In another thread, http://www.part15.us/forum/part15-forums/general-discussion/more-crowding-fm-band, there are some suggestions on how to reduce the possibility of adjacent channel interference from Part 15 FM transmitters which may use higher power than is now permissible. Rather than interrupt that thread with a technical topic it seems best to present some information here.
Harmonics, splatter, and spurs are words often used to describe interference and filtering is offered as a solution but from the context of the use of these terms there may be some confusion about this. The comments that follow are not textbook or dictionary definitions but represent my attempt to discuss this clearly so the terminology used can have some consistent meaning. Keep in mind that harmonics, splatter, and spurs can cause interference but each has different causes and different solutions.
HARMONICS are signals which appear on integer multiples of the carrier frequency and therefore are widely separated and will not cause adjacent channel interference. All transmitters produce harmonics and since they are widely spaced from the operating frequency they can easily be prevented from causing problems with proper and simple filtering. The filtering is simple because low Q filters can be used and often the use of a tuned antenna is sufficient.
SPURS, short for spurious emissions, appear at somewhat random frequencies around the operating frequency and my be in or out of band and are caused by such things as improper transmitter component layout, weak power supply decoupling, or improper neutralization or tuning of the transmitter. Though filtering is offered as a solution, the proper solution is to correct the deficiency in the design or operation of the transmitter.
SPLATTER happens when a transmitter is overmodulated or has severely distorted audio which causes the bandwidth occupied by the signal to widen beyond the "channel bandwidth" and this will cause adjacent channel interference because the effect is centered around the operating frequency (harmonics and spurs are not). Audio filtering and limiting of the modulation can be used to reduce this problem but filtering the RF signal out of the transmitter is not practical. Such filtering would require such high Q (selectivity) that expensive components such as cavities or crystal arrays would be needed. In addition, such filtering would introduce phase shifts which distort the modulated signal so RF filtering is not a viable solution. Careful control of modulation is required.
Interference can also happen even with a perfectly clean signal produced by a transmitter if the signal overloads a receiver or if it mixes with another signal producing heterodyning or "intermod" which is heard at the receiver. There is nothing that can be done at the transmitter to prevent this other than to reduce the transmitted power and this is one of the reasons for the FCC limits on Part 15 operation.
Preventing interference to other radio services requires addressing all of these different problems and each requires its own solution. Some are hardware issues and some are operational issues. If higher power transmitters are to be allowed then both hardware design and proper adjustment and operation of the equipment will be needed. Even with good equipment and practice things can go wrong and the operator needs to know how to detect and correct problems.
Neil
Splatter is one reason I don't like taxing my Audio input too high. I can tell when my aidio is too hot because I'll hear a Th sound wen S words are spoken. Trying to match the same audio level as a commercial station is often not a good way to measure this and if you don't have a modulation meter you really need to have your Audio slightly under the volume level of a commercial station. Pay close attention to the rated MV level for your audio and don't try and go over the millivolts that its rated at and that will be a good start. Make sure your music on your mixer doesn't go over +0 and talk should be a -3 to -5 db to avoid over modulation. Some FM transmitters can only handle about 60% modulation and be careful of that BASE. Really a flat frequency response it best, but if you do use some EQ be sure to watch your levels.
If you can afford it always use well regulated power supplies. This was a lesson I learned with the SainSonic AX-05B. That ticking sound from the switching power supply could be heard outside my fundamental frequency. This causes spurs within the FM band. You can hear it as hum, clicking sounds and can really cause havoc to your neighbors. And I've read how the CZH-15A caused spurs below its fundamental frequency. Radio Brandy attempted to post that on HB to warn folks because even though they could reduce the power from 0-15 Watts, the issue was a huge issue that no one who owned that make and model of transmitter could solve. This is why some part 15 operators have mentioned getting a part 73 transmitter and attenuating the power to meet part 15. then there was a huge squawk about not having a certified part 15 transmitter. However the operator can run whatever transmitter they want so long as no more than 250 uVm @ 3 meters is emitting from the antenna. So in a situation where as the signal is attenuated to meet part 15 levels with proper measuring equipment they would be far better off than a cheap part 15 transmitter and this it was too bad that site didn't explain how to do this legally. To the best of my knowledge you can own a part 73 transmitter and operate it so long as you don't exceed to 15:239 field strength rule. If not I'd like to see where this is the case. Many part 15 hobbyists who have the $2K to spend on a good solid transmitter such as that along with the proper attenuaters would do so to get better audio and without splatter.
