Transmitter and antenna tweaking

Mojoe,

The range you reported is certainly decent. About the only thing that will increase the range is to add more ground length and considering recent posts on this subject this may not be the wisest approach. You are probably at the point of incremental gain in return for much effort but experimentation may be worth while.

Neil

If you are getting that kind of range with the SSTran you have done an excellent job of optimizing it! MoJoe is right about being careful about ground lenghts.

I have the SSTran and then bough the rangemaster - certanly more money, but it can modulate up to 125%, where the SSTran maxxed out about 100%. The extra modulation probably added a mile in all directions. To take advantage of the extra modulation you will need something to drive it hard and limit the positive peaks to 125% and the negative to 95%. I use an inovonnics box to do that. This is not cheap to do - the Rangemaster + Inovonics will cost between $1,000 and $1,300 together, so you have to decide how much that extra range is worth to you.

JGanley,

I am really interested in how going from 100% modulation to 125% modulation increases the range from about 1.25 miles as Mojoe reported to an additional mile as you claim which would be 2.25 miles. Perhaps there are some confounding variables such as installation details which account for the range advantage?

If I calculated this right, 125% modulation on a 100 mW. transmitter would yield a power of 100 mW. in the carrier plus the sidebands power of 78 mW. for a total of 178 mW. compared to a carrier of 100 mW. at 100% modulation which would produce sidebands power of 50 mW. for a total of 150 mW. Since the field strength is inversely proportional to the distance, the 125% modulated transmitter shoud produce an increased of range on the order of 25% assuming that the sideband power provides the range advantage. This does not reconcile with what you reported unless there are other factors to consider.

Comments?

Neil

Radio8z,
Probably I should have better phrased it as, "increasing your usable range"...
While your math may be right, I think the simple answer here is not that it increases the actual range by a mile, but that it increases the *usable* range by about a mile.
When I first installed the Rangemaster and tuned it up according to the direction, and made sure I was putting out legal power, I had range around 1 mile in two directions and a little more in the other two directions. Beyond that mile the signal was buried in the noise floor; it was there, just not usable. However the extra 25% modulation, which translates to about 2db, raised it enough above the noise floor to add about another mile all around. If 2db does not sound like much, remember that decibels are logarithmic in nature and a change of 3db will double the perceived loudness of audio.
Now if it were just an issue of more modulation than just increasing the gain would yield the benefit. However the rules allow for 125% positive modulation, negative peaks must remain under 100%. This is asymmetrical modulation. So we need a limiter that can process the positive and negative peaks differently. Many sounds, including the human voice and many instruments are asymmetrical in nature.
Some processors like the Inovonics 222 provide good asymmetrical limiting functions and minor compression, more advanced units can actually force more asymmetry into program material as well as performing DSP functions to sweeten the audio.
Hope this helps!

JGanley,

Thanks for your response. I am interested in your comment about about "putting out legal power" since the part 15 rules do not specify this. Part 15.219 references the input power to the final amplifier and that is all regarding power. No mention of modulation.

I am also interested in your interpretation of of "actual range" verses "usable range". What is the difference? Care to elaborate?

By the way, a 3dB increase in audio power does not double the perceived loudness of the audio; it is the minimum increase in power with which most subjects can perceive an increase in acoustic volume. According to many studies, ten dB equates to perceived doubling the loudness acoustically It has an entirely different meaning when applied to electronics, namely 3 dB is double the power. I am very well aware of the logrithmic nature of dB but it is nice that you are trying to educate me on this. Correct me if I err.

To quote you:

>Now if it were just an issue of more modulation than just increasing the gain would yield the benefit. However the rules allow for 125% positive modulation, negative peaks must remain under 100%>

I really don't understand your first sentence, but continuing, the rules do not address modulation percentage so what are you talking about? What do you mean when you say the negative peaks must remain under 100%? Don't they do that by definition?

Please help me understand your post. I cannot understand what gain your are referencing.

I still believe your claim to extended range using the Rangemaster (R) is most likely the result of the use of an extended ground lead rather than 125% modulation.

Your comments will be carefully considered and appreciated.

Neil

I'm certainly no expert, but let me try and address the asymetric modulation issue as I understand it. You can certainly overmodulate an AM transmitter. Even the SSTRAN can be adusted so. However, you will notice when you get over 100%, as your sound will be distorted.

