Wired links

Are these to be on the same or different frequencies?

Neil

Neil is asking a good question. If the coverage areas of the transmitters don't substantially over lap and are crystal controlled, it is possible for both to be on the same frequency without destructive signal mixing. So, actual signal coverage MUST be known, to be on the same frequency. Rangemaster transmitters have this capability in "daisy chain" mode. (See the Rangemaster web site for details.) In many cases, Rangemaster transmitters are placed at least a mile apart on a common frequency.

Otherwise, transmitters on different frequencies can be any distance apart. The wild cards in this question are coverage area, crystal controlled oscillators and syncing the audio (adjust for delay).

Let me suggest you take a look at linking the transmit audio via a Barix Instreamer/Exstreamer network. Part 15 multi-transmitter broadcasters across the country have found real success with this system, plus its affordable.

More expensive; you might consider a Trango Wifi system.

"Let me suggest you take a look at linking the transmit audio via a Barix Instreamer/Exstreamer network. Part 15 multi-transmitter broadcasters across the country have found real success with this system, plus its affordable."

I was hoping for close proximity, like 10 feet, using the same type of TX and split audio feeds from the same source, but I guess there'll be phase cancellation, right?

Barix units have to be linked somehow as well, like routers, wireless or otherwise. There will still have to be distance between the TX' for no overlap, and the only other ways are (expensive) wifi, or phone lines.

I was thinking there might be a way to matrix the signal to avoid overlap mixing/comb filtering problems. Looks like I'll have to use different frequencies. With no possibility of adequate ground radial system, I wanted to mount the second unit on my boat as well. Guess I'd better separate the freq's at least several channels apart in that case.

On PDF page 35, document page 32, of the publication Carrier Current Techniques by E. Wilson you will find a description of a system that uses one transmitter to feed a string of repeaters, each with a 100 mW input to its final stage.

This might be the answer, as only one transmitter means a greater chance that all the repeaters would be in phase over short wire distances.

SCWIS has a good suggestion as long as the transmitters are all on the exact (or very close-within 100 Hertz or so) frequency and all the audio is close to being in phase. Otherwise the audio high frequency audio will disappear at some point in between each pair of transmitters, plus the difference in audio phase can cause an echo effect in the receiver. These are all receiver products on the AM band only. Being able to adjust these parameters is critical to success with multiple transmitters.

Happy to tell you the caveats do not apply in this instance.

The master transmitter sets the AM frequency and the repeaters do not generate frequencies of their own, but follow exactly the frequency set by the master transmitter.

The audio modulates only the master transmitter, and no audio is separately run to the repeaters, the repeaters send only the modulation coming from the master transmitter.

An applicable caveat might be this: the repeaters should probably be located one wave length apart, otherwise they might cause phase shifting which may possibly bend the directionality in unexpected ways.

This is an opinion and all comments are welcome.

The diagrams show mostly distances of 200' between repeaters ... that's a lot more wiring than I can get into, plus it's all obviously intended for a campus type situation, which is nothing like I have here. There really isn't a level flat street in this town. Everything would have to be on tops of buildings, which are all private- or corporate-owned.

I guess the only thing that would stand a chance will be some sort of line-of-sight wifi or A/V wireless, if we are looking at these sorts of distances. I was just hoping for another water situation, since the grounding is so good.

How are the repeater antennas grounded?

I can see the roof of the Whale Museum building from my boat (of course), but also from my studio. Not sure how I would ground a repeater antenna up that high, though ... any ideas?

I get so enthusiastic when a question gets asked that I think I know the answer to. The question was: "what kind of grounding with the repeaters."

Carl's opinion: I think each repeater gets treated exactly the same as a typical transmitter installation. That is, you go for the best legal grounding you can get, which usually means ground radials. Trouble is if you're up high, then the classic conundrum comes up when your ground wire and antenna and lead-in cannot exceed 10-feet.

Ground radials do not get counted toward length IF and ONLY IF they are underground and out of sight.

Was I right? Was I right?

Excuse me for acting a little silly. There was just a thunderstorm and I think there's nitrogen in the air.

In the treatise on Carrier Current Radio, "a repeater is a LINEAR AMPLIFIER." The transmitter is considered a low level EXCITER (100 MW). The distance from the exciter to a repeater would be a part of the transmitter and by precedent included in the transmitter (exciter-final amplifier), antenna and ground package and must be kept to the current 10 meter length requirement. You see, under this scenario, the repeater becomes the final amplifier for the transmitter. And its 200 feet away.

