 dB greater than that of the conventionally matched antenna.
This is the azimuth pattern of the mismatched dipole at the middle of the band for a signal arriving 5° above the horizon. The antenna is 20′ above ground. The gain figure is with respect to a free-space isotropic antenna and does not include balun loss.
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
½″ EMT 0.706 119.0 -1.63
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.
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
A rectangular loop provides even better performance. This design uses ½″ EMT, which should be available at any hardware store. The galvanized steel tubing is heavy but cheap. You need two 10′ lengths.
This compares the loop with a mismatched dipole made of ½″ EMT. Loop impedance at resonance is about 210Ω. Using mismatched 300Ω feed increases worst-case gain 0.5 dB. The curves include ohmic, mismatch, and balun losses.
The azimuth pattern is somewhat broader than that of the mismatched dipole. The gain figure does not include balun loss.
Loop width is 65″ and height is 40″, center-to-center. Flatten the tubes at the corners and bolt them together. Split the lower tube at its center and feed with 75Ω coax through a 75:300Ω balun.
Low-VHF TV Loop Free Space Symmetric 54 88 MHz 4 resistivity 5.92E-8 wires, inches ; Zinc (galvanized steel) w = 32.39582 h = 19.98988 d = .706 ; Nominal outside diameter of 1/2" EMT 1 0 -w h 0 w h d 1 0 -w h 0 -w -h d 1 0 w h 0 w -h d 1 0 -w -h 0 w -h d 1 source Wire 4, center
I optimized all designs with the AO 8.50 Antenna Optimizer using 30 segments/halfwave.
88–108 MHz