- January 28, 2020 at 9:26 pm #113993RFCCRadioAudioParticipant
Total posts : 34
Over the past few months, I’ve picked up a few more FM transmitters originally designed for cars without 3.5mm audio input/aux in jacks.
Here’s how much max range I got out of them:
Belkin Tunecast Auto Universal FM TX – 50 feet
Griffin iTrip Universal FM TX – 25 feet
Auvio Universal FM TX – 50 feet
and the big winner was the Scosche FMFreq with 220 feet max range.
Of course, all these transmitters needed a cigarette female to AC male wall adapter, or a cigarette female to USB adapter with the USB end connected to a USB female to AC male wall adapter.
Long story short, if you want a decent Part 15 FM transmitter, get a Scosche. The TuneIt and FMFreq have the same max range. The only difference is the FMFreq needs a 2.5mm male to 3.5mm cable or adapter to help get the audio you want to the transmitter.February 10, 2020 at 4:06 pm #114052radio8zSenior Moderator
Total posts : 247
Thank you very much for report on your testing of various FM “car” transmitters.
Sorry, but I think it is not workable to promote range beyond a few hundred feet on the FM band since the regulations (15.239) technically and severely limit FM transmission range.
It is also necessary to consider the other requirements for 15.239 regarding purity of the signals in addition to the range achieved. FM legally applied under the rules is essentially a “yard-casting” endeavor.
Please convince me that I am wrong.
NeilFebruary 14, 2020 at 11:06 am #114093
you are pretty much right meal but I will tell you this much the FCC agent that I talked to did say under optimum conditions you can get up to a thousand feet legally. He also mentioned the decade ms100. He said he knew it went a thousand feet but would not bust anyone for using it because it had been certified and tested. so I think the certification sticker on the transmitter does mean quite a bit for the FCC. If an inspector comes to your house and you have a certified transmitter show him the transmitter with the sticker. Worst comes to worst you will be told to get it replaced or repaired if it proves to be a problem.February 14, 2020 at 9:21 pm #114141
I don’t think that RFCC Lebanon was encouraging operation in excess of part 15.239 just reporting on a few models of off the shelf car plug in FM transmitters people would get to here an MP3 player in the car which I’m sure have the FCC # on it. He did not say they were modified in any way. Just saying which one worked the best. Although 225 feet is more than most of these go as the usual is much less than that max to even the best receiver.
Assuming RFCC Lebanon is in the USA.February 14, 2020 at 9:24 pm #114144
Second Look At Certification
I looked on Ebay for the Tuneit with the 3.5 mm jack on it. It was digital tuning and looks like it covers 88.1 and 99 frequencies past that. I don’t know if it uses .1 .3 .5 .7 and .9 or if it is in steps of .1 .2 .3 ect after the frequency. but there is something more important here.
I saw No part 15 Certification label on the package which is one of those blisters you have to cut open. The label talks all about how you can use it in a vehicle by using the 3.5 mm male to male patch cord. However nothing at all about FCC rules part 15. This is interesting since places like Walmart and dollar stores sell these and if you buy one and its past part 15 and it bares no sticker you are open to a NOUO or worse if the agent is having a bad day and rather or not it causes tremendous amounts of interference. One has to be careful with certain switching power supplies when used with FM transmiters as the ripple effect can cause the transmitter to emit unwanted spurs due to the AC hum. This is part of what goes on with the Chinese transmitters we see many issues with spurs and harmonics. In a car you have metal around you limiting the range so the FCC does not see a field strength issue. But now in your house or out on your porch without certification on FM is really dangerous. The FCC does not have much leniency for non certified FM transmitters from what the agent told me. If it does not have a sticker you will get a NOUO unless you can Prove your transmitter is not above 250 uVm @ 10 Ft and passes +- 75Khz for deviation. Over deviation is yet another issue for many FM transmitters and still will get you busted for not being on spec.
