Hey everyone, I'm trying to build a 50 ohm antenna for 75-108 Mhz at 15 watts. The impendance (50Ω ) is important since if its not right it might blow out my transmitter. Any ideas on how long it should be or what materials I should use? Is making it very very long bad or is leaving it short bad? Like if I connect it to 100ft of wire?

That's a rather strange band to be building a transmitter for (including the FM broadcast band, but also going below it?), and it's also very, very wide. You won't get optimum performance across the band with one antenna, that's for sure.

The "optimum" antenna (depending on what you're trying to do, anyway - there are other types of antenna that trade omnidirectionality for gain in a certain direction) is a 1/2 wave dipole (1/4 wave will also work if you have a ground), matching the polarization of your receive antennas. Using a coil to match impedance (of the same size on each pole of a dipole), you can shorten the antenna while keeping the same electrical length, but you reduce the amount of metal actually in the air to radiate signal.

Calculating wavelength at a given frequency merely requires knowing that light travels at approximately 300 megameters per second, so divide that by the frequency in MHz, and you get the wavelength. 75 MHz == 4 m wavelength, 108 MHz == 2.78 m wavelength. (And now you should see why I used 1/2 and 1/4 wave terminology above, and that's what it's referring to.) I hope you can see why that's too large of a band to efficiently cover with one antenna.

If it's some sort of low-power FM transmitter kit, does it not have matching coils? If it's something higher power, stop, get an appropriate amateur radio license, and do this in amateur radio bands according to the regulations in your country.

If the antenna is 4 meters and I move to 107.9 mhz will it hurt the transmitter because its too long? I'm afraid power will feed back from the antenna into the transmitter. Also are those 50 ohm?
Im asking this because I have steel rods at specific lengths and can't cut them 1.5 meters and 4 meters. Could I use a long 15meters copper extension coord as an antenna?

SWR will likely be unacceptably high and could damage the transmitter, if you do that without matching.

To use an antenna over that wide of a band, or an antenna of an incorrect length for the band, you'd need an antenna tuner to impedance-match things (whether you build your own or buy it). Otherwise, cut the antenna for the right length for the frequency you're transmitting on. And, of course, make sure your operations are actually legal in your jurisdiction.

Thanks for the info, ill try and get a different length antenna. I using 1/2 wavelength better than full wavelength?

Also does the material matter? Iron/Copper/Steel/Rusted Iron?
And does the thickness of the cable going from the transmitter to the antenna matter (shielded coax or just a copper wire)