Micro-Broadcasting 101
by J. Hale / equinox17
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Micro-Broadcasting 101 |
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Thanks for dropping by!
This page is meant to assist anyone interested in getting started with "Micro-Broadcasting", aka "Hobby Broadcasting", etc.
Whatever you call it, in my own humble opinion, micro-broadcasting is a communications medium that has great potential for many practical and worthwhile uses in every sector of daily life: personal, public service, religious, commercial, you name it. I firmly believe that micro-broadcasting is a medium that not only can be but should be embraced by people everywhere across this country in a grass-roots effort to bring back the presence of real community radio service - a valuable asset that has systematically been taken away from us by the large corporate broadcasting conglomerates in recent years.
This isn't intended to be super technical or comprehensive "everything-you-need-to-know-in-one-page" kind of thing, but it should help to get you going in the right direction. If you have any questions, feel free to contact me.
Getting It All Hooked Up
Crossing The Line
Random Radio Ramblings
FM Frequency Survey Log Sheets
Useful Links
The first question usually asked:
Is it legal?
Answer: Yes, it is.
OK, that's the quick answer. More specifically, "low-power non-licensed transmitters" are expressly permitted by what is known as Part 15 of the Federal Communications Commission's regulations. Part 15 regulations govern all the many varieties of low-power RF transmitters in use today, including "cordless phones, baby monitors, garage door openers, wireless home security systems, keyless automobile entry systems and hundreds of other types of common electronic equipment".
If you plan on operating a "low-power non-licensed"
AM or FM band transmitter for personal use, such as broadcasting your favorite music or other
audio to radio receivers around your own house and property, please be mindful
of the fact that your transmissions must not cause interference with someone
else's
reception of a licensed broadcaster's signal. Basically, as a casual "home
broadcaster", you should select a frequency that isn't already in use by a
nearby station, stick with using the simple antenna that came with your transmitter, and
keep the output power set to the
lowest level that produces the desired range of coverage. Then, just relax and
enjoy the tunes.
The commentary below is intended primarily
for those who want to advance to the next level of micro-broadcasting:
Taking it to the limit of what is permitted by the FCC.
FCC Part 15 regulations contain some very specific technical restrictions concerning the level of radiated power and types of antennas allowed for use by low-power non-licensed transmitters intended for personal use and operated in the standard AM and FM broadcast bands. As a very general rule-of-thumb, a transmitter operating with the maximum power allowed by FCC regulations and using the antenna that came built-into the device can be expected to have an effective transmission range of at least 150-300 feet. In practice, the transmission range will be affected by many variable factors such as the local terrain (clear flat land vs. land with many structural obstacles, power lines, trees, hills or mountains, etc.) and by building construction (is the transmitter located in a wood-frame house vs. being in or near metal-framed or metal sided/roofed buildings, etc.)
Also, keep in mind that the quality of the radio receiving system on the listener's end will play a major role in how far away your broadcast signal can be heard. In fact, someone with a very sensitive receiver connected to a high-gain antenna might be able to receive your signal from considerably farther away than 300 feet. Under ideal circumstances at both the transmitting and receiving ends, your signal could be accessible from a quarter mile away, possibly more. And remember, all these measurements can generally be applied equally in all directions. In other words, 150', 300', a quarter mile - whichever measurement applies - this can be considered as being the radius of a rather large circle with your transmitter's antenna at the center.
At any rate, my point is that a non-licensed but FCC-legal transmitter can definitely pack enough power to cover not just your own house and property, but say, for example, a school campus, a business lot or shopping center, a church, a campground, a retirement home, a dorm or apartment building, perhaps even an entire community.
Before we can move away from the question of "legality" however, let's discuss the aspect of FCC Part 15 regulations which requires that your radio transmissions must not cause interference with the reception of a signal coming from a licensed broadcaster.