I’ve talked to the FCC too about my concern of transmitters that transmit on even frequencies such as 100.2 not 100.1 or 100.3. They do agree this can be an issue but they have not looked into rewriting part 15 rules to stop this.
Be careful on FM. If you find the certification please share it.February 14, 2020 at 9:40 pm #114146
Surprised as I always though those were approved with number. On the label or something. Guess they all aren’t.February 17, 2020 at 6:48 am #114186RichPowersParticipant
Total posts : 421
It’s because your transmitter was intended to be used in a different country:
Why do all [US] FM radio stations end in an odd number?
“FM radio stations all transmit in a band between 88 megahertz (millions of cycles per second) and 108 megahertz. This band of frequencies is completely arbitrary and is based mostly on history and whim. Inside that band, each station occupies a 200-kilohertz slice, and all of the slices start on odd number boundaries. So there can be a station at 88.1 megahertz, 88.3 megahertz, 88.5 megahertz, and so on. The 200-kilohertz spacing, and the fact that they all end on odd boundaries, is again completely arbitrary and was decided by the FCC. In Europe, the FM stations are spaced 100 kilohertz apart instead of 200 kilohertz apart, and they can end on even or odd numbers.”
That’s from “How Stuff Works” website, tried to provide a link, but it caused the forum software to flag it and grabbed my post for moderation.February 17, 2020 at 8:13 am #114184RichPowersParticipant
Total posts : 421
It’s because your transmitter is from another country:
“..FM radio stations all transmit in a band between 88 megahertz (millions of cycles per second) and 108 megahertz. This band of frequencies is completely arbitrary and is based mostly on history and whim. Inside that band, each station occupies a 200-kilohertz slice, and all of the slices start on odd number boundaries. So there can be a station at 88.1 megahertz, 88.3 megahertz, 88.5 megahertz, and so on. The 200-kilohertz spacing, and the fact that they all end on odd boundaries, is again completely arbitrary and was decided by the FCC. In Europe, the FM stations are spaced 100 kilohertz apart instead of 200 kilohertz apart, and they can end on even or odd numbers.”February 17, 2020 at 8:21 am #114188
The 100 klz spacing would allow for a while lot more stations but I assume not as good fidelity.February 17, 2020 at 8:35 am #114189
Going Back To Certification Of FM Transmitters
This has been discussed several times in these forums and on HB that no Fully Assembled transmitter that does not bare a certification label and absent of being listed in the FCC database shall be sold in the USA. So again this could end up that the user of such a transmitter especially in an open environment whereas maximum range could be achieved a citation from the FCC or worse. The fact that the transmitter was originally meant for a car which in most cases the car is made of metal so rather or not the transmitter is above the field strength in open space it would not be relevant since the metal in an automobile would greatly absorb the RF energy.
This is where things can get sticky because the transmitter is not being used in the way the manufacturer intended so rather or not the transmitter is certified on FM may not be relevent in this case of going over 250 uV @ 3 meters from the transmitter. Since the transmitter would be normally in a car the field strength may be way less than 250 uV at 3 meters. Thus could be why we see these non certified car transmitters sold whereas a transmitter that is meant to operate anywhere has to be certified.
Id like to know what the FCC’s take on that would be because if you read the part 15 rules section 239 and take it in face value by letter of the law these transmitters are illegally sold on store shelves. So why then has the Radio sherriff’s forum not covered that one?
It covered AM transmitters so much that SStran and others may have chosen to close down due to liability. Yet these car transmitters without certification are sold and now that hobbyists have figured how to make these work in the house what types of interference could these transmitters cause compared to the Decade MS 100?
I’ll bet that you put these against the MS 100 and I can already tell you which one would come out cleaner on a spectrum analyzer. Plus add the ripple hum that cheap cigarette adapters could cause to a transmitter actually causing an otherwise clean FM transmitter to now emanate spurs all over the band in the same way that the poorly made power supplies cause FM transmitters to do and you may have yet another problem. At $7-20 a transmitter I can already see serious issues here.
Any comments or thoughts to this? Really ponder on it.
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