Clustering

The clustering is possible on the Ragemaster because it can have the slave units synchronize with the master unit's oscillator. It forces the slave unit's oscillators to switch perfectly with the master so that all of the oscillators are timed precisely in their phase and rise/fall times. This prevents "beat frequency" problems when two or more oscillators on the same frequency are operating in close proximity.

The other units could be synchronized too, with considerable modification that is. They could be set up so that the slave units get their oscillator signal from the master through a coax, which feeds each additional unit with the operating frequency signal. The slave units then just act like amplifiers and only need power and the signal from the master unit. No need to run audio to the slaves since the master signal is being modulated at the master unit.

RFB

Question 1, if the slave units get their oscillator signal from the master, but the oscillator's place in the AM circuit is BEFORE the modulator stage, how can the slaves sense the modulation from the master unit?

Question 2, coaxial cable lengths become critical with high frequency signals, so how far apart can the slave and master be located?

Question 3, what kind of splitter is needed to drive more than one slave?

"Question 1, if the slave units get their oscillator signal from the master, but the oscillator's place in the AM circuit is BEFORE the modulator stage, how can the slaves sense the modulation from the master unit?"

Answer 1. The slave units, when being fed by a master oscillator with modulation, will not use their modulator portion of the circuit. You inject the master oscillator signal AFTER the modulator circuit of the slave unit.

/"Question 2, coaxial cable lengths become critical with high frequency signals, so how far apart can the slave and master be located?"

Answer 2. Since this kind of setup uses coaxes to send the master oscillator signal to the slave units, it does not matter how much signal level is traveling through that coax. Just make sure you do not use "leaky" coax, but regular shielded coax to feed the slave units. Separation of the units will depend on the local situation and ability to spread these things out, as well as budget considerations for the coax, and getting permissions to hang this coax and TX units where they are needed.

For example, if there is a section of the entire whole where the distance is considerable from one unit to the next, then you can use an amplifier to boost the master oscillator signal down the coax to the next unit, and if the signal is too much, simply attenuate it to properly drive the next slave unit without saturating it. Simple resistor networks can accomplish this.

Power for all of the units can originate at the master location by feeding the DC power down the coax and use that, along with DC isolation at each slave unit to separate the RF from the DC. So no need to string out another set of wires just for powering the slaves.

"Question 3, what kind of splitter is needed to drive more than one slave?"

Answer 3. The only time you would need a splitter is if your having to drive the coax that feeds the slaves with considerable RF power drive, like in the watts range. This will not be needed IMO. You can use simple coupling techniques like a pad resistor and a capacitor to couple from the coax into the slave unit. The pad resistor can serve two purposes here, attenuate the incoming master signal if needed, as well as provide proper loading characteristics before the oscillator signal is fed into the slave unit's final.

As long as each slave unit is operating within the limits per 15.219 and 209, it does not matter what is going on inside the coax feeding each slave unit. It can have 10 watts or more on it. But as long as each slave unit is 100mW at their output, your fine.

A lot of college CC setups did this very thing when feeding multiple buildings and using one transmitter. LPB made custom splitters and attenuators for this type of setup. But the concept can also be applied for a multi 3 meter antenna setup spread out over a given area.

Think of it as your own little tv cable system, which works basically the same. Instead of having cables run into homes, each coupling point along the feed coax has a small 100mW unit and 3 meter stick. In tv cable, there are booster amplifiers along the main cable feed line to maintain signal levels along the entire length of that feed line. Inside each booster amp and drop box, are attenuators that can be selected and inserted into the signal path so that each residence is not over saturated with cable signal.

The same technique can be used for clustering 100mW transmitters from a master oscillator/modulator unit.

RFB

Thankyou for the response, sir. Really appreciate it! Would you say that it'd be difficult to modify the other transmitters in order to accomplish what you posted regarding my using 'other' tx's in a cluster (2 or 3) as opposed to the more expensive Rangemasters?

Answer 1. The slave units, when being fed by a master oscillator with modulation, will not use their modulator portion of the circuit. You inject the master oscillator signal AFTER the modulator circuit of the slave unit.

This would not work unless the final amplifier is a linear amplifier.

It would seem that the unmodulated master oscillator signal could be injected in the other units after the oscillator stages without much modification with the audio signal applied normally.

Neil

"Would you say that it'd be difficult to modify the other transmitters"

Should not be too difficult. If you have all of the schematics on hand, and have electronic skills, should be a piece of cake.

There is a way to do this without even spending 80 or 100 or 240 bucks on additional kits for the "slave" units.

If you take the main oscillator/modulator feeding a main feed coax to all of the slave points, each slave merely needs to be a linear amplifier module with coupling circuitry for the 3 meter radiating element. The "slave" module could consist of a simple two transistor amplifier powered via the feed coax as well as getting its main oscillator signal with modulation.

