Gerry wrote I'm not that familiar with this requirement and wonder how many people would want to do this?? - I don't know enough about it. From what I have read, it is complicated to set up and there is the expense of extra equipment and recurring charges for STL feeds etc.
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An issue about doing this that seems to get overlooked on these boards is the way that the radiation patterns of synced, multiple systems interact with each other, even when they are rather far apart geographically.
The link below leads to a net azimuth pattern plot of two identical Part 15 systems 2 km apart. Both transmitters supply equal and synchronized r-f power to each radiator, and the two radiators are driven in the same r-f phase.
So while this system would have no "zero beat" issues between the two carriers, and the modulation of each transmitter could be of identical phase, amplitude and polarity -- the coverage it would provide certainly would be less than desirable.
And if the carriers were not synced on the same exact frequency, then the net radiation pattern would not be stable. The peaks and nulls would be constantly rotating to face different compass directions. Using more than two synced systems can be even worse than shown in this analysis.
Not to discourage anyone from trying this, but knowing a bit about what may result should be useful.
Here is the link (paste it in your browser):
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Once again, Rich states a valid technical point. Having studied this type of spatial phasing interaction and having used spatial diversity antenna systems I can attest to the effects that he mentions. Keep in mind that changing the phasing only changes the pattern...it does not eliminate the nulls, they just move around.
I would also welcome any real life experience reports from those who have done this.
That aside, I still believe a good approach to multiple transmitters is to use different carrier frequencies where the signals overlap. This would eliminate the carrier signal interaction, audio phasing problems, and the need to link a sync. signal.
Gerry: it would seem that it is not too difficult to provide an external clock input to the transmitter for sync. but phase adjustment may be a bit more involved. I wonder if it is worth the effort. Also, having read this thread and having learned that you have been "down the road" with certification before, a couple of things come to mind.
If you provide an antenna then that is plus for certification. It would seem that certification under part 15.219 would then only require documented final input power and acceptable out of band radiation limits. I would think the cost of these tests would be modest as compared to the 15.209 field strength tests. This may be naive and perhaps other fees dominate the cost.
Neil
Well,I can relate my experience with setting up and running a simulcast FM paging system,basically,they suck! Sure,you can spend thousands on a GPS based phase locked high stability oscillator and many more thousands on audio delay modules,but when you get out in the overlap areas,you get anything but a clean signal. It was so bad,we had the manufacturers send their engineers in and after their best efforts,it was horrible sounding. Music quality? Not a chance! Voice was barely adequate! The signals bounced and tumbled off of everything,twisting the audio terribly! We switched to a multi-channel zoned system and forgot all about simulcasting forever. The next best thing is to minimize the overlap with your radiation pattern. Like putting them on opposite sides of big buildings with some separation in between. Or what we did in the 60's,alternated between two frequencies in an in-line fasion. You could be assured of a good signal on one or the other and no worries about sync of the audio or carrier. Regards,Lee
I know that the syncing of transmitters is not ideal. My point is that, considering this is the only way we Part 15'ers have of covering a larger area, it an option worth having.
As for using multiple frequencies, yes, that can be done. However, expecting listeners to keep track of which frequency you are on in various parts of town is asking a lot. Rememeber, if we make it difficult for them to find us, many won't bother. Also, having one frequency makes the station appear more professional and is less of a logistical problem. And what about people in their vehicle? Forget it.
As for internet vs STL feeds, I'm not saying that both can't be used. Whaterver suits a particular installation. Don't limit our choices to just one. No one is saying that you have to provide the STL equipment. In fact, your transmitter shouldn't care where the audio comes from, so this should be a non-issue.
As for syncing carriers, that is a separate issue from the audio and is not necessarily an STL specific issue. As long as the sync signal gets to the transmitter, it doesn't matter how it got there, although I will admit that it will most likely arrive via an STL type setup.
The comments about syncing multiple transmitters as being "less than desirable" are certainly true. However, what other choice do we have if we want to cover a larger area with a single frequency? We either use multiple transmitters that are synced or not synced. The synced option is the better choice of the two. If anyone has a better idea, I'm sure everyone here would love to hear it.
