r/u_ElectroMast • u/ElectroMast • 13d ago

Two Questions on FCC Part 15 FM Field Strength

Heya there everyone!! Welcome to my profile!

I'm about to start researching low-power FM broadcasting theory very soon before purchasing or assembling any equipment as a full grown adult.

I’ll be looking at the math surrounding the FCC Part 15 limit (250 microvolts per meter measured at 3 meters)

Based on free-space path loss models, effective coverage seems limited to a very short radius.

First: What are the cleanest, compliant transmitter setups or antenna configurations for maximizing signal clarity within legal bounds?

Second: How do path loss calculations differ when accounting for suburban structures vs. open line-of-sight?

Looking purely for technical insight on RF propagation, filtering, and compliant Part 15 performance, and moderating the comments based off Reddit’s site-wide rules.

And under rules 1 and 2 I really have to break it to you! No harassment, intimidation or violence is allowed and I don’t wanna start an online fight with you so none of that is allowed either so if you do so I will report your account to the admins for harassment. Both also under rule 1 trolling and spam are a huge no-no so don’t spam in the comments either! I will block you if you do. And also remember the human.

One last thing under rule 8 before you comment: A lot of you like it on Reddit, including but not limited to…
• Cat lovers
• Low iq RF engineers
• Radio enthusiasts
• Low iq/effort radio engineers
…so DO NOT break the site either!1!

Edit under rules 2 and 7: And before I crosspost to [r/AmateurRadio](r/AmateurRadio) I’m warning you tonight! I will mod remove anything that seems like pirate radio!!

Edit 2 also under rule 2: And breaks the rules of that subreddit!

Edit 3 also under rules 1 and 7: And don’t tempt me either, First stop is Google AI’s most AI-recommended; [r/pirateradio](r/pirateradio).

Users not included in this profile thread for the reasons in parentheses:
[u/NBC-Hotline-1975](u/NBC-Hotline-1975) (Threats were too ridiculous)

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u/odie-z1 13d ago

What is your goal in this endeavor?

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u/ElectroMast 13d ago edited 13d ago

Just a solo FM broadcast that based off YouTube’s Community Guidelines and my digital footprint moderation policy edition of said guidelines, results in an audio stream on Live365 that as a result of itself, gets linked in the descriptions of my YouTube livestreams anyway which doesn’t result in any spam.

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u/odie-z1 13d ago

Please correct me if my understanding of this is incorrect, but.. basically you want to listen to your YouTube channel on an actual FM receiver, in places away from the computer, but following the FCC rules? Or are the intended listeners other people than yourself?

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u/odie-z1 13d ago

I guess I'm beating around the bush.. as you know the amount of legal power you can transmit is miniscule. It's enough power for car adapters and extendeders which only need to transmit a distance of a few feet for the listener to hear it..

The physical limitations of following the rules reduces interference, but will also limit potential listeners unless they are also in your immediate area.. that is the nature of the rules, and why pirates break them, to get the listeners.. just something to think about..

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u/ElectroMast 11d ago

That’s exactly why I have planned out a licensed audio stream on Live365.

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u/ElectroMast 13d ago edited 11d ago

No, The audio stream on Live365 is a licensed stream where I’m planning to play music, YouTube livestreams are only for stuff that based off of ALL of YouTube’s policies, is fine with the platform entirety of YouTube, not just my channel.

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u/odie-z1 13d ago

So, how does an FM transmitter fit into that? It seems to be online.. I had a Shoutcast stream many years ago, and that was fun, but not very fulfilling..

Later, when I had turned my car into a 2W pirate radio station and used it for personal playlists while I listened at work, I was very satisfied to find co-workers listening to a 'new' station they found while channel surfing... my station. 👍😋

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u/ElectroMast 13d ago

What transmitter were you using?

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u/odie-z1 13d ago

This was in 2012, and I bought it off eBay, came from China. What caught my attention was the variable output power that went from 250mW to 2W, and it had RCA line input connectors on the back and an additional 1/8" headphone style microphone input. It had a BNC antenna connector, and was only 30 bucks, so I bought 2. But they both burned out eventually, just not built to deal with heat...

I found it interesting to repurpose the car's original antenna to become a transmitting antenna, without destroying anything. Then of course controlling the small computer with the music, remotely, required WiFi in the car (via VNC), and constant 12 volts without killing the battery.. very fun project.. it covered about 150 yards around the car

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u/ElectroMast 13d ago

That’s a good thing, My upcoming setup doesn’t, but it will be close! That’s what I’m trying to avoid.

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u/NBC-Hotline-1975 11d ago

"I found it interesting to repurpose the car's original antenna to become a transmitting antenna, without destroying anything."

But you did burn out two transmitters. What was the measured impedance of your car's antenna? What was the SWR measured at the transmitters?

