Here's an excerpt from post in 2019 where Tim in Bovey clarifies why modulating 100% or 125% (which only the Rangemaster is capable of) results in increased range with a part 15AM transmitter. There's no big revelation here, most of us already know this but it's still an interesting point and Tim explained it well:
https://www.part15.org/community/transmitter-talk/designing-the-perfect-part-15-transmitter/
Excerpt from Tims post:
"... People will tell you that modulation more (100 % or even 125% positive) does NOT increase your range or signal strength. Well, yes, it does. For one thing you can WATCH on an analog field intensity meter the signal peaking higher with more modulation. Sure, it's a teeny tiny bit, but it is some. But what you're NOT seeing is that the louder signal increases your range not by giving you more field strength, or by giving you more output, but by giving you LOUDER output that covers up more NOISE increasing your effective range, simply because you can be HEARD over the noise floor. When I was working on my initial setup, I have a test spot about 1 1/3 miles from the transmitter as the crow flies. This is what I consider to be the fringe of my coverage. When I drive past this spot I get an acceptable signal in any one of 8 different cars, and when I park there I can listed on a $4 portable transistor radio, to a boombox, to a GE Super Radio, to a Tecsun, etc. But if I reduce my modulation to 100% I start to lose it at this spot, and if I let it go down to 80% peaks the sound is lost in the noise -- and I live in a tiny rural town with a VERY low noise floor. I have set up at this spot and had my wife adjust the modulation up and down back at the studio and I can hear the station come and go. While field strength for all practical matters remains constant, the increase in modulation greatly increases the ability to listen to the station, by increasing useable, listenable range.
So, while a Rangmaster may have a better signal strength by a smidge, with it's lack of processing my money says it won't be listenable at the same distance as a Procaster using it's built in processor. Yes, of course you can add an external processor to the Rangemaster. But this adds quite a bit of cost depending on what you want."
I wonder if John Sees ever designed and followed up on making a "perfect" transmitter.
Another way to try to squeeze out a bit more range is by increasing positive peaks which the Schlockwood has but I never tried it. But the transmitter has to be able to accommodate that.
As for the modulation with the Procaster processor I had to get a balance between that and the audio input level as the modulation control has the same effect as the audio in control has and I get as loud on air as I can without overmodulation distortion. A louder on air signal also gets you better signal to noise ratio at the fringe reception areas that Tim was talking about.
Overmodulating your signal is effectively adding distortion. Music purists will shudder at the thought. But yes, if you do overmodulate, you will increase your listenable signal range, but there is a cost. I've also wondered occasionally if that overmodulation results in more harmonics and spurs from the transmitter.
Going to mono in FM vs stereo is similar in concept. Although the overall field strength is identical, you reduce the potential interference from stereo (the pilot & difference), and get greater listenable range. Again, there is a cost (losing a channel), unless your source programming is all in mono (such as 45rpm oldies, vintage jazz, OTR, etc.).
I wonder if John Sees ever designed and followed up on making a "perfect" transmitter.
I was wondering the same thing, I haven't even heard is name come up on years.
Going to mono in FM vs stereo is similar in concept. Although the overall field strength is identical, you reduce the potential interference from stereo (the pilot & difference), and get greater listenable range. Again, there is a cost (losing a channel), unless your source programming is all in mono (such as 45rpm oldies, vintage jazz, OTR, etc.).
I always thought with psrt15FM you got more range by using only one channel because its more efficient to do so since it takes more energy to feed both channels.
@artisan-radio The FM transmitter like the Decade for example joins the two channels from the input to mono if selectable or the mono version does the same thing. Nothing is lost.
Same with the AM Procaster, the studio interface does that and its a mono balanced to the transmitter. The Schlockwood takes the 2 channel TRS input and joins the two channels to mono balanced out.
Just saying that transmitting in stereo will result in lower range, as you use some of the signal power for the other channel and the pilot. You also get more interference, sooner.