FM Frequency

fm is limited to 250 uV/m @ 3m whether at 104 or 88 most of the part 15 fm's get peak range in the middle of the band.

From what engineers have said at radio and tv stations in my memory, the "skip" and "long distance" behavior so typical of shortwave and medium wave, sometimes gets up into the 50MHz region, where (analog) channel 2 is located, so tv stations assigned to channel 2 (and maybe channel 3) often had "ghosts" or interference from out-of-town-stations, but up in the 70 to 80MHz region things evened out for channels 4, 5 and 6, and as most of us know, channel 6 audio is receivable on an FM radio around 87.7.

So far as I know the FM band, 88-108MHz, is uniform in behavior all across the band.

BUT, the antenna lengths on FM are far larger at 88MHz and much shorter at 108MHz, with an average being 10-feet (unless I'm wrong).

The coincidence, or irony, is that part 15 AM is only allowed a 10-foot antenna, even though that's a terrible length for the very lengthy wavelengths of AM-Medium Wave.

On AM, the proper antenna lengths actually result in better distance out-reach at the lower end of the dial, 550kHz. But with only a 10-foot antenna, part 15 works best up top around 1700kHz. The AM band in general is less productive at the upper-end for licensed stations.

The FM part 15 rules allow ANY length or type of antenna, with only the resulting field strength being of any importance.

So ; putting FCC rules aside for a moment (I am really sure I am within the rules, I just dont want to get into another dogmatic diatribe), I wonder why I was once able to hit a couple of block (not AWESOME, but good enough to listen to) on 104.1 FM and now that I moved to 88.7, not so awesome.. maybe just background noise on 88 vs nothing on 104?

Carl, the difference in sizes of resonant FM antennas across the band is only a matter of inches.  I don't think that's a factor.

My experience has been that the 'best' frequency depends on the transmitter you're using.  It's rare that these transmitters will operate uniformly over the 20Mhz spread of the band - each will have their 'sweet spot' - the frequency at which they perform the best.  Sometimes it's difficult to tell that sweet spot, however, as other factors, primarily other stations on the frequency (or very strong stations adjacent to your frequency), have far more influence on your range than anything else.

I've found that my Decade MS-100, as an example, does best at the top end of the band (right now, I'm experimenting with 103.9 and 105.9).  But other transmitters that I've tested do better at the bottom or middle (sometimes they come with fixed length antennas, and you have to choose a frequency that is closest to the resonance of that antenna).  Like most things in Part 15 radio, there is no easy answer; you have to try it out.

And whatever works best for you - THAT'S the answer.

Artisan, you have touched on something I wondered about, but never before heard anything said about...

FM transmitters have a "sweet spot," even when they are capable of being set for any frequency. I have noticed that a few times.

Right now my Wholehouse 2.0, which in every other way is just like the certified C.Crane, is set on 89.5 MHz, and it works fine, EXCEPT that it also sends out a ton of spurious frequencies ALL OVER THE DIAL.

Fortunately those spurs are weak and don't go very far, but the same transmitter didn't do that when it was set at 107.1 MHz.

THAT is a major observation about the FM Part 15 transmitters.

For mir, the different experience at different frequencies is something I am unable to comment about, but it is interesting.

I have an interesting story, somewhat related to the 'sweet spot' of a transmitter.

Decade publishes a table for the MS-100 which lists, by each frequency on the FM band, the length that you should have its built in telescopic antenna set at.

I noted that the lengths were not exactly what a resonant antenna should be for those frequencies, sometimes off by several inches or more.  So I asked them about it.  They replied that yes, theoretically, the antenna should be at resonance for each frequency for maximum range, but the lengths they published were what they actually measured the strongest signal at.

The experience overcame the theory.

I suspect that interence is likely the cause of the differences in range that mlr experienced between the two frequencies.

Just as a Part 15 FM station will fade in and out depending on obstructions, topography, etc., so too do licensed stations.  And while at your transmitter site you may not hear anything on your frequency, there may be something there half a block away, ready to trounce on your already weakened signal.  Or something on an adjacent channel may strengthen and bleed over into your frequency.

You almost have to drive around your entire coverage area, listening, to determine the best frequency to use.

Without knowing anything about the transmitter being used, and its antenna, it's difficult to say if it could be something else, such as an antenna resonance issue.

Awesome.  These are some of the best responses I have seen yet.

"And whatever works best for you - THAT'S the answer."

Brilliant - I love this answer.  This is what happens in Part15 across the board.

"The experience overcame the theory."

And this, another great answer. 

We seem to have gotten so buried in what the calculators are saying, and relying on what has been proven on paper, rather than what actually "IS".

When I see an answer like "I don't know, it seems that the theory is disproven in this particular case by what has been tried, and here's what I did to try.." it makes me happy.  The whole reason I got into this is because of a conversation with a quantum physicist. Most people on this board are hardcore newtonian people (wether you know you are or not).  This means you look at "The apple fell to the ground at this velocity, and created this much energy, and heres the math to prove it."  The quantum folks are the opposite "Hmm.. I rebuke that theory, and I say the earth came to the apple."

I think the earth came to the apple, and this probably explains why I am always looking at numbers and saying "meh.. so what.. thats not at all what happens when I try it."

😀

Three baseball pitchers each threw an apple and their velocities measured.

An apple dropped from a helicoptor fell to earth and its velocity measured.

A Decade FM transmitter was broadcasting at the time and its field strength observed at the time of the apple experiments.

The results are posted on the ALPB How To Do It page.

lame plug 😛

I run 4 different frequencies using two different types of FM transmitter.  The Decade CM-10 gives much better coverage at lower end of the band.  In my case... Set to 91.9 or 92.7, the coverage is significantly better than when set to 99.5... which is slightly better than when set to 103.7.

The BroadcastVision Entertainment AXS-FMTD seems to perform about the same coverage-wise throughout the band.  The BVE is definately the winner in distance.  Side by side at any frequency, the BVE outperforms the Decade.  I also lean towards the BVE for fidelity and modulation.  It can be driven louder while remaining clean with great stereo seperation.

The higher the frequency, the more line of sight the recever needs to be to the xmtr. The higher the frequency, the less knife edge effect.

The range could be a little better at the lower end of the FM band but not noticeable, I'm sure..

Going from the Am band to the FM band, huge difference..

I can't comment on the Decade CM-10, which is basically a consumer grade transmitter (read, cheaper than their professional gear, such as the MS-100).  I've used the BV and it performed fine.  However, the MS-100, at least in my experience, vastly outperforms it.  And the Decade is tough.  I had one survive for 2+ years in a weatherproof box on top of a rooftop, running 24/7 during that time.  Nary a hiccup.

Of course, it's also a lot more expensive.

i have a BV BR8000 similar to the BV3001. i just don't have an extra $515.00 + shipping price tag for a Decade.