This approach keeps things simple and easy to service in the field, as well as keep the slave units as small as possible so they do not take up too much room, or look out of place with the rest of the surroundings.

I believe there is a diagram of this kind of setup somewhere in the library resources.

RFB

I certainly appreciate the feedback. Gonna try this thing out. Will keep the forum posted on all of the goings on. Now, to figure out which of the 'other' tx's to buy to try it on. I like the simplicity of the I.Am.Radio units, but I'm going to do my due diligence on them all.

Yes the library source is a document by Ernest G. Wilson titled Carrier-Current Techniques subtitled: wired-wireless broadcasting.

On p. 32 is the diagram and text for a 100 mW Repeater which will rebroadcast your signal from a shielded coax cable, complete with a power control for the input to the final.

"I like the simplicity of the I.Am.Radio units"

The TH, or I.AM. units would be a very good choice as a master oscillator/modulator for the system. Simply hook up to its external antenna port with the main feed line. And the feed line can be the much cheaper cable tv coax, which the TH and I.AM. units are set up for a 75 ohm output anyway.

Then all that would be needed are the slave amplifier units and their 3 meter stick.

But you can use ANY transmitter as the main oscillator and modulator unit you want. Although the units like the Rangemaster or Procaster, and a few kits, do not have a proper output connector such as a BNC or F type connector, making the build of the system easy and redundant throughout. Although a connector can be installed in those units, a couple of them are meant to operate directly into their 3 meter stick instead of a connector or coax. Those are also carrying a certification number which means they must operate with the supplied parts/antennas/connector etc. Rangemaster was certified using a 102 inch CB whip, and the Procaster was certified using its supplied 102 inch aluminum rod.

So modifying those two units will no doubt void their certification. With this in mind, I would consider all the options very carefully before committing to the build of the system.

RFB

"This would not work unless the final amplifier is a linear amplifier."

Ya I left that part out simply because most here know already that a linear amp is required for low level modulated signals through a balanced modulator for AM. The amp must be linear across its gain range or the audio of the amplified signal will distort like crazy, not to mention cramp the signal itself.

"It would seem that the unmodulated master oscillator signal could be injected in the other units after the oscillator stages without much modification with the audio signal applied normally."

Sure can. One little problem though. Your going to have to run another set of wires just to carry audio to the slave units. That means a coax carrying the master oscillator signal and power, and another pair for the audio.

Why do all that when you can modulate the master oscillator through a balanced modulator IC, then feed that into a distribution amp that feeds the main feed coax, then on to all the slave units/amplifiers. With this method, only 1 (ONE) piece of cable will be required for the whole mess, and far far less expensive to boot.

ADDED 10/31/11

Besides all that, linking the Rangemasters up requires a cable between each transmitter anyway. At 750 per unit, then throw in the cost of the wire for the link, one could accomplish this with spare junk parts and surplus tv cable and build the repeater system and save a ton of money, and get the same if not better performance.

RFB

I'm really disappointed the new SSTran 5k did not include sync capability, it has everything else. Same disappointment with AspiSYS, there are several GPS units that you can pick up cheap and use for reference with a counter/divider on the pulse pin. I'm still working with Jack on the new PLL boards that have external 10khz reference inputs, and these would be great for syncing older transmitters. A simple comparator to look at the on-board oscillator vs the external ref could be used to stop runaways. In my opinion, the only solution to Part-15's archaic limitations, is having multiple sites linked together, manufacturers need to step up.

Tom Johnson
http://cat-am.com

one recommended upgrade is a high stability tcxo if these will be installed outdoors. this is a flaw with the hamilton pll option. syncability can easily be added to any am medium wave transmitter with just a few dollars in parts. gps sync can be added to any tx and a search of the fs/ft forum will yield a source for cheap gps receiver boards.

"Same disappointment with AspiSYS"

I actually spoke with one of their on-site engineers who designs their transmitters and they stated that putting in such a feature as transmitter synchronization would crank up the cost of the unit along with the added costs of the cabling.

The engineer said that it is far cheaper, and more reliable if you use a single transmitter and feed a coax which feeds repeater amplifiers at key points to extend station coverage.

I tend to agree 100 percent and then some. To me having 700 dollar transmitters in areas I would not always be at to ensure the transmitter's well being and operational status makes me just a bit nervous.

If $$$$ is not an issue..go for the more expensive option by all means! It works either way. 🙂

RFB

Two or more synchronized transmitters operating on the same frequency will produce nulls at certain points where the signals overlap. The position of these nulls can be controlled to an extent but they will still be there.

An alternative is to operate the transmitters on different frequencies. This may not be convenient for mobile listeners but would work for those who are stationary. It would be a matter for the listener to find the frequency which gives the best reception such as is done with FM translators.

Neil