I have a non-modified SainSonic AX-05B using the antenna that came with it and I also have a Procaster for AM. You may have answered my question, Thelegacy. When I plug audio into the Sainsonic, I get hum on my nearby radio from the Procaster. If the Sainsonic is off and audio feed unplugged to it, no hum on the Procaster. Does this make any sense? If that is the issue, should it go away with a regulated 12 volt power supply?
My SainSonic AX-05B would cause an analog TV audio signal to hum anywhere near the transmitter within 20 feet. The SainSonic AX-05B to me was not a clean transmitter and that is why I no longer use it. I went with the Whole House FM transmitter 3.0. There are a few other nice clean FM units. Now if someone built a cavity filter for you then maybe it would clear that up. But a better designed clean transmitter is always best.
radio8z Thank you for posting this information.
The talk about purchasing and operating cleaner transmitters, whether it be AM or FM is something we have to keep on the front burner.
My posts in particular, may have lacked better wording or usage of the words to describe the issue and how to resolve it in the most effective way.
Your post has cleared up some misconceptions and pens the proper way to resolve each possible issue that we are faced with. Your efforts, I think, covers what we must keep in mind when discussing th problem and how to resolve it when that problem arises.
Thelegacy Said:
This is why some part 15 operators have mentioned getting a part 73 transmitter and attenuating the power to meet part 15. then there was a huge squawk about not having a certified part 15 transmitter.
MrBruce Said:
I did not quote your entire post in regards to why you said what you said above, because, I wanted to put my emphasis on just that line of words.
It appears, since you menioned another website forum board, in your post, that you might be referencing my thread called "Attenuating High Powered transmitter for part 15 operation" Am I correct?
I asked, because I wanted to purchase a high quality transmitter and a 25 watt part 73 certified LPFM transmitter is what I was in the process of purchasing, it costed me $4,000 and had the same circuitry the licensed LPFM stations would use, or even the high powered ones, minus the larger amplifier section.
Yes, I was crapped on for even mentioning that transmitter in that owner's forum board, that's when I went under the PIRATE spot light over there and the owner even said so, before totally banning me. Owning a 25 watt LPFM transmitter should not automatically put you under the pirate spot light, specially when I asked if putting a device in the antenna line, would help attenuate the signal down to the legal part 15 RF power level, if the trasnmitter's output stage was also turned way down to 0.001. Well long story short, he scared me and I got cold feet, I called the USA ran compnay and asked if I can send the transmitter back in the same unopened shipping create and since they said yes, I sent the box back the same day it arrived without even opening it to see what it looked like up close.
Anyways, that's all I have to say.
Bruce.
Yes that was the post I was talking about. But here is the thing: You can run any transmitter you want as long as you keep the levels to meet 15:239. Since you didn't get the amp with it and only had the exciter, you could have ran the transmitter through a dummy load if you wanted or attenuator and that would have been sufficient enough to bring it down to 15:239 rules. Its unfortunate that some don't know this and immediately jump to conclusion. But to be honest you'd have to possess and know how to operate an FIM-71 field intensity meter to really get the power to the right level. It can be done however and should not make you a Pirate because you possess a part 73 transmitter.
On paper, coaxial cable (coax), attenuators, and a dummy load will move RF or simply turn RF into heat. In the real world, coax can leak or radiate signal. Attenuators or a dummy load can radiate signal. So while what Thelegacy said is true, verification of your actual field strength is important. It would be unfortunate if someone has an exciter feeding an attenuator with a known loss, coax with known loss, and an antenna with a known gain (all of which should result in just under 15.239 field strength), but then a NOUO was issued because RF was leaking from the dummy load causing the real field strength to be over the limit.
Coax leaks; systems of RF cable and connectors together leak. The cable TV industry has trouble with it to the point the FCC has set limits on the allowable leakage, especially in the air band.
Stvcmty: you mentioned the leakage of cable TV onto the air bands and all the way to around 154 Mhz. I know that all too well. I remember the birdy signal they sent on 107.9 Mhz ghat sounded like a kids Star Track Walkie Talkie signal. With a goo police scanner I was able to track down the leakage and a lot of the times when I could pick that up for hundreds of feet I'd call the cable company and get a tech who knew what he was doing. I'd show him with my scanner what was up. And in the early days I could sometimes watch HBO on channel 2 on a portable watchman tune to channel 2. When I had a color watchman I could watch movies in color and sometimes in perfect picture. Yup you have to be careful your coax is not leaking too bad. Better off just making a leaky coax network all the way to the back yard and no antenna with just the exciter and no final amp. Even without a Patomac FIM 71 you should get pretty close if you had a prior certified part 15 transmitter to compare. Its possible to get pretty close to the mark.
