![]() In this way, the interference signals present have free play to reach the antenna. If you look into the base of the antenna, you notice that usually the ground wire (shielding) is not even connected, which is an additional problem. Moreover, this small antenna needs a good base (mass), by e.g. There is also no resistance (impedance) adjustment to the 75 ohm cable, so that a part of the received signal is already short-circuited. This also applies to the included SDR stock antenna.Īlthough positioned vertically, and therefore all around sensitive, this is still a simple dipole, and therefore can only be used on a very limited number of frequencies. Moreover, a horizontally suspended dipole is only sensitive in 2 opposite directions.Ī vertical dipole receives more around radio signals. The length of the dipole determines which radio wave (wavelength), or frequency is best received on the antenna.Īll other radio waves are then weakened. Many fixed radios still have a 300 ohm antenna input, to connect a flat cable antenna.Ī standard dipole will be of little use as an SDR antenna, because in principle it is sensitive to only one frequency. Most receivers are adapted to this resistance, so that low-loss signal transmission is possible.Ī standard SDR stick also has an input resistance of 75 ohms (just like the smaller TV sets). In this way, the 300 ohm resistance can be converted to usually 75 or 50 ohms. with an extra piece of coax cable, or with a (toroidal) transformer. Otherwise, a part of the radio signal is short-circuited to the ground (shielding), and we do not want that. If (as nowadays) a coax cable is used (= asymmetric cable), a transducer (balun) must be placed near the antenna to use an asymmetrical power supply cable. The transmitter / receiver input is adapted to this, for low-loss transport of the radio waves. This antenna system with symmetrical power cable has in practice a transport resistance (impedance) of approximately 300 ohms. ![]() Remember that in the past the TV sets also had that flat cable (color in black or white). This type of power cable was formerly the most used in transmitters / receivers. This line of supply is always symmetrical to a dipole, so with 2 parallel wires to the receiver. In the middle of a dipole, the bars / wires are connected to a power cable that goes to the receiver / transmitter. If you just want to receive, this problem is not there, but the reception sensitivity without a tuner can decrease. If you do not, then there is a chance that you will damage the transmitter during broadcasting. In order to solve this, the dipole is made slightly more broad-band with some artificial handles.īut the transmitter / receiver usually requires an antenna tuner to be able to tune exactly to a certain frequency. In order to receive another frequency ideally, we would therefore always have to adjust the length of the dipole, which is not possible in practice. ![]() ![]() Now it is possible to receive current and voltage on our dipole at a frequency of 10Mhz ,īecause the current in the middle goes through the zero point and thus is also low at our feeding point. So we divide the dipole of 15m into 2 pieces of 7.5m. We must be able to feed the dipole in the middle, because there is too high a voltage on the ends, and we can not connect our antenna cable to it. So we would need a wire of 30 meters !įor practical reasons, a dipole is made at half of one period (1 period = 1 wavelength = here 15m for 10MHz), because of the changing field. Wavelength in meters = 300 / Freq (in MHz) = ~ 30 m. Radio signals travel at about the speed of light or 300,000,000 meters / sec.Īt a reception frequency of 10MHz we can then calculate the wavelength of our antenna with the formula: So also the signal that is received on the dipole.Ĭalculate the length of our antenna or dipole. Radio waves, however, have an alternating voltage, which changes on frequency.įor example, a radio wave at 10MHz (10 Megahertz) alternates 10,000,000 times per second, or 10,000,000 periods per second of polarity , In the first place, EVERY antenna is derived in practice from a DIPOLE.Ī DI-POLE, that are 2 equal wires or bars in one line, one receives the negative part, and the other the positive part of the radio wave. To answer this question, we first have to briefly explain how radio waves can be received. For whom SDR radio is new, or has no radio technology background, it is best to read this article. However, many are only interested in scanner receiving or weather-photos.īut it does not work like that. I get a lot of questions about SDR antennas. SDR Antenna choices, the simple explanation
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