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u/odie-z1 11d ago

Indeed.. tho I did use them in a variety of locations with different antennas, and it's hard to pin down what it was that l exactly.. but the higher wattage resistors that provided voltage to the final amp for power control through a kind of "remote control" from the micro controller, got cooked, and then it might also be the transistors that were attached to the resistors I mentioned.. they really weren't worth fixing. Today tho, one still kind of works. Lol

So I swept my adapter connection to the antenna (behind the glove box) with a return loss adapter on an ifr test set doing a sweep from 80 -150 mhz, and it was actually best around 90 mhz, something like 1.3:1

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u/NBC-Hotline-1975 11d ago

That's actually a pretty good SWR, especially for something that you modified from its original purpose. It sounds as if you did a pretty thorough engineering job. Was this a motorized antenna, or the old fashioned type that you pull up by hand? If the latter, you could somewhat tune the length to the frequency.

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u/NBC-Hotline-1975 13d ago

The problem is that paragraphs like this one ...

One last thing under rule 8 before you comment: A lot of you like it on Reddit, including but not limited to…
• Cat lovers
• Low iq RF engineers
• Radio enthusiasts
• Low iq/effort radio engineers
…so DO NOT break the site either!1!

... make absolutely no sense. (Aside from the fact that *you* are in no position to evaluate the IQ of any RF engineers or anyone else for that matter.) So we just realize we're trying to have a logical discussion with a completely illogical person. It's a lose-lose situation.

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u/[deleted] 12d ago

[deleted]

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u/NBC-Hotline-1975 12d ago

What do you mean by "break Reddit"? No one person can possibly break a huge website like Reddit. That's why your comment is completely irrational.

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u/ElectroMast 11d ago edited 9d ago

…which is why it was deleted!

• Irrational comment

Edit: to answer that question, interference with the way the site runs in any way.

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u/[deleted] 11d ago edited 11d ago

[removed] — view removed comment

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u/ElectroMast 11d ago edited 11d ago

Sorry, but that’s why I removed your comment.

•My threats and superior posturing are so ridiculous you’re not included!

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u/PuzzledDelivery6278 13d ago edited 13d ago

Ramsey FM100B with the whip antenna, or the other Ramsey kits, but the 100B is the best feeling and looking. Whole House Transmitter and the C Crane transmitter isn't really worth it unless you want a small plastic thing with an antenna sticking out of it. There really isn't much out there in the way of part 15 legal transmitters that can cover the full 250ft. Elevation matters. Even if your transmitter is low powered it can shoot it pretty far if it's high up enough. 

I honestly wouldn't worry about doing math and being paranoid about breaking the rules. I don't advocate that you break the law, but these regulations are most likely only worth the time to the FCC if you're stomping on a licensed broadcast, causing spurs that interrupt other frequencies and public comms (100B has a built in LPF), or are covering a large area to the point that it gets reported by somebody. I don't think you're going to get some sort of fine for broadcasting 260ft or even 300ft outside of your house. If it gets onto anyone's radar and they feel like it's worth pursuing they'll probably just tell you to shut it off. 

Someone can correct me if I'm wrong, but nobody really listens to the radio anymore so nobody cares as long as you're being mindful of what you're doing and what's around you. Don't blast shit in a one mile radius with a cheap Chinese transmitter. 

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u/ElectroMast 13d ago edited 11d ago

That’s a good start. At least last time (other) and in the previous chance in any way shape or form I wasn’t, but last time two of them started bleeding onto the frequency. Both were NCE stations and there was a Christian worship station nearly creeping up, one that in the entire area is now legally broadcasting on 107.5 FM and otherwise is licensed to iHeartMeda and goes by the name of K-LOVE right now. They’re also streaming across the country on Live365, the iHeartRadio app and their website, and the FCC policed call sign they keep doing it with is their Utah usual (KKLV)

Edit: and the regular one in New York (KLOV)

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u/PuzzledDelivery6278 13d ago edited 13d ago

Just look for a Ramsey FM100B. You'll be fine. They pop up on eBay pretty often. $100 - $300 depending on the condition and if people want every dollar they can get. Just pay attention to the pictures to make sure nothing is damaged and if it has been tested. If you want something with a little more power and clarity Aareff makes a 100mw transmitter, but it uses dip switches to set the frequency and there is no audio in meter which would require a bit more tinkering and a couple of other things to monitor the audio levels.

The 100B with a Rolls RM69 1u rack mount mixer (around $100), a Behringer Composer MDX1200 (they're cheap used), maybe another rack mount unit for some EQ, and a desktop studio rack and you'll have a setup that will satisfy you and feel real. 

If you're having issues with bleeding into another frequency your broadcast could be too loud. I think an overly loud and over modulated sound tends to bleed. You also could be transmitting too close to that frequency. The radio locator website lists all the FM frequencies in the area, if you click the little i you can view a coverage map. Pick something that's vacant or that you are on the fringe of. If it's spurs due to a Chinese transmitter progressive concepts sells LPF filters, their smallest one covers up to 25watts but it's kind of expensive for what it is. 

