Difference between revisions of "FM Transmitters"

We only need 250-300 feet and 500 ft., is pushing it. The FCC will bust those who have these .5 to 5 Watt systems with dipole half wave antennas sticking up in the yard or on the roof. If you are hanging Antennas up on the house and transmitting more than 100 yards there will be a big target on your light show. Antennas should be kept out of site, in the house; not on the house or in the yard. FCC PDF

Vacant Radio Station Locator

before setting up their transmitter. You input your ZIP code and it tells you the three or four frequencies that work the best in your area

The 1/4 Wave Ground Plane antenna is probably the easiest to build and least complex of the antennas. The quarter wave antenna has an omnidirectional pattern; it radiates equally well in all directions. This type of radiation pattern is preferred in mobile applications as the bearing from the device to the reciever is unknown or randomly changing. The quarter-wave antenna is well suited in fixed applications having a good ground plane and adequate signal strength. The quarter-wave requires a ground plane to work properly. A ground plane is a flat surface designed to limit the downward radiation of the antenna, and is generally 4 grounded piece of metal wire bent at 45 degree angles.

If a 1/4 wave antenna is used without a sufficient ground plane, two things will happen:

(1) Radiated power will tend to go out equally in all directions. Up and down as well as towards the horizon. Because there are no radio receivers up in the sky and down in the ground, energy is wasted which would be better used transmitting toward the receiver audience.

(2) An improperly installed quarter-wave antenna does not establish a good reference plane for the radiating field. When this happens, the antenna is said to be mismatched and the transmitter will see a high SWR (Standing Wave Ratio). Much of the power that the transmitter generates will then bounce back into the transmitter and be dissipated as heat causing damage to your nice FM transmitter. This reduces the signal that the audience receives and may cause improper decoding in fringe areas, along with spurious emissions. LOTS OF NOISE.

Amazingly, there is a trick to make them work better. Most people do not know how, but I will give away the secret. An expedient often improves performance of your flexible antenna and is a wire counterpoise that is an 18" piece of wire for VHF FM frequencies.

A counterpoise prevents transmitted RF from coupling with your body or other surrounding elements. Your antenna now performs like a center-fed dipole (See Dipole), instead of an "end-fed dummy load!" Simply cut some AWG18-22 gage stranded, insulated wire to 18 inches, crimped and then soldered to alligator style battery clip, which will fit over the outer shield of a the BNC connector on the antenna. This will make a huge improvement that will amaze you.

Ideally, a dipole antenna is fed with a balanced, parallel-wire RF transmission line. However, this type of line is not common for our FM transmitters. An unbalanced feed line, such as coaxial cable, can be used, but to ensure optimum RF current distribution on the antenna element and in the feed line, an RF transformer called a balun (contraction of the words "balanced" and "unbalanced") should be inserted in the system at the point where the feed line joins the antenna. This is where it gets complicated and because of this, I would stick with using the 1/4 wave antenna.

For best performance, a dipole antenna should be more than 1/2 wavelength above the ground, the surface of a body of water, or other horizontal conducting medium such as sheet metal roofing (another issue with this antenna because it would require outside installation and we don’t want to do that). The element should also be at least several wavelengths away from electrically conducting obstructions such as supporting towers, utility wires, guy wires, and other antennas; even more issues that are inherent with this antenna.

The Folded Dipole: A folded dipole is a dipole antenna with the ends folded back around and connected to each other, forming a loop. This antenna would not be easily matched to your FM transmitter but is perfect for your FM receivers. For this reason and the complexities that occur during transmission, I would highly recommend you not attempt using this kind of system for your light show. Enough said.

The J-pole antenna is an end-fed omnidirectional dipole antenna that is matched to the feedline by a quarter wave transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until a VSWR as close as possible to 1:1 is obtained. Because this is a half-wave antenna, it will exhibit gain over a quarter-wave ground-plane antenna. The J-pole is somewhat sensitive to surrounding metal objects, and should have at least a quarter wavelength of free space around it. This is another complicated fixture and adding any gain to our FM transmitters is not the direction the FCC would like us to go. For these reasons, I do not recommend using this system.

They discovered that by adding "elements"of various lengths and spacings in front of and behind a dipole antenna that the performance and effectiveness of the dipole could be greatly increased and the pattern of the dipole RF energy could be "beamed" or focused in one direction, with the resulting "effect" of making it appear that the transmitter was running lots more power than it actually was, yielding much stronger signals both on receive and transmit! Do we need to go any further here? This antenna looks like the old TV antenna and is way more complicated than we need and we certainly don’t need to beam our music across town. Lets move on.