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Transmitters feeding a balanced transmission line will need a balun. This transforms the single ended output of the transmitter to a higher impedance balanced output. High power short wave transmission systems typically use 300 ohm balanced lines between the transmitter and antenna. Amateurs often use 300–450 ohm balanced antenna feeders.
A modern high-gain UHF Yagi television antenna with 17 directors, and one reflector (made of four rods) shaped as a corner reflector Drawing of Yagi–Uda VHF television antenna from 1954, used for analog channels 2–4, 54–72 MHz (U.S. channels).
The balun is a 16:1 ratio, thereby transforming the 50 Ω (ohm) coax to an 800 Ω feed at the antenna. The resistor load is also 800 Ω, non-inductive. This allows the antenna impedance to swing from 400–1,600 Ω over the frequency range intended and thus keep the SWR at the transmitter 2:1 or lower.
A coax balun is a cost-effective method of eliminating feeder radiation but is limited to a narrow set of operating frequencies. One easy way to make a balun is to use a length of coaxial cable equal to half a wavelength. The inner core of the cable is linked at each end to one of the balanced connections for a feeder or dipole.
A balun / ˈ b æ l ʌ n / (from "balanced to unbalanced", originally, but now derived from "balancing unit") [1] is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. [2] A balun can take many forms and may include devices that also transform impedances ...
The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a shorted quarter-wave parallel transmission line stub. [5] [1] [6] For a transmitting antenna to operate efficiently, absorbing all the power provided by its feedline, the antenna must be impedance matched to the line; it must have a resistance equal to the feedline's characteristic impedance.
At these lower frequencies, due to atmospheric noise (static) and man-made noise (interference), even a weak signal from an inefficient antenna is far stronger than the internal thermal or Johnson noise generated in the radio receiver's own circuitry, so the weak signal from a loop antenna can be amplified without degrading the signal-to-noise ...
The term "voltage balun" is used twice and the two usages conflict with each other: one says a voltage balun is an autotransformer, and the other says it's a classical transformer. 192.91.173.36 ( talk ) 21:43, 6 March 2019 (UTC) [ reply ]