Rhombic antenna . Its installation demands a large extension of physical area, with it you get high gain for VHF and UHF, it provides magnificent image sharpness for all channels in a band.
The rhombic antenna is ideal for television in places with weak signal reception. This antenna has two characteristics that make it possible: that its gain is of the order of 20 db and that its frequency range reaches between 0.5 and 1.5 times a central design frequency , these two characteristics make it ideal for television in places of weak reception.
From a physical point of view, its installation occupies a large area when compared to other types of antenna.
Its impedance reaches approximately 800 ohm, so a line coupler must be made to obtain 300 ohm, from the TV tape; with the mounting of two antennas in parallel that would offer 400 ohm and thus provide some additional gain: almost 3 decibels.
The making of these antennas requires wire number 14 for UHF , and number 18 for VHF .
A non-inductive 820 ohm (two watt or more) carbon resistor will be connected at the opposite end of the supply, so that the atmospheric discharges do not affect it.
A ten winding of number 14 wire turns is manufactured, tinned in the form of one centimeter in diameter for VHF and six millimeters in diameter for UHF. The length of the coil is three centimeters, so the spines must be spaced until that length is obtained.
The line coupler consists of a coil with two leads. These will be made two turns from each end, leaving six for the common 300 ohm line. This achieves the necessary impedance transformation .
The rhombic antenna for UHF is 1.5m long by 1m wide. A wooden or plastic crosshead can be made with these measurements. At the crossing point, a plywood or plywood plank is screwed to give the assembly more rigidity, as well as serving as a platform to mount it on the mast and to direct the antenna.
When it is VHF, the same assembly cannot be used due to its large dimensions (5.19 meters long by 3.46 wide). Four posts will be used to support the wire that makes up the antenna.
The sides of the rhombus of at least two wavelengths. The center frequencies for UHF and VHF are 665 and 192.5 MHz respectively. The UHF measures 0.90 m per side and the VHF 3.12 m. The rest of the measurements appear in the construction figures.
The frequency range to be covered by the UHF rhombic ranges from 332 MHz to 997.5 MHz and that of VHF from 96.25 to 228.75 MHz.
As can be seen, both antennas extend above and below the frequency range of two TV bands. The VHF antenna was designed for channels 7 to 13 which is the range of 174 to 215 MHz.
Both the line coupler and the resistor must be protected against the effect of the weather.