Jumat, 21 Desember 2012

Building a good Antenna System

Outside antenna systems are considered electrical installations that must comply with safety regulations that may vary depending on the area where you live. Enquire at your local government, fire department and home insurance broker about the regulations regarding construction of outside antennas, proper grounding and connection of radio equipment to the mains. You could be held responsible, legally and financially, for any damage that is caused by your installation (lightning strike, fire, injuries) when you fail to comply with local regulations.
The antenna systems, presented on this page are for receiving purposes only with a portable receiver that is either battery operated or connected to the electrical mains by an adaptor with galvanic isolated transformer without ground plug (not the electronic adaptor type).
Good reception is only possible when you have both a good radio and a good antenna system. Even the best receiver is useless without a good antenna system. We should point out the essential difference between "antenna" and "antenna system": an antenna captures radio waves and an antenna system is the comprehensive installation that brings the radio waves in good condition to your radio.
Now, there's often some misunderstanding about antennas. If you have an antenna that has the correct length, you can never make it receive more or better. What you can do is to place it in the ideal position to capture signals, and you can reduce the noise level so that the signal is more readable. The signal-to-noise ratio is what makes the signal readable. Even very weak signals are more readable than strong signals if they carry less noise: you simply turn up the volume. If you experience too much noise, turning up the volume will turn up the noise equally, even on strong signals. The good news is that anyone with basic soldering experience can homebuilt a good antenna system that provides excellent reception for less than 100 Euros or $120. You don't need to be familiar with electronics or radio technology.
IMPORTANT! The antenna systems, describes on this page, are only suitable for receiving purposes and should never be used to transmit. The toroids are not suitable to dissipate the power and the impedance of the system is not adapted to the transceiver output.
Below the circuit diagram of a commonly used long-wire antenna system: the wire antenna is connected to a 9:1 transformer, also called UNUN (Unbalanced to Unbalanced) that is grounded to a 2,5 meter (8 feet) long copper tube, driven in the ground. The 9:1 transformer brings down the very high impedance of the end-fed wire antenna to an impedance that is suitable for both coax and receiver. The feedline transports the signal to a wall connection box. From there, a coax brings the signal to the external antenna jack. The transformer ground, and optionally receiver ground, are connected to the grounding rod with the shortest possible wires, all near the entry of the coax into the house.

Inside the house, the coax that comes from the transformer is connected to the connection box with a BNC chassis connector which in turn is connected to a second BNC chassis connector at the front of the box. The easiest way to connect the external antenna jack to the inside coax cable is to use a small box with on one side a BNC chassis connector that is soldered to the wires that go to the antenna jack. Make sure that the coax ground (shielding) is connected to the sleeve of the jack and the core wire of the coax is connected to the tip of the jack.
WARNING: this is how the external antenna connection works on my Sangean ATS 909X and on some other brands. However, other radios might have another antenna socket wiring setup! Inform yourself about how to connect the external antenna to your radio! I'm not liable for any damage, caused by an incorrectly connected antenna.
Never accidentally insert the external antenna jack into some other (audio) jack. They look the same and are mechanically the same, but I don't know what it could do to your radio, since the transformer output virtually short-circuits LF and DC signals. I accidentally inserted the antenna jack into the headphone socket of my 909X a few times (stupid me) without any damage. Nevertheless, be careful not to do so. You might attach a flashy warning label to the cable, next to the jack, as our brain is conditioned to put such jacks always into an audio socket.
If your receiver has a separate grounding connection (often with butterfly nut), you can connect that ground to the ground connection of the wall connection box. It is strongly advised to use one single grounding point and to connect each of the separate parts of the antenna system, each with its own heavy gauge or flat cable grounding wire, to that single grounding point, and to keep these wires as short as possible.

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