The following guide mainly deals with modifying low power, short range FM adapters commonly used for allowing things like ipods and computers to output audio through an FM radio. This is based on personal experience and is illegal in most parts of the world, although depending on where you live they may not monitor for illegal broadcasts and in that case try to keep a low profile and not advertise that you are a pirate radio station, as amateur radio operators commonly report such things :mad:. The range of such a transmitter will generally average around 3 Kilometres ( about 2 miles )
Most low power off the shelf FM transmitters can have an external antenna attached if you have the time to study where you can and can’t connect an external antenna. Most transmitters have an internal coil antenna next to the main board which is ok if you have a legit use for the transmitter but for most is pathetic. Most can be upgraded by soldering the core conductor of a 75 ohm coax cable to the point where the internal antenna is soldered to and the shield wire should be connected to the negative battery terminal as that is generally the common point of the circuit. I would actually remove the internal antenna so all radiated power goes to your amp and antenna and not leak out through the internal antenna
The signal from the transmitter wont be strong enough on its own to cover any real distance so the signal needs to be boosted to be of any real use. A cheep way is to use a TV signal splitter/amplifier which most electronics stores sell. Although not as good as a proper linear amplifier it should give descent range depending on antenna and terrain. A linear amp can be built from plans on the interwebz or purchased from stores that deal with radios. eBay generally has amps for sale for a decent price if your budget allows.
Believe it or not your TV antenna will not work and will provide poor results in the long run. Search the net for Helical Antenna designs see Helical Antenna for details on this antenna design the picture above is an example of the design. I made mine out of PVC drainage pipe and fencing wire and it works very well for my needs. the antenna is circularly polarised allowing for better signal propagation over hilly terrain. The antenna needs to be impedance matched with amp and transmitter to give excellent results. The antenna will still work well unmatched but range is reduced due to wasted power. Online calculators will calculate impedance depending on number of loops and loop diameter. Size is crucial to the design and needs to be calculated for the exact transmission frequency of your transmitter.
As you can see in the next pic, this is an example of a homebuilt antenna and it is the antenna I’m currently using.