Shocking News from the FCC (Good News)

There are a few charts floating around out there. I seem to recall one that showed dbf relative to distance and showed the strongest radio would have issues at about 1000feet where the dbf drops below either 11 or 8 dbf.

Here is that chart from Ramsey Electronics, it was printed in the manuals that were included with the kits, i am not sure how accurate they are.

The point is that the formulae stated by Mighty1650, as well as the chart posted by BlueBucketRadio (which is really just the tabular form of the inverse linear relationship between field strength and distance, which Mighty described in his first post) is the absolute best you can hope for with a Part 15 compliant transmitter. Both describe physical laws in free space, with no obstructions, ground reflections, etc. In other words, an FM transmitter on a pole, very high up with nothing surrounding it, is about as close as you can get to ideal (unfortunately, you can't do much about the atmosphere). Even a transmitter on the 24th floor of an apartment will be affected by the building surrounding it.

Temperature, humidity and a whole host of other atmospheric factors will certainly affect range, both negatively and positively relative to each other, but the stated ranges in that chart, associated with receiver sensitivity, are not going to be exceeded, no matter how much experimentation you do (unless, of course, you use a non compliant Part 15 transmitter). Experimentation would be useful in determining just how much temperature, humidity, geographic location, etc. do affect a Part 15 FM signal - like TheLegacy and others have reported, I do see significant differences in range depending on the current weather. Does ground conductivity have an affect of FM (we know that it affects AM)? What kind of building materials impede a Part 15 signal the most, or the least? At what height can you eliminate ground reflections? What are the effects of hills? Etc.

Also to be considered from Part15.us and Radio8z - it takes more than a 1uv field strength to generate a 1uv signal at the receiver antenna terminals. That's another factor which is certainly not going to allow you to increase your range. Even ignoring that (but in the real world you can't, obviously), if you have a very good receiver of 1uv sensitivity, the best you can hope for is up to 2520 feet range (maybe 1/2 mile). It is highly unlikely you're going to be able to get that kind of range due to all those other factors described, but this is the upper bound.

It's a lot more than 200 feet. But that's never been part of the rules, just a guideline from the FCC to those who don't own a FIM. The maximum field strength is the only Part 15 FM rule.

Going from the chart again, if you have a good portable with 7.5uv sensitivity, and once again ignore the fact that you need more than a field strength of 7.5uv to generate that kind of signal, you're looking at a maximum range of 96 meters or 328 feet. A more typical home/portable receiver might have sensitivity of 15 to 30uv, with corresponding maximum ranges of 24 to 48 meters, or 78 to 157 feet. That's why the FCC advisory or bulletin says what it does.

In a study of receiver sensitivity that I did recently, some of the lousier ones had sensitivities of well over 50uv, which translates to ranges of 12 meters or 40 feet and under (maybe a yard, sometimes barely across a room).

I apologize if this analysis doesn't meet the expectation of some who hope for more range. This is not being negative, it's actually putting the most positive spin possible on the science and physics behind what we do. It actually does show that if you can get a transmitter up high enough, with absolutely no obstructions, to the right receiver you'll get a lot more than 200 feet range.  And with experimentation, we might be able to nail down exactly what effects weather and other factors have on range.

I still remain very interested in exactly what the FCC agent was asked, and his/her responses. Perhaps there is a little bit of wriggle room between the rules on one side and what FCC inspectors look for in the field on the other side.

Of all the factors that determine Part 15 FM range, the receiver sensitivity is by far the most important, and it's also highly variable.

Here are the results of that receiver sensitivity test that I mentioned (posted over at Part15.us). In the tests, stereo signals 20db SNR & 46db SNR were injected into the receivers:

Car Radios (9 in Total)

20db SNR: Average 1.3 uv, Best <1.0 uv, Worst 6.3 uv

46db SNR: Average 80 uv, Best 56 uv, Worst 800 uv

Portables (5 in Total)

20db SNR: Average 4 uv, Best 2.3 uv, Worst 5.6 uv

46db SNR: Average 100 uv, Best 56.2 uv, Worst 141 uv

Handhelds (6 in Total)

20db SNR: Average 8 uv, Best 2.3 uv, Worst 28.2 uv

46db SNR: Average 400 uv, Best 200 uv, Worst 1000 uv

It should be noted that 1 car receiver, 5 portables, and 4 handhelds could not even receive a stereo signal at the specified levels, so were not included in the results (i.e., the results are skewed towards the best performing, and even then, highly variable).

It should also be noted that 20db is still a relatively noisy signal, which listeners might tune away from.

Hi Bruce, A couple of questions.  Is the 107.7 station HD? My experience is that the potential for creating interference to adjacent channels is much higher.  That being said, you might still want to contact the FCC and mention the interference caused to other licensed stations on the adjacent channels. How is your planned return to the airwave coming? 

I know I have even complained about this, as far back when I was still active at HB, my post over there, was moved to the principle's office because I complained about a licensed radio station on a part 15 site. It was frowned upon over there.

Druid Hills Radio, I know WBMW 106.5 has several HD channels, again WBMW and WWRX are owned by the same person, he has one under his real last name the other under the name of his broadcast franchise name. I am keeping that info out of this thread, you people can look it up if you wish to.

I am not sure if 107.7 has HD channels at this moment and time, I am not really sure, without an HD radio how to find out.

Troy, I do not think the HD channels are what is causing the audio to vibrate, it is actually the music, that is deliberately OVER PROCESSED to appeal to the popular that like music that vibrates their car's to hell with BASS!

Now to be truthful, most of the African American community here, listens to foul language filled CDs, not WWRX, but then again, if need be for FM they do listen to this station for the clean Hip-Hop version of those songs.

Now at the transmitter site, they heavily process the music even more, specially in the BASS region, to make the music sound more BASSY than it does for the Highs. That is why it vibrates, it is processed to sound that way, plus they take full advantage of all the modulation they can push before reaching that illegal limit, but when doing sound level tests with VU meters, WWRX pushes the meters over in the red, where WBMW is close, but not quite as extreme as WWRX.

You can look both stations up if you wish to at the FCC data base, WBMW 106.5MHz is the other station I spoke about in my previous posts in this here thread. Both stations are in the SAME studio and same tower location.

In fact here is a photo showing their studio located on Colonel Ledyard Highway in Ledyard, CT. Notice the WBMW Soft Rock 106.5 and Jammin' 107.7 on the signs in the photo: CLICK IMAGE TO ENLARGE!



Bruce.