I AM NOT COMFORTABLE suggesting to anyone on this site to play with linear RF amplifiers that are not occupying a common chassis with the exciter. Even the new TH transmitters don't have any active components in the ATU. Under the current Part 15 rules, this would most certainly attract a citation. Plus for the repeater system to work each repeater would require its own output network and antenna or radiating system. This would compound the violation because each repeater would become part of the exciter and extend the length of the composite device. Other wise we could all build large empty boxes 200 feet by 200 feet by 200 feet with a 100 MW exciter and a 102 in ch whip attached to one of the outside surfaces. Does anybody think this would violate the intent and letter of Part 15? You betcha. In 1990 this was acceptable practice because it was outside the scope of the rules at that time; not any more.

The only exception would be an installation of the federal government or a station on a military reserve or base. Not even schools could get away with this one under the current rules. Remember, even though schools can operate with more power (as long as field strength limits are observed), they are still subject to all the rest of the FCC rules and regulations.

And just for reference, you might explain why the caveats have nothing to do with repeaters. In AM broadcasting, the design and construction of booster installations (repeaters) eventually deal with both caveats. Even wire links with analogue signals over 200 feet have delay when you are working in the RF and audio spectrums. Delay is just another word for phase shift or timing error. Delay degrades the signal and can cause distortion. On the motherboard of the Rangemaster you will find networks designed to adjust the delay.

All of these reasons, and more, have resulted in the reason that Keith Hamilton has chosen a much different approach to multiple transmitter networks than this idea. He has been synchronizing multiple transmitters for years with GPS, daisy-chained wired links and digital access points like the Barix system and Part 15 unlicensed Wifi. I have no doubt there are other ways to extend coverage. The question still begs an answer, how do you accomplish extending the signal and do it in compliance with the rules of Part 15 and others?

If this thread is just a mental exercise and asking the "What if?" question, then lets continue. But, if this is going to potentially lead to some reader building and operating a repeater system like the one mentioned here, I suggest we take a different tack.

Yes, the repeaters in this exercise are linear amplifiers, which is essential to their being able to amplify the previously modulated signal without distortion. But I tend to believe that this method of operation would be perfectly legal if properly engineered. Here is a crucial point, quoted from the previous message from Marshall Johnson, Sr.:

"The distance from the exciter to a repeater would be a part of the transmitter and by precedent included in the transmitter (exciter-final amplifier), antenna and ground package and must be kept to the current 10 meter length requirement."

Several points to register regarding this:
1.) the 3-meter antenna limit applies AFTER the output of the final, not prior to the input to the final;
2.) the 200' or whatever the length happens to be from the exciter (originating transmitter) to the final is a closed circuit system contained within the sealed environment of a coaxial cable. To fortify this point we consider cable technology:

Radio or TV transmission by cable does not require FCC authorization per se, EXCEPT to the degree leakage may occur, and there are guidelines provided by the FCC as to allowable leakage from cable systems. I don't know that these are Part 15 issues, I'm not sure of the rule section. The same responsibility holds true for leakage from carrier current systems.

Here's an example of a real life situation. When we produced audio/video here, I installed a house cable system which includes Blonder-Tongue television modulators for channels 7 and 13. The same company manufactures FM cable modulators, and these devices may be operated without a license provided they are used properly in a well designed cable environment that is shielded, terminated, and grounded according to standards.

Those same cable modulators are also capable of feeding up to 10-watts into an open antenna, which would be entirely illegal.

Therefore, in the case of driving Part 15 repeaters by way of a cable installation, we have a hybrid made up by combining two different RF technologies.

Carl commented earlier "An applicable caveat might be this: the repeaters should probably be located one wave length apart, otherwise they might cause phase shifting which may possibly bend the directionality in unexpected ways."

Even at 1 wavelength spacing with phase synchronization there will be mutual interference between the wavefronts of the two transmitters which will result in loci of points where there will be nulls in the signals. The spacing between the antennas affects the total radiation pattern and this, applied with phasing controls, is how directional AM stations control their patterns.

Neil

Under the current Part 15 rules, this would most certainly attract a citation. Plus for the repeater system to work each repeater would require its own output network and antenna or radiating system. This would compound the violation because each repeater would become part of the exciter and extend the length of the composite device.

Ahem - in your opinion, that is.

And opinions are welcome, as long as they are stated as such.

I just don't see it that way at all. The only relevant limit appears to be the "no more than five user constructed units in use" and I definitely agree that each repeater would be considered a final amplifier and one of the five permitted.

I also agree that each repeater would need to adhere exactly to the 3 meter radiator rule with no tom-foolery about the grounding - in other words, no unbalanced dipoles masquerading as a long ground lead 🙂

So, I see that as one transmitter and four repeaters, certainly fewer than the number shown in in the book.

I think it might be fun to use the instructions on building a splitter and to place the transmitter in the center and have two repeaters equidistant, say 100 feet in each direction.

