I need to change the frequency of my transmitter to another spot to avoid interference with another station. I see that there are no stations at all from 102.1-103.1 (in Portland). Is there a reason for that, like they're saved for emergencies or something?
How likely is it for my transmitter (150 Watts) to bleed into other stations?
I need to change the frequency of my transmitter to another spot to avoid interference with another station. I see that there are no stations at all from 102.1-103.1 (in Portland). Is there a reason for that, like they're saved for emergencies or something?
Probably frequency protection because Portland is close enough to Canada to cause problems in both nations on the FM band, and as crowded as it is on the East coast with point after point of stations from one end to the other, special care is taken to avoid interference between the two nation's broadcasters along and near the border with Canada.
Or it could be a group of stations all decided to pull maintenance at the same time.
How likely is it for my transmitter (150 Watts) to bleed into other stations?
Well let's hope your transmitter has at least a 7 pole low pass filter on the output. Also let's hope the antenna system is well tuned and VSWR is less than 1.5:1. And let's hope that your transmitter's exciter is feeding that 150 watt amp with a nice clean free of spurious and intermod clutter signal..if not put a low pass filter or a band-pass filter between the exciter and final amp.
Checking the above will ensure that your system is running as clean and correctly as possible which will reduce the chances of it "bleeding" onto or over top of other stations along with reducing the potentials for clogging up aircraft/tower/approach communications.
Also it's a good idea to run your system at least 10 percent below it's maximum rated power level. This provides headroom during mains power surges as well as power-on surges. If the antenna system is tuned properly and one with gain like that found in a J-Pole, SlimJim or 5/8 wave ground plane, the antenna will do most of the work so the transmitter and it's power supply system won't have to work as hard.
It's not just watts that get's the signal out there. Antenna type, it's gain factors, angle of radiation and beam tilting (if any) all play a role in how far that signal will reach. Antenna height is also an important factor. The higher the better. FM broadcast frequencies are line of sight so if your TX antenna is up high enough (at least 1 full wavelength above the dirt) and clear of obstructions, your system should provide the same coverage running at 80 watts with gain antennas mounted up high versus a system running a no gain antenna mounted fairly low to the ground, then the transmitter would have to push extra power to push the signal through all the obstructions in the line of sight path between the transmitting antenna and receiver antenna.
Standard channel spacing between stations is 400Khz from center to center, leaving only 200khz of channel separation. This is why you found that it was necessary to choose another frequency because of the co-channel located station and interference problem.
So if you want to operate on 90.1 Mhz, but there is a station on 90.3 Mhz, your going to run into a problem. But if you stepped down 1 point, say 89.9, then you would be 400 khz center to center spaced from the other station on 90.3.
Good luck with the operation! Let us know how things turn out!
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
Probably frequency protection because Portland is close enough to Canada to cause problems in both nations on the FM band, and as crowded as it is on the East coast with point after point of stations from one end to the other, special care is taken to avoid interference between the two nation's broadcasters along and near the border with Canada.
Oh gawd, the Seattle-Vancouver corridor has jammed every channel except one little hole around 98.1, protection for some univeristy fleafart (Simon Fraser, IIRC) - add Portland and it's a mess.
I can sit on one frequency, drive through three towns (about an hour) and have picked up five different stations on both sides of the line in that drive >.<
It's not just watts that get's the signal out there. Antenna type, it's gain factors, angle of radiation and beam tilting (if any) all play a role in how far that signal will reach. Antenna height is also an important factor. The higher the better. FM broadcast frequencies are line of sight so if your TX antenna is up high enough (at least 1 full wavelength above the dirt) and clear of obstructions, your system should provide the same coverage running at 80 watts with gain antennas mounted up high versus a system running a no gain antenna mounted fairly low to the ground, then the transmitter would have to push extra power to push the signal through all the obstructions in the line of sight path between the transmitting antenna and receiver antenna.
I know this post is a bit late, but just wanted to add that RFBurns is dead on the money in regards to wattage on FM.
General rule of thumb with FM: Power means nothing without line of sight!
FM signals travel in a straight line towards the horizon. As the Earth is round, naturally, the higher you go, the further out you will get out. But...this only works to a certain point. Since the signal is traveling in a straight line, it will eventually go out into space. Same theory works for the reverse. The further away you get from a station, the higher your receiving antenna needs to be. But, eventually you will lose the signal completely.
AM antennas sit on the ground as medium wave propagates via ground wave and the signal will generally follow the curvature of the earth in daylight hours. However, at night, a different story. As the sun sets and ionosphere conditions change, the medium wave signals shift from ground wave propagation to sky wave.
Simple illustration for reference:
So as you can clearly see, with FM, there comes a point where no matter how much power you pump out, your range will not increase.
With medium wave frequencies, generally your range is more directly related to how much power your pushing out. The more power, the further the signal.
Excellent graphic demonstrations for MW and FM propagation sam!
With FM, the technique of "Beam Tilt" is used to bend the signal back down to the horizon. This can be done with varying the phase of a feed harness to a multi-bay antenna system, or using directional antennas and mounting them at a negative degree of angle from the center of radiation.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