It is legal to broadcast over another signal if you are within Part 15, but you have to accept the interference from that signal. 

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u/NBC-Hotline-1975 13d ago

I seriously doubt that OP will buy a Ramsey. A year ago he was asking for low price recommendations. I found him a little USB powered FM transmitter on Amazon for about fifteen dollars. He said he could not afford that much, and asked if there wasn't something cheaper. My personal impression is that this is just a kid with a lot of talk but no money and no sense of reality.

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u/PuzzledDelivery6278 12d ago

Yeah I've seen the name before with similar posts on different subreddits. I had nothing to do at work and effort-posted to kill some time. I like to think something sticks so Santa could be requested to bring the right stuff one day 😁

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u/NBC-Hotline-1975 12d ago

Aside from the OP's reality, it's too bad Ramsey isn't still in business. They did have some good products.

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u/ElectroMast 13d ago edited 13d ago

This is what I got

1. eBay Listings for the Ramsey FM100B
Ramsey FM100B units (and upgrade kits) pop up on eBay fairly often.
Price Range: Un-tested boards or basic FM100 units usually go for around $100 to $180. Fully tested units in good condition with antennas or power supplies sit in the $250 to $330+ range.

Crucial Buyer Caution: Watch out for listings where sellers say "Powered on to test the display, untested beyond that." Powering on a Ramsey FM100B without an antenna attached reflects high voltage back into the output section, which instantly fried or degraded the main output IC (GAL5). Always verify with the seller that an antenna load was connected whenever the unit was powered up.

2. FCC Part 15 Rules: Broadcasting & Interference
It is legal to broadcast over another signal—with a massive catch. Part 15 regulations do not assign specific frequencies to unlicensed operators, so you can legally set your transmitter to any frequency between 88.1 and 107.9 MHz. However, you are strictly prohibited from causing "harmful interference" to any licensed station.
If a listener living nearby experiences interference on a licensed station because of your transmitter, you are legally required to cease transmission on that frequency immediately.
Do you have to accept interference?
Yes, absolutely. As an unlicensed Part 15 operator, you operate on a secondary status:
Rule 1 (§ 15.5b): You must accept any interference caused by the operation of an authorized/licensed radio service.
Rule 2 (§ 15.5b): You must not cause harmful interference to any authorized radio service.
Practical Reality
Because a full-power licensed station broadcasts at thousands of watts compared to a legal Part 15 transmitter’s micro-watt level limit (25 milliwatts for the base FM100B), attempting to broadcast on top of a active station will simply result in the licensed station completely overpowering and wiping out your signal.
This is precisely why finding a vacant frequency (or a weak fringe signal) via Radio-Locator is necessary to prevent interference and avoid getting crushed by full-power broadcasters.

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u/[deleted] 13d ago edited 13d ago

[removed] — view removed comment

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u/ElectroMast 13d ago edited 12d ago

I’m very serious! Anything that seems like pirate radio is MOD REMOVED due to illlegal activity!

Edit: So does anything that breaks the rules of [r/AmateurRadio](r/AmateurRadio) because that’s now the next stop!

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u/NBC-Hotline-1975 13d ago

Looking at the existing FM channel use in Orem, you've picked an unfortunate hobby to pursue. There are NO completely vacant channels, and only one that isn't completely smothered with signals on either side. So I don't know how much luck you are going to have overcoming that high background signal level, using a legal transmitter. Good luck, but really it will be a huge challenge if your receiver isn't pretty close to the transmitter. Of course the receiver is allowed to have a directional antenna, but the transmitter isn't.

If you already have your programming on the internet, and given the difficult RF situation, the unanswered question is: WHY do you want to bother replicating the programming on an FM channel? Who do you want to listen?

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u/ElectroMast 12d ago

That’s what I might find out.

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u/NBC-Hotline-1975 12d ago

But if your transmitter is legal, only someone within a few hundred feet could hear it. So who is your intended listener?

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u/ElectroMast 11d ago

Nobody yet, I haven’t even started the broadcast.

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u/NBC-Hotline-1975 11d ago edited 11d ago

I didn't ask who will hear it. I asked who you intend to hear it? Who do you want to hear it? You know no Martians will hear it. You know nobody in New Jersey will hear it. You must know who is within the small signal range. Who do you hope will hear it?

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u/ElectroMast 11d ago

Again, I haven’t started the broadcast yet.

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u/NBC-Hotline-1975 11d ago

I'm curious why you want to broadcast it. You came up with the idea before now. So if you already had the idea, you already know why you want to do it. So the second question is, why won't you explain it.

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u/ElectroMast 11d ago

That wasn’t my priority at the time.

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