I'm not trying to knock your kit or your ideas. I'm just hoping you'll be able to provide an alternative to the Rangemaster, at a more affordable price. In my view, the Rangemaster has only two things going for it. One, it is type certified. This is certainly desirable, but not required in many situations. Two, it has the syncing option. As far as I know, it is the only unit that has this capability. Personally, I think that the sync option is his biggest selling point. Look at that station south of Atlanta (mentioned here recently) that is using 11 Rangemasters. In fact, every story I read about someone using multiple transmitters mentions the Rangemaster. I don't think the type certification is the only reason.
If (and when) LPAM becomes a reality, then that may be an option for us. If it isn't screwed up like LPFM, if it doesn't cost an arm and a leg to get a license, if you can even get a license, etc. Even then, Part 15 will still be the way to go for many people.
My two cents worth about the stereo suggestion - I'm sure it sounds nice, but how many of our listeners actually have an AM stereo receiver? I would guess almost none. I actually do have a SW receiver that can decode AM stereo on the BC band and I wouldn't care about a stereo transmitter.
From your previous comments, you obviously have at least built a prototype unit. Do you have any pictures? I'm sure we'd all like to see them.
Gerry, as you say, of course it costs more to put a SSTRAN in a weatherproof box, build the base-loaded antenna, etc. Like many others, I have done exactly that. Having to buy parts at the local Lowes and Home Depot in small quantity costs more, and I also had to buy a few tools and accessories. However, including the cost of the SSTRAN kit, I spent about $400 total.
Having a kit with everything you need in one package for a similar price would obviously be desireable. I also spent a great deal of time figuring out what I needed, buying parts, fabricating the antenna, assembling everything, etc. Although fun the first time, I'd rather not have to do it multiple times.
A kit that had everything but the copper pipe and blacl iron pipe would be nice. I would still want to build all of it, but getting all the PVC and adapters and wire, etc. included in the kit would be a big help. When I was researching the antenna for the SSTran that I wanted to buy I went to Homedepot. They didn't have all the adapters that I needed, so I would have had to put a couple extra pieces in the puzzle to get it all together, yet I could order all the pieces from different online companies so I know they are out there.
Thought you would like to see the current prototype transmitter.
It has built-in compression/limiting, auto power adjust, microprocessor auto servo tuning (so you don't have to be at the top of a pole to tune it). It's lightweight and rugged. The only problem I might have is that auto tuning of an AM low power transmitter has been patented by the ACTradio guys (I found that out after the fact). I have changed it to a second version that has a manual tuning arrangement: it has 2 buttons to tune the loading coil together with a digital signal strength readout. Remote tuning is very nice to have. The advantage of the second method compared to the first is that you can monitor the signal strength.
If I leave out the servo tuning I could reduce cost, but then you have to be at the top of the pole. The arrangement I have now is a 10ft 1.5" dia. pole that is hinged at the bottom. I can swing it down, work on the transmitter and then swing it back up again. This is on my kids old swing set - imagine if it was on the chimney :). The range at the moment is 1 -2 miles using a single 5ft ground rod. I can't pound the rod in any further because my clay is so hard.
Gerry
Neil, I'm really hesitant to add synching because it's such a can of worms. I could add RS485 master/slave circuitry but then you have to fiddle with delays to get everything just right. I have not tried any of this, but I cannot imagine that 2 transmitters separated by a fair distance would have the issues that the plot showed earlier. If I was doing a large area, I would still consider separate transmitters servicing select areas of higher density listeners running off Exstreamers via the Internet. This way, everything is straightforward; there are delays but no one would notice and the frequency would stay the same. True, some listeners would not get service.
For TalkingSign I got FCC and Industry Canada approvals. The tests involved intentional and unintentional emissions, and conducted emissions (through the AC adapter - which for AM at these low frequencies is hard to suppress). It cost about CDN$6K back in 2000. With a non-detachable antenna added, they would have to do an open field test which would add even more cost 🙁
Gerry
I appeciate the picture, but could you post something that would show the transmitter board and a closeup of the coil and tuning arrangement? You can't see anything of interest in that one picture, other than it looks nice and sleek.
