Going back to square one, is there any way of "clustering" Part 15 transmitters on a single frequency so that they can cover a large area without nulls? Expense being no obstacle.
Most common PLL chips have external ref pins, adding $10 worth of parts wouldn't break the bank IMHO. What PLL does the sstran and/or aspisys use? I'll draw it for them if they send me a test unit.
Tommy Johnson
http://cat-am.com
PS: Robert, I lost your number, plz resend it.
is there any way of "clustering" Part 15 transmitters on a single frequency so that they can cover a large area without nulls?
Simple answer, no.
Detailed answer, reinforcement or cancellation of multiple same frequency radio signals results from the wavelength distance each signal travels from the transmitters to the receiver. If the wavelengths traveled are such at the signals arrive 180 degrees out of phase then there is reduction of the received signal. If the amplitudes are equal then cancellation occurs. Otherwise, if not equal, the received signal is diminished. The same applies if the waves arrive in phase (0 degree shift) except here there will be reinforcement. The relative phase shift is determined by the frequency (in this case equal) and the distance from the multiple transmit antennas each signal travels. Phase shift is also caused by reflections which alter the distance the signals travel to reach the receiver.
Eliminating this effect would require that somehow each signal arriving at the receiver be phase controlled so they are additive (no phase difference) but since the phasing depends on wavelengths traveled and hence position this cannot be done for a single receiving antenna. Spatial diversity reception can be used but this requires equipment which is not typically used by the intended part 15 audience.
Short wave listeners have heard stations where the signal fades in and out. This is the same principle but in this case the signal is from one transmitter site but the paths due to reflections from the ionosphere differ in length creating phase shift which causes the signal to vary in strength as the reflections change.
Neil
Again this goes back to the point about a single transmitter feeding a main coax which feeds the 100mW repeaters placed at distances from each other to provide uniform coverage.
This arrangement can also give some phasing effects and null effects if not done properly, but the effects are minor compared to two or more transmitters combined without proper phase matching. And the latter is still incredibly expensive compared to simply setting up little repeaters being fed the main TX signal via a coax.
RFB
How is the use of repeaters different from using multiple transmitters?
Do not both radiate signals which might overlap?
Neil
"How is the use of repeaters different from using multiple transmitters?"
The main difference is that your not using multiple transmitters. Your using one transmitter feeding a coax which provides the signal to the repeaters.
And of course they are going to overlap. You get that either method you choose to use.
The point with either method is to extend the range, and your going to get overlap no matter what. And with the use of a single main transmitter, you will not have to deal with issues of synchronization accuracy, which you know as well as I, if that synchronization is not precise, you end up with an array of beat frequency problems and audio phase bucketing between each unit as you pass from one to the other.
The other method..the one I keep pimping here, has no issues like that at all. And I point out once again, is cheaper and far easier to maintain.
Sometimes it is not necessary to needlessly complicate things just to say to one self "I have a complex system".
But if funding is not an issue, go for the multiple transmitter approach by all means. I am merely offering an alternative that DOES work and has been PROVEN to work and is documented by one of the best that ever existed in this low power radio world. Maybe ask him?
RFB
Spreading your transmitters around town whether they are synced or not seems like a safer alternative to clustering several transmitters in close proximity to boost the signal.
5 transmitters feeding one antenna or each its own, I would think the Inspector would feel you are pushing the intent of the Part 15 rule by purposely exceeding a calculated norm under Part 15.219. I don't think you can even get away with inductively feeding a tower by placing the 10 foot antenna in close proximity to take advantage of reradiation.
By the way, there is an old publication I've seen that addresses the idea of repeaters interconnected with coax. Somewhere around here I have the schematic for that if anyone is interested.
I'd appreciate it if you could post that particular schematic, sir. You and RFB seem to answer the question forthrightly and direct to the point. I appreciate all of the other responses posted here as well. I knew this topic would generate this kind of a reception. I love it! I am going to set up my cluster in approximately two weeks; I'll use 3 AM transmitters in all on a singular roof.
I am leaning towards the I.Am.Radio units to accomplish this goal. I will be utlizing their tunable, FCC certified antenna extenuator as the sole antenna for them all. One unit will serve (as mentioned by RFB) as the 'Master' from which the computer generated audio will come from. The other two, linked via cabling, will become the amplifying 'Slaves'. I will be using a linear amp as well.
The I.Am.Radio units are unique iin that grounding them only requires plugging the issued wall wart into an electric outlet. Now, that being said, when I use the antenna extenuator, it will have to be traditionally grounded. I already have a bead on how and where I'm going to ground them up there to remain compliant. (However, Other rooftop suggestions are welcomed). I will keep everyone abreast of the results of the experiment.
Quoting RFB, "The main difference is that your not using multiple transmitters."
Yes you are. The "repeaters" radiate signals equivalent to those of transmitters. You wrote "And of course they are going to overlap. You get that either method you choose to use." with which I agree so what's the difference except what each radiating device is called?
The point is that multiple same frequency radiation sources within range of a receiver will interact to affect the signal at the receiver. As was discussed, this interaction depends on the relative signal strengths and phase relationships and one system will not have an advantage over the other in this regard..
Another consideration in choosing which system to use is the efficiency of a linear RF amplifier will not be as high as that of a class C or E transmitter, so the output signal for a fixed 100 mW final input will be less.
If it is desired to use a single coax to connect synchronized transmitters then the power and audio could be frequency division multiplexed with the carrier reference on the same coax.
I am not arguing one system over the other but am sharing the thoughts I have as I consider the two possibilities.
Neil
look in the library there are old ernest g wilson and jr cunningham books there that explain all this including the repeater concept.
"Yes you are. The "repeaters" radiate signals equivalent to those of transmitters."