My guess would be any Part 73 transmitter or exciter, set to it's lowest output and run into nothing more than a dummy load will output more than legal Part 15 FM limits. This is only a calculated guess.
As I've mentioned before using a certified FM transmitter as the standard to then compare output on a device such as a spectrum analyzer is not going to help you. As we've seen in my certified FM transmitter field tests the odds that your certified unit is legal is unlikely. Of the three "name brand" and somewhat popular certified transmitters I tested, two were way over the legal limit right out of the box, and one was at about half legal limit. Which one might you have chosen to be your standard?
Now, I KNOW that we should expect a certified unit to be near the legal limit, but this is clearly not the case.
TIB
The point has been made previously that leakage from an attenuated output transmitter can be a problem. To illustrate this I draw from my experience with building and operating an amateur radio 2 meter repeater.
Since the repeater has to have both the receiver and the transmitter operating at the same time some means must be used to keep the transmitter output from overloading the receiver, essentially you need to attenuate the transmitter signal reaching the receiver by 70 to 90 dB. This is a tremendous attenuation and tuned cavities are required to get the proper numbers. Even with tuned cavities I found that the transmitter signal was unacceptably high at the receiver input. The reason was leakage from the coax cables between the cavities and the receiver input and leakage directly into the receiver enclosure.
The coax was replaced with double shielded type and this reduced the signal but to keep the transmitter signal (which is "in the air") out of the receiver required some heroic effort. The receiver enclosure was a cigar box size metal box with a lid secured by many screws. Even with the screws secured RF would get in. The solution was to make a gasket from coax shield braid and install this between the box and the lid to completely seal the box from RF which confirmed that the signal was getting in through the gap in the sheet metal between the screws which were placed every inch. The audio, control, and power lines entering the box needed coaxial feedthrough capacitors to keep the RF out. It turned out that it was extremely difficult to shield the receiver from the unwanted radiation.
Thinking about this in reverse where it is needed to keep RF inside a transmitter to prevent exceeding Part15 limits it is clear that this is not a trivial task and simply attenuating the output is probably not sufficient. It is not just a case of putting the lid on tight. In normal installations leakage from the transmitter is not a concern so extreme effort is not made regarding shielding. A few milliwatts leakage can be tolerated but not if one is trying to comply with 15.239.
I conclude that the only way to gain compliance with an attenuated over power transmitter is to measure the field strength and it is likely that without added shielding the unit will fail to meet the requirements. This is not something which I would waste my time on.
Neil
If you spent $4,000 for a part 73 25 watt transmitter you got screwed.
i contacted micheal and i will look into investing in a couple of decades with the mono balanced input and telescoping whip. they are built to part 73 specs (and sound as good as a part 73 exciter) and will handle our wild temp swings here in colorado when mounted in a weatherproof box outdoors. that is my main reason for using the MS15 it can be put indoors and not subjected to our wild weather. the Waio just doesn't sound all too great and doesn't handle our weather well.
WDCX said "If you spent $4,000 for a part 73 25 watt transmitter you got screwed."
I'm saying our Part 73 certified Nautel VS300 LPFM transmitter adjustable from 8 to 300 watts with the Orban Processor option cost about $6000. A bit less without the Orban option card. 2 years later they're even less.
wdcx Said:
If you spent $4,000 for a part 73 25 watt transmitter you got screwed.
MrBruce Said:
He's talking to me.
Total cost was close to $4,000 included all accessories, also included a 5/8 wave vertical GP antenna, included 100 feet of double braided coax, N connectors and shipping fees and taxes.
Why are you so negative towards me?
It's all good I can take it.
And no, I didn't spend anything, like I said, like 100 times now, I sent the package back to the company UNOPENED.
I seen a LPFM transmitter sold to a Spanish station for way more than that, look up W283BW, LaBomba! bought it from Revival Christian Church in Groton CT and moved it to Montville, CT up top WCTY's tower on Cook Drive. It says city of license is New London CT, but the tower is in Montville CT.
Class: D, A Noncommercial educational operating with no more than 10W
That is on the application, 10 watts, sold for over $400,000.00 that's a lot to pay for a transmitter and license don't ya think? It was listed on the FCC daily digest back in 2014 sold at that price.
It's am FM translator for LaBomba radio, which is allowed to play commercials, and I mean real 30 to 60 second commercial spots, several played every station break like any other full powered station. There's no underwriters spots, they play commercials. I believe they are carried on tunein dot com, tune them in and listen to a LPFM play commercials.
Here is the current station info for the station in question.
http://www.fccinfo.com/CMDProEngine.php?sCurrentService=FM&tabSearchType=Appl&sAppIDNumber=1544231
Bruce.