The FCC FM search lets you find all the FM stations within a radius of you. https://www.fcc.gov/encyclopedia/fm-query-broadcast-station-search
The FCC has rules about how far spaced FM stations need to be based on class.
https://transition.fcc.gov/mb/audio/includes/46-73207.htm
http://www.ecfr.gov/cgi-bin/text-idx?node=sp47.4.73.g&rgn=div6#se47.4.73_1807
In the distance table it is pretty close between any X-Y class paring if you take the X-X spacing, half it, take the Y-Y spacing, half it, and add the results.
Since part 15 is practically no power, my gut feel is we could be right on the edge of a full power station’s separation.
| Separation (all values in km) | Co channel (same frequency) | 1st adjacent channel | 2nd/3rd adjacent channel | 10.6/10.8 MHz up/down (IF) |
| Class A to Part 15 FM | 57.5 | 36 | 15.5 | 5 |
| Class B1 to Part 15 FM | 87.5 | 57 | 25 | 7 |
| Class B to Part 15 FM | 120.5 | 84.5 | 37 | 10 |
| Class C3 to Part 15 FM | 76.5 | 49.5 | 21.5 | 7 |
| Class C2 to Part 15 FM | 95 | 65 | 29 | 10 |
| Class C1 to Part 15 FM | 122.5 | 88.5 | 41 | 17 |
| Class C0 to Part 15 FM | 135 | 103.5 | 48 | 20.5 |
| Class C to Part 15 FM | 145 | 120.5 | 52.5 | 24 |
| LPFM to Part 15 FM | 12 | 7 |
The only difficulty comes from translators. Some of the 250W translators with a large HAAT approach the service are of a class A, so to be safe the separation for a class A could be applied to a translator. Alternatively there is a rule based on distance from the service contour of the translator. That is less able to be done in excel.
So, If I were looking for a frequency to do Part 15 FM on, I would get the list of all the FM’s within 145km of me, making sure I put my transmitter site’s location in using NAD27 coordinates. Then I would sort the list by class and for each class put in a new column the values from the above table. If the station was farther away than the above values, it is not a concern. If it is closer than the above values, it is too close to put a part 15 transmitter on that stations channel. Eventually you will have a list of channels that do not work for co channel operation. Anything that is not on that list is worth checking the distance to 1st, 2nd, and 3rd adjacent stations, as well as IF stations.
Your approach is reasonable and I have used a different method. First I checked with http://www.radio-locator.com/ to get an idea of where to start. Listening to this frequency on a car radio revealed that there were two very weak stations audible which were switching back and forth being received.
Assumption 1 is that no one will be listening to either station in my area.
Checking using the link above it was found that I am surrounded by stations 50 to 120 miles away on this frequency which leads to
Assumption 2 that using this frequency will not cause problems in their service areas and it is unlikely that a future station will be licensed on this frequency in this area.
Checking up and down on first adjacent shows the nearest stations are at least 90 miles away and the signals heard were very weak so unless my neighbors are listening with outdoor antennas there should be no problem.
The down side is that since there are two stations heard on "my frequency" this could reduce my range but I reason that when far enough away from my transmitter when my signal is as weak as theirs it would not be listenable anyway.
I am only house and yard casting with limited range so this works for me.
Neil
I tried to search 150 miles from me as that is about the furthest must folks can receive during a temperature inversion. During an Inversion I can receive 96.3 in Morehead City but only during extreme inversions usually after 9PM at night however when I broadcast anytime after 6 I'll go silent for a few seconds every hr or sometimes 1/2 hr to be sure its not trying to come in. 96.1 is a Very faint station out of Va Beach that only comes in at night and the next station that is licensed is 96.7Mhz. 96.5 gets bleed from that Rap station on FM which is 96.7 Mhz. I have looked at all stations in the area. Interesting however that I found on the database an 87.9 station several miles from me. Maybe I'll try and narrow my search in the database and see what happens but that wide of a search really messed things up.
I too have used Radio Locator to find my blank frequency and by listening. I've listened at different times a day and by getting folks to use a car Radio too.
I can see Translators are really getting to be an issue on FM and its gonna get worse. We always need to check our frequencies and these data bases do help.