Again, as I understand it, it is the negative peaks of the over-modulated signal that cause the distortion, not the positive peaks. I assume this has to do with the design of an AM receiver.

Since increased modulation is good for the loudness of your signal (thus more "usable" range), but distortion is bad, how to solve this problem?

I don't know exactly how the Inovonics 222 does its magic, but it manages to shift the audio so that the negative peaks never exceed 100%, while allowing the positive peaks to go up to 130% (according to the spec sheet). This gives you increased modulation without the distortion.

As an example of the benefit of increased modulation, I noted that there are many areas in town where I can hear my transmitter at 2+ miles. However, the audio is very low. Not in the noise, but not much above it. A higher modulation level would give me more volume at the receiver, and so a greater "useable" range.

There is no reason that an Inovonics 222 couldn't be used with a SSTRAN transmitter. That capability isn't unique to the Rangemaster. In fact, I'm looking to pickup a used Innovonics 222 (anyone have one for sale?). The Rangemaster does have a few other features that are useful, if you can afford the price.

I used the old PanAxis AM 100 (now out of production) that also managed more than 100% modulation. My experience with that transmitter might shed some light on this discussion.

For the engineers in our membership it might be easy to fall into the trap of calculating the signal strength of the transmitter without thinking about the receiver.

When I was under driving my AM 100 I found the range was a bit of a disappointment. I defined range based on how far I could walk away from my apartment and still hear my station on a radio receiver.

When I bought the PanAxis compressor and added that to my audio chain, finally driving the transmitter to the max, I was delighted with what seemed like an increase in range.

In reality there was no increase in range from a mathematical perspective because the uV/m was exactly the same with or without the compressor.

What was different, and what made me feel like my range increased (called "useful range" here by some) was that the increased modulation gave the receiver more to work with, in that the modulation of the weak signal was much stronger. That gave the receiver detector enough information to tell the noise control circuit to unclamp this signal because it's not noise, it's a modulated RF signal to be passed to the audio section and amplified.

Without the compressor and the with an under modulated signal my receiver interpreted my signal as noise and did not pass the signal to the audio section, hence the perception of "less range."

Another example of the notion that at part 15 power levels there's just a bit more to consider than the traditional, calculation driven assessment approach. It's all about maximizing effectiveness across a wide range of variables, many of which aren't even a factor at higher power levels.

Experimental broadcasting for a better tomorrow!

SCWIS: In reality there was no increase in range from a mathematical perspective because the uV/m was exactly the same with or without the compressor.
________
This is true only for the field produced by the a-m carrier, alone. But within certain limits, the peak and average r-f power in the a-m sidebands still can be practically and usefully increased by using a suitable audio processor.

The r-f/i-f AGC circuits in an a-m receiver are driven mostly by the value of the received carrier, and very little by its sidebands. But increasing sideband power means that field strengths of the sidebands have increased relative to that carrier, and therefore the detected audio S/N ratio will increase for a given carrier level and AGC value. This permits a somewhat better coverage radius for a given audio S/N ratio at the receiver output (other things equal).

Positive peak limits for amplitude modulation are set by legislation in some cases, and/or by tx and processor equipment performance capabilities. Maximum negative-going peak limits for a-m are set by physics and equipment capabilities, but in no case can exceed 100% without creating high audio distortion and r-f interference to other users of the radio spectrum above and below the normal r-f bandwidth of that AM station.

Many commercial AM broadcast stations use elaborate modulation processors to produce maximum perceived loudness at the receiver without violating the limits of the previous paragraph. Quite often this results in a modulation envelope whose average value rarely falls below ~100% on their modulation meters.

For a given set of positive/negative peak modulation limits, it is the average value of the modulation that determines its perceived loudness to a radio listener. However the actions of the audio processor in generating a high average modulation value also introduce various audio distortions into the signal -- so as with many things, "tradeoffs" are necessary.
//

I'm getting ready to assemble the Panaxis AM100, just waiting for the last components to arrived before I create the board. Cost me about $35 in parts plus a power supply.

Well said mojoe, scwis & Rich!
There are many nuances to coaxing the most range out of our little rigs, proper tuning, good ground conductivity, good modulation and interfernece free channels. Some of these things we can control, others are luck of the draw!