- May 22, 2019 at 6:09 pm #111242radio8zSenior Moderator
Total posts : 232
A recent thread here reported on a NOUO (Notice of Unlicensed Operation) regarding interference to the aircraft band caused by a FM transmitter. Let’s do a review of what technical problems can cause interference, first for FM.
Harmonics are frequencies which are integer multiples of the intended carrier frequency and which are produced by all transmitters, for example, the harmonics of 100 MHz are 100 MHz, 200 MHz, 300 MHz, and so on. Though these are possible sources of interference they are predictable and can easily be mitigated with filtering. A certified transmitter likely will have filtering built in to suppress harmonics.
Short for spurious emissions, these are not predictable and can occur at any frequency and cause serious interference since they usually contain significant energy and cannot be effectively filtered. These are caused by deficiencies in the transmitter design so the best prevention is to use an unmodified and certified for Part 15 FM transmitter. The practical limit of range of about 200 feet gives some assurance that even if spurs are in the signal that the range will be limited which minimizes the chance of widespread interference.
Splatter happens when the transmitter is overmodulated. There is no intrinsic limit to the modulation percentage on FM but what happens is if the DEFINED deviation is exceeded the bandwidth of the signal widens and can interfere with adjacent stations. A simple check for this problem is to tune a few hundred kHz off the carrier frequency with a receiver and listen for noise which should not be there. A good way to set deviation is to adjust the transmitter audio to match the volume of a commercial station on the same receiver. Though not exact, it is almost always close enough.
These are usually not a problem of AM if the transmitter is well designed. Even when present, the 100 mW input power limit for Part 15 AM, if observed, will produce milliwatts or microwatts of harmonics. A base coil tuned antenna system will greatly reduce harmonics.
Also a rare problem with a well designed AM transmitter. Listen up and down the band around your carrier frequency and you will hear them if they are present. You may also hear the most common form of interference which is:
Unlike FM, AM does have a physical limit on modulation which is at 100 percent when the carrier amplitude is zero. If this reaches 100 percent before the audio peaks then it is overmodulated which causes RF frequencies surrounding the carrier to extend beyond the intended bandwidth and which will interfere with adjacent stations. Splatter can be detected by tuning up and down the band around the modulated carrier frequency and listening for interference. If you are 20 or 30 kHz away from your carrier frequency and you hear noise which correlates with your audio you are likely over modulating. This results in distortion of the audio signal as well as nearby channel interference and is to be avoided. Even certified transmitter can produce splatter so it us up to the operator to make the proper modulation adjustment to prevent this.
This is just an introduction to some of the possible problems which can affect Part 15 (and all other radio services) and I kept it as brief as I could. Some depend on the transmitter system design but, especially, modulation is under the operator’s control. Please add to this or ask questions as you wish.
- May 22, 2019 at 7:24 pm #111244ThelegacyParticipant
Total posts : 271
Thanks Neil for the post. This should solve a Lot of issues.
- May 23, 2019 at 3:37 am #111246rock95sevenParticipant
Total posts : 56
When Free isn’t always better
Back in 2016 when Rock 105.3 FM became Blue Bucket Radio of London Ky, I won a Whole House 2.0 FM Transmitter from the Whole House Facebook page.
I thought that’s cool, now i don’t have to plunk down the $$ for one of these and can finally try one out. Boy was I ever disappointed. It sounded pretty decent, the AGC didn’t work and channel preset buttons became harder to push after a month of use.
But that’s not the worst of it, one day I fired it up to listen to one our shows over headphones so as not to bother others in the room with me, when i pressed a preset the dam thing didn’t show the preset of 105.3 mhz but rather 163.500 mhz!
Immediately and almost without thinking I grabbed a police scanner and punched in the frequency 163.500 however the scanner wouldn’t take that channel so i just let it scan the channels I programmed into it.
The signal showed up across the GMRS/FRS bands at around 462.6625 mhz!
UH OH! Not only that, but i was able to press the channel selector up or down on the WH 2.0 and it would slide around the band above and below FRS which means this sucker was probably messing with ham and business 2-way radio’s in the London area.
Needless to say, I stopped using it and stored it away for safe keeping.
Below is a link to a YouTube video I did to show the interference in action.
- May 23, 2019 at 1:45 pm #111251
- May 25, 2019 at 12:05 pm #111273ArtisanRadioParticipant
Total posts : 498
Every transmitter generates harmonics, and can also generate spurs as well. Part 15 certification means, among other things, that the spurs and harmonics generated are relatively small compared to your fundamental frequency (so many db down).
If the Whole House 3 transmitter is indeed certified, and it is both for Canada and the U.S. (you can look it up in the equipment databases), you could still pick up those spurs and harmonics on a receiver if that receiver was relatively close to the transmitter. They are still there, but they’re muted sufficiently to eliminate interference in the eyes of the FCC and Industry Canada.
That’s the beauty of certification – as long as you’re not transmitting over top of an existing station, you can be pretty sure that you’re not generating interference.
Not all Chinese transmitters are junk. The Decade CM-10 that I own (and that I’m growing fonder of, particularly if I transmit in its ‘sweet spot’ in the FM band, is just a reworked CZH I believe (power output tuned down) and it’s both Part 15, RSS210 and BETS certified. The Whole House 3 is Part 15 and RSS210 certified.
Neither holds a candle to the MS-100, but then, they don’t cost nearly as much either, and they do carry that all important certification.
- May 25, 2019 at 1:35 pm #111275MarkParticipant
Total posts : 510
Check out my report on using my new scanner to check my MS-100 for anything spurious in the communication bands as my station is operating. Confirms what the FCC agents told thelegacy about liking the Decade.
That malfunction in the other post regarding the Wholehouse isn’t good at all.
- May 25, 2019 at 8:34 pm #111277ArtisanRadioParticipant
Total posts : 498
Depends on where the receiver is in relation to the transmitter. If they’re beside each other, there’s probably nothing to worry about.
- May 26, 2019 at 11:22 pm #111292radio8zSenior Moderator
Total posts : 232
Artisan wrote “If they’re beside each other, there’s probably nothing to worry about.”
This is certainly true since receivers in a near field of a transmitter, and those in high strength fields can produce many artifacts, so what is a license free broadcaster to do to separate artifacts from reality?
Say I am using a transmitter for license free FM, what suggestions are offered to enable me to try my best to operate without causing problems?
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