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30 Maret 2008
Idiot's guide to antennas
Preferred type of antenna is affected by several factors, but mostly by transmitting site. In the middle of the area you want to cover you'll need an omnidirectional antenna which transmits equally all ways, while outside your coverage area you can beam the signal in with a directional antenna. Before going on air get a low VSWR by adjusting the position of the antenna and any of it's adjustable pieces. Aim for 2:1 or less. Use low power into the antenna when tuning it up and making adjustments. If you were using 100's of watts and a bit of the antenna came off in your hand the VSWR could be so bad as to blow the final transistor. For the same reason check the DC continuity of the antenna with an ohmmeter before plugging it in, to be sure it's what it's meant to be, either a short circuit or an open one, depending on the antenna type. A dipole shown below should be an open circuit.
A PIECE OF WIRE OR TV ANTENNA IS NOT SUITABLE FOR FM BROADCAST BAND TRANSMITTER!
You have to realize that antenna was, is and will always be crucial part of the system. Special care has to be taken! It is usually good idea to place antenna away from your transmitter, power supply and audio system. If you cannot meet these requirements, you could experience feedback and other RF problems. Interestingly, RF energy can make CD players and other digital devices go bezerk. Try placing 30W-driven antenna next to yours.
Dummy Load Dummy load is not actually an antenna, it dissipates all transmitted power in a form of heat. So what's the use of it? Well, it is presents an ideal match for an output of your transmitter (usually 50ohms). Since all power (virtually 100%) is transverted into heat there won't be any interference to your neighbors while you do tuning and testing. This is what dummy load is usually used for; testing and tuning transmitters. If you don’t have dummy load, you can build one easily from a BNC or other RF connector and the proper wattage/value of CARBON resistor(s). DO NOT USE WIREWOUND OR METAL FILM RESISTORS! A useful one can be constructed with 4 -220 Ohm 1/4 watt resistors in parallel (220/4 = 55 Ohms) with center conductor to outershell (ground) of an RF connector. That is pretty close to 50 Ohms and if you use 1/4 watt resistors you get a nifty 2 Watt Dummy Load for testing your equipment without an antenna. Commercial Dummy loads are available at our website, check under Transmitter accesories.
The simplest possible antenna for VHF is known as the Half-wave Dipole:
Both elements can be either aluminum/copper tubes or wire. The lengths of each dipole, L, is calculated from your transmitting frequency by this formula: L = 71/F (meters), where F is operating frequency in MHz
A half-wave dipole used vertically is omnidirectional, but when used horizontally it has a figure of eight coverage like this (view from top):
Note: A dipole needs a Balanced Feed as it is symmetrical, but a coaxial cable provides an Unbalanced Feed. What's needed is a Balun (BALance to UNbalance) transformer. These can be made out of bits of coaxial cable. If you don't do this power will be radiated from the feeder. You CAN use it without BALUN, but you might get lower-than-expected performance and unusual radiation pattern due to interactions with the feeder. We recommend this antenna to beginners, as your knowledge broadens go for one of the following babies:
Click here for alternative image. Most designs on the web don't compensate for the fact that GP antennas are not wideband antennas. Here is a Freq./element length chart for this simple GP antenna, all elements are in millimeters:
Radiator - B
Radials - A
If you have SWR meter (and you SHOULD have one) leave a few inches extra for the radiator and adjust it later by cutting to achieve minimum SWR.
Slim Jim This is a vertically polarized omnidirectional antenna.
Radiation efficiency 50% better than ground plane antenna, due to low angle radiation Unobtrusive No ground plane radials, so low wind resistance Fully weatherproof 50W input impedance Low VSWR - 1.5 to 1 or better Integrated balun Construction Details
J- Pole We will publish a number of antenna designs here, these have all been submitted to our forum by our forum members, most notably NormB in his best days. A number of links is provided at the bottom, leading to design sources or other interesting resources regarding J-pole antenna construction. Some of the designs were made for 144MHz (2m ham band) and need to be scaled down for 100MHz operation. This usually means increasing element size for roughly 144/100 = 1.44 (44% increase).