SCWIS is correct in pointing at 15.23 - "Home Built Devices" in our discussion of 100-mW repeaters fed by an RF cable. The repeaters can only be home made, as no commercially available such device exists, and this rule specifies "Devices that are not marketed, are not constructed from a kit and are built in quantities of 5 or less for personal use."

The originating transmitter for feeding the cable is an entirely other question, as it will probably be above 100-mW so as to overcome cable losses, so it is a different class of device, NOT an intentional radiator! It could be home made or obtained commercially. If home made would it count as one of the five? I would doubt it, since it is not subject to Part 15, because it is a cable device.

The water is murky full of oil and the rules seem vague.

I want to perfectly clear as to "my opinion" as to the use of external power amplifiers for use with Part 15 AM BCB transmitters (intentional radiators).

"Subpart C_Intentional Radiators Sec. 15.204 External radio frequency power amplifiers and antenna

modifications.

(a) Except as otherwise described in paragraphs (b) and (d) of this
section, no person shall use, manufacture, sell or lease, offer for sale
or lease (including advertising for sale or lease), or import, ship, or
distribute for the purpose of selling or leasing, any external radio
frequency power amplifier or amplifier kit intended for use with a part
15 intentional radiator.
(b) A transmission system consisting of an intentional radiator, an
external radio frequency power amplifier, and an antenna, may be
authorized, marketed and used under this part. Except as described
otherwise in this section, when a transmission system is authorized as a system, it must always be marketed as a complete system and must always be used in the configuration in which it was authorized.
(c) An intentional radiator may be operated only with the antenna
with which it is authorized. If an antenna is marketed with the
intentional radiator, it shall be of a type which is authorized with the
intentional radiator. An intentional radiator may be authorized with
multiple antenna types.
(1) The antenna type, as used in this paragraph, refers to antennas
that have similar in-band and out-of-band radiation patterns.
(2) Compliance testing shall be performed using the highest gain
antenna for each type of antenna to be certified with the intentional
radiator. During this testing, the intentional radiator shall be
operated at its maximum available output power level.
(3) Manufacturers shall supply a list of acceptable antenna types
with the application for equipment authorization of the intentional
radiator.
(4) Any antenna that is of the same type and of equal or less
directional gain as an antenna that is authorized with the intentional
radiator may be marketed with, and used with, that intentional radiator.
No retesting of this system configuration is required. The marketing or
use of a system configuration that employs an antenna of a different
type, or that operates at a higher gain, than the antenna authorized
with the intentional radiator is not permitted unless the procedures
specified in Sec. 2.1043 of this chapter are followed.
(d) Except as described in this paragraph, an external radio
frequency power amplifier or amplifier kit shall be marketed only with
the system configuration with which it was approved and not as a
separate product.
(1) An external radio frequency power amplifier may be marketed for
individual sale provided it is intended for use in conjunction with a
transmitter that operates in the 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz bands pursuant to Sec. 15.247 of this part or a transmitter that operates in the 5.725-5.825 GHz band pursuant to Sec. 15.407 of this part. The amplifier must be of a design such that it can only be connected as part of a system in which it has been previously authorized. (The use of a non-standard connector or a form of electronic system identification is acceptable.) The output power of such an amplifier must not exceed the maximum permitted output power of its associated transmitter.
(2) The outside packaging and user manual for external radio
frequency power amplifiers sold in accordance with paragraph (d)(1) of this section must include notification that the amplifier can be used only in a system which it has obtained authorization. Such a notice must identify the authorized system by FCC Identifier." - [69 FR 54034, Sept. 7, 2004]

Unless I am misreading this rule and don't understand its meaning, this clearly says that from 2004 until this date, external power amplifiers are not legal to use with Part 15 AM BCB transmitters without the proper FCC authorization and marketed as a complete system. I am unaware of any company foreign or domestic that sells such an authorized system.

Additionally, "Carrier Current" AM BCB rules are NOT specifically a part of the "Sec. "C", Part 15 rules with the exception of 15.221 where field strength limits are mentioned for carrier current stations in the AM BCB (15uv/m @ 47,71456/f meters from the cable). And thus, I welcome the discussion of carrier current systems for the purposes of education, but not for encouraging the practice with Part 15 transmitters and equipment.

And finally, most of us know the rule, Section 15.219(b) of the Rules states "the total length of the transmission line, antenna, and ground lead (if used) shall not exceed approximately 3 meters." This is not just the antenna, but the whole system in its entirety. It should be noted that the precedent established by enforcement of these rules may clarify and change the character of certain parts to the total body of Part 15 rules at any time. CFR 47 is a dynamic document. Otherwise, can we agree to disagree?