In the Baltimore area I am stuck between the DC, Baltimore, Philadelphia, York/Lancaster/Harrisburg and Fredric stations. The Chesapeake Bay makes an excellent medium for radio to propagate over, so even stations from the Eastern shore come in quite strong. So if I want to use FM to transmit around my yard/court, I had to find the least bad frequency. Radio Locator’s unoccupied channel tool does not work for my area, most of the channels it says are open are not because of stuff coming in from across the bay. So I used my method described in post 1 to find the channel that was most removed from where the FCC might consider a service area for a full power station. Going up and down the dial in my car I hear something on every station, unless there is a local IBOC station, in which case I don’t hear anything on the adjacent channels to the IBOC local. Even 87.7 has two signals on it around here, an analog CH6 LPTV on the eastern shore being used for Christian broadcasting, and an analog CH6 LPTV in DC that has a Spanish format. 87.9 is useless from the IBOC sideband on 88.1 NPR.
Now that would be a case where you'd have to go AM and there will be some of you that this is your ONLY choice. And that is sad because you may not get too many listeners. The Talking House if you could get the hum out if it does look nice and easy for AM broadcasting and can achieve a 1 mile range from what some are reporting. We'll have to see how the AM petition goes as well. If passed and you can have a better antenna and ground you may get 2 miles on AM and that would be awesome. But keep in mind those Radio's today generally have poor reception on AM. I do wish you the best for you'll need it where you are.
We are probably lucky that "quad-sound" didn't go anywhere. There were some quadrophonic LPs released, but the equipment to reproduce all four channels was costly and nobody showed interest, so it vanished.
Not to be undone they tried to promote 5.1 channel sound, which would have required standing in the center of the room lfacing a single direction. Some TV stations were even mapping plans to implement it, but it went the way of 3-D TV.
Even stereo is sometimes more than needed, given the fact that human speech is by nature a monaural phenomenon, and acoustic musical instruments are by nature monaural.
Stereophonic sound only matters when groups of musicians perform ensemble style, and requires a special enclosed listening environment. Automobiles are good for stereo, and concert halls.
A past client wanted "surround sound" in her auditorium for single speaker events and she was unable to grasp that spoken language is clearest and easiest to understand if it comes from a single "point-source."
Birdies flying around her brain might sound good in surround-sound.
It was such fun for me to go frequency hunting that I have often "done it again" and amazingly things have sometimes changed and I discover new channels or lose old ones.
For AM broadcast over the air by antenna of course the search was mainly up above 1500 to 1600 kHz, and the X-band in particular (1610 - 1700) has seen some changing. For example 1640 from Wisconsin went quiet for awhile when the Disney station went silent pending sale, and is now back as a Catholic station but not as good a signal as before. That frees me to use 1640 which will be the AMT3000 Official Indoor Antenna Experimental station KHZ.
1680 was my main AM channel until I observed that the IBOC splash from the 2nd harmonic (1700 kHz) of local 5 kW 850 kHz was splashing on 1690 and brushing me, so I side-stepped to 1670 kHz which works fine.
Finding an AM carrier current frequency was a bit trickier and this is best done on the lower side of the band from 530 to 1,000 kHz. Right now local 590 kHz is silent because of financial trouble and would be a great CC channel, but before that I ended up on 970 kHz, everything below that was crammed.
The FM frequency search was even more complex than that and has put me through several frequency moves as the translators keep multiplying.
Things are pretty good right now, but watch out for an invasion of orange mushrooms from outer space,
88.1 and 88.3 were blank FM frequencies until 2006 I think when a God caster came on 88.3 and my frequency was taken thus FM cannot be used. I would have had to go Am probably if I still have lived with my folks or in Lansing. But its the capital city.
I think the room is closing in on open FM channels. I am in a rental house on the Blue Knob ski resort in PA and from the deck with an Insignia HD Radio Tabletop Radio I could receive something on nearly every channel from 88.1 to 107.9. I did not get any HD stations so I could even hear signals next to stations that filled the signal strength meter. There are many religious stations, some with the same content on multiple frequencies (translators). I am not sure if that radio is particularly good or if the FM dial here is just packed. Either way if I lived here I would have to find a least bad part 15 FM transmit frequency rather than an optimal one. If the FCC opened a translator window I would guess what open channels there are here would be snapped up, either by religious groups or by stations in Altoona (AM or FM) trying to better serve listeners due to the mountains and valleys.