Im sorry, but technically the repeater amplifiers are not transmitters by themselves. They cannot transmit anything without a source signal. They radiate a signal just like a transmitter does, but by definition they are not a transmitter.
A transmitter has the ability to generate its own frequency of operation, or multiple frequencies such as that with a variable tuned transmitter or one with a PLL system or DDS system or capable of accepting various crystals. All of those generate the carrier to transmit.
Those repeater amplifiers cannot do anything but amplify a signal being applied. Without that source signal, they are merely taking up space and doing nothing....unless poorly built or installed and its self oscillating.
I wont debate the issue either. After 48 years of learning RF electronics and engineering, and after being told the same thing by countless professionals and "old tymers" of the HAM community, I tend to stick with what was taught and what was learned.
In the "big boy" world of radio, an amplifier cabinet for a broadcast transmitter is indeed called a "transmitter". But it is not. It is merely a big amplifier..fed by what.....an EXCITER. Now without that exciter...that "transmitter" cannot transmit anything..right?
So technically what is that big cabinet next to the rack holding the EXCITER? An amplifier..nothing more.
Even units combined into a single cabinet, ie amplifier and exciter in one box, they are still separate units by themselves doing a specific job.
But I am not the one to debate with in reestablishing a new definition to what the device actually is. I just stick to what I have been taught and what I have learned. It is others, far more up the chain that you should convince that an amplifier meant to amplify that has no ability to generate a signal is a transmitter in by itself.
I do not discourage anyone from trying these various approaches to expanding their station coverage. We are all tossing out ideas, and they are all good ideas. Some simple and easy to implement, others way to complex and out of most people's leagues here, not to mention out of budget ranges.
As the FCC said back in the 80's, so should we all say here and now with this issue.....LET THE MARKET DECIDE.
RFB
Taken from Earnest G Wilson's Carrier Current Techniques.
Page 26 in the original print, page 29 on the PDF.
It begins like this:
Exciter: An exciter contains at least 4 DISTINCT circuit functions. These include the oscillator (and buffer), a radio frequency amplifier, and audio amplifier-modulator and a power supply. The output of the exciter is normally a low RF power. In all other respects, it functions as a transmitter.
Power Amplifier: A power amplifier increases the small amount of input RF power to a much larger output power. When used to amplify an already modulated AM signal it must do so without distortion. An amplifier that can faithfully reproduce its input signal while increasing power is said to be a "linear amplifier"
And for the BIG GONG...
Repeater: A repeater IS A LINEAR AMPLIFIER. It is designed to accept low power inputs which are already modulated and provide an increase in that power. It is used at the end of a long run of transmission line to boost signal levels up to a usable value.
And for the ultimate definition of the two separate devices combined as a whole.
Transmitter: An exciter can be used as a transmitter but usually provides a low output level. When FOLLOWED BY A LINEAR AMPLIFIER...the entire circuit becomes a transmitter with greater output power level. Monitoring meters are included to facilitate tuning and other adjustments.
Ahh....so there we are...out of the mouth of the babe himself...as well as countless other far more experienced engineers than anyone here today. That last paragraph I quoted is saying that unless all these things are combined, only then can the entire combination be called a transmitter as all the pieces are functioning as one. But without each individual component, to which DO have their own definition as to what they are..the entire mess becomes not only useless, but refines as to what EACH individual device IS.
Putting all the eggs into one basket is convenient. However convenience does not define nor has ever defined the established standards to what individual devices actually are or what they are defined as. It is much easier to say in one sentence "transmitter" than it is to name out all the individual components that make up the end device..ie transmitter. I rarely hear conversation where the combined device is segmented into its individual parts in said conversations. It all gets compressed down to the convenient single word...transmitter. But just because there is convenience, does not mean the convenience overrides the actual term and definition for each of the components that make up the whole.
trans·mit·ter/transˈmitər/
Noun:
A SET of equipment used to generate and transmit electromagnetic waves carrying messages or signals, esp. those of radio or television.
A person or thing that transmits something.
http://en.wikipedia.org/wiki/Linear_amplifier
No where in that entire wikipedia page does it have the word "transmitter" in its describing the separate device, which in this case is an amplifier. Nor does the referenced publication by Jerry Whitaker "The RF Transmission Handbook" refers to an amplifier being a transmitter.
RFB
It is the mission of a transmitter to provide transmission.
Interpretation is as vast as individuals on the planet. Each has their own they stick with. Perfectly fine for each individual. But not perfectly fine for everyone. If it was perfectly fine for everyone..aka one blanket covers all, then what is the purpose of discussion, debate, discovery and learning, much less open forums like this one?
It is the mission of each of us to share what we know and help others. Everyone here that has participated in this thread has done just that. It is the perfect example of diverseness in this field, and is a gold mine of information. Each member is a contributor...no matter how complex or how simple the advice is. It is that information that is important, not what one individual believes is the right interpretation or the other. It is up to each one who is taking in this information to sort it out for themselves and learn from the process.
Maybe I am just too old school...or stuck in the old ways or something. But I will state this for the record....it is the old school stuff that paved the way for the new stuff, without that established path, there would not be any path at all. It would be starting from the beginning all over again.
As new stuff comes about, new paths are created for even newer things. Its called progress. Its called advancement. But even with advancement there is always that back reference to turn to..aka a starting point.
With the clustering/spreading/repeating, all of the ideas presented here are worthwhile to investigate and experiment with. The more ways to do a thing means the better chances of accomplishing something. Isn't that the entire point?
RFB
I managed to find the schematic and description of the BCB repeater circuit.
Here is a LINK TO THE FILE with the info. It's a little fuzzy but I think you can read it.
If you can't view it there, I can email it to you.