As far as the auto-tuning arrangement, I would vote to leave it out, unless the additional cost is minimal. Although convenient to have, tuning is only something you're going to do at initial installation. I wouldn't want to pay a lot of extra money for a feature I would only use once.
Gerry wrote: "The arrangement I have now is a 10ft 1.5" dia. pole that is hinged at the bottom. I can swing it down, work on the transmitter and then swing it back up again. This is on my kids old swing set - imagine if it was on the chimney :). The range at the moment is 1 -2 miles using a single 5ft ground rod."
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Hi Gerry,
The description of your prototype transmitter reads well, especially the part about reducing the effort of getting a good impedance match between the transmitter and the 3-m, base-loaded radiator connected to it.
If you were to proceed with testing for FCC certification for your design, would you expect it to meet the applicable Rules in the installation configuration you outlined in the quote above?
Unless effective means were included in your transmitter design to electrically isolate the transmitter at radio frequencies from its mount, and from all DC, "ground" and program input wires connected to it, wouldn't the physics of this situation mean that the effective radiating length of the antenna system exceeds the 3 meters permitted by 15.219? Paper 3 at the link below goes into more detail about that.
http://rfry.org/Software%20&%20Misc%20Papers.htm
Just wondering what your viewpoint might be on this.
Rich
Gerry wrote: "...but I cannot imagine that 2 transmitters separated by a fair distance would have the issues that the plot showed earlier."
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Just to note that AM broadcast stations licensed for directional radiation patterns generate those patterns by using two or more vertical radiators separated by some physical distance, where each radiator is synchronously fed with the r-f power and phase needed to generate the pattern licensed by the FCC.
This is the situation in the plot I linked to earlier, although a broadcast station would never be licensed for the pattern in my plot 🙂
But the physics is identical.
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For FCC testing, I would just use the transmitter, attached antenna and short ground connection: I would not include the mast. I'm sure that most wiring going to these transmitters radiates something, but I don't believe the FCC is concerned with this. They are more interested in reducing complaints due to interference which is more likley to come from FM and not AM devices. R. Fry's plot is very interesting and goes to show how range can be increased using an elevated transmitter (and of course, a longer ground lead - which is in itself a grey area).
As I mentioned earlier, I do think it is advantageous to have remote tuning because then you can mount the transmitter up high and tune it properly. A properly tuned antenna increases range. Also, because these are low power devices, they are more susceptible to changes in the seasons; therefore requiring occasional re-tuning for peak performance. Maybe there should be 2 versions of this transmitter: one with with manual and the other with remote tuning.
Gerry
So what would happen to a cluster of synched Rangemaster transmitters in reality - would it work OK?
Wow!
The designer of the talking sign transmitter visitng Part15.us? HOLY COW!!!
Hi Gerry! Welcome!!!
I think this is pretty darn cool!
Multiple Rangemasters - back in about 2003 a person posted of their experiences with that on another board that's now gone (the pre-break-up version of Radio-Info). That person set up five Rangemasters at a school. The project was discontinued because the range wans't what the school had hoped for.
There was also another issue that arose in that the person who posted felt that while the RF syncing worked OK, the modulation needed to be similar across all five units or there was audio distortion in the receivers. The original post indicated that adjusting modulation effectively required an oscilloscope, which this school did not have access to.
This poster reported that a volunteer had visited this school and help adjust the modulation levels using the volunteer's oscilloscope an that this was helpful, but listeners still weren't satisfied. Furthermore, this adjustment needed to be checked periodically. IIRC, support for this project waned and the operation was terminated.
One wonders if five might have been severe overkill, and that perhaps two might have been better.
Rangemaster never responded to those posts (perhaps they don't as a matter of policy, I don't know) so we don't know what Rangemaster's response to that might have been.
Back in the early 1990s a Rhode Island outfit called DiPonti sold a pair of part 15 transmitters to be spaced 50 to 100 feet apart and sync'd with modular plug telephone cable.
Experimental broadcasting for a better tomorrow!
So, what is the status of the kit? I am certainly interested, but would like to see some more details, pictures and of course, a price.
If you need someone to do some comparison testing, I am getting about 1.25mi with a SSTRAN transmitter and antenna. I would be interested in trying your kit under the same conditions.