What this means is that you must, first of all, select a transmit frequency that's not already in use by a licensed station whose signal is being received in your area. You wouldn't want to try to use such a frequency anyway since the signal from another station would seriously interfere with the reception of your own signal, but bear in mind that choosing the frequency you will use is a responsibility you shouldn't take lightly. My suggestion would be for you to sit down with a radio receiver at your intended transmitter location and do a complete survey of all the frequencies at which you either do or do not hear a station that's already on the air. For this survey, a good quality and sensitive receiver, preferably one with a digital tuning display and an external antenna (at least a telescoping whip antenna) should be used.
FYI, the standard FM broadcast band is divided into 100 channels at 200 KHz increments between 88 and 108 MHz. The lowest channel is designated as number 201 and it's center frequency is 88.1 MHz. The highest channel is number 300 at 107.9 MHz. Channels 200-220 are reserved for use by non-commercial stations including educational, public (NPR), and religious broadcasters. Channels 221-300 are reserved for commercial stations. There are some exceptions to these "reserved" frequency allocations and certain specific restrictions will apply in different parts of the country. (Of course, your Part 15 transmitter can use any of the so-called "reserved" frequencies in the FM band, or it can even transmit "in-between" the standard frequencies, as long as the non-interference requirements are met.)
You should note that the FM audio transmission for a channel 6 television station lies at 87.7 MHz. That's just below the standard FM broadcast band, but many FM radios can still tune it in and it is conceivable that a low-power transmitter operating near that frequency could cause interference with someone's TV set tuned to channel 6. So, when you're doing your survey of the frequencies being used in your area, be sure to consider whether there's a channel 6 TV station within range. Furthermore, the Air Band, used for communicating with private and commercial aircraft, lies just above the FM band. You definitely do not want to take any chances with causing interference up there, so under no circumstances should you try to transmit at a frequency higher than 108 MHz.
In all cases, a good quality transmitter, preferably one with Phase Lock Loop (PLL) circuitry that will stay tuned to the correct frequency and produce less "harmonics", can help to prevent unexpected interference outside the intended FM band frequencies. This is another reason the Ramsey FM25B and FM100B make great choices for the serious micro-broadcaster. These units offer rock solid tuning at a very reasonable price.
Now, back to our frequency survey ...
Assuming that you have a receiver with a digital tuning display, it's a simple matter to start at 88.1 MHz and then tune slowly across the entire FM band up to 107.9 MHz. Many receivers will tune in 100 KHz increments (i.e., 88.1, 88.2, 88.3, 88.4, etc.) but keep in mind that licensed channel allocations are always in 200 KHz increments (88.1, 88.3, 88.5, 88.7, etc.) In other words, the digit you see after the decimal point on your receiver's display should always be an odd number when the radio is correctly tuned to a standard FM channel.
Again, you're permitted to transmit on one of those "in-between" frequencies, but if you choose to do so, your signal won't be picked up very well by radios that only tune in 200 KHz increments - and there are many receivers of this type. Of course, you might consider this particular "problem" to be an advantage, depending on the overall goals and target audience for your station. Maybe you don't want everyone to be able to hear you quite so easily?
Sorry, I keep getting sidetracked ...
As you tune your receiver across the FM band searching for an available frequency, make notes as to which standard frequencies appear to be "open". If possible, it's a good idea to select an open frequency that's centered between two adjacent channels that are also open. Unfortunately, if you live in a more populated part of the country, you might not find any spots on the dial where you can't hear some kind of a background signal. For now though, at least make a list of where the weakest signals are found, and perhaps add a few comments describing what you were able to hear on those barely audible channels. As you go through this process, whenever you do come across a potential available frequency, it's a good idea to rotate the radio's antenna, or even move the radio itself, to see if a change in position changes the level of the background signal. Again, make notes of your observations.
~ NEW ~
I've recently compiled
a set of Log Sheets to help with your FM Survey.
Click here
when you're ready for it.
OK, do you have a list of potential transmitter frequencies? Good. The next step in your survey will be to go mobile. Either by car or on foot, skateboard, whatever, travel around the area which you anticipate as being your intended coverage zone and check again, from various points within the zone, to see if the frequencies on your list still appear to be open throughout the entire target area. As before, a sensitive radio with digital tuning display should be used. A car radio is usually a very good choice for this, but try not to cause any wrecks or run over anybody while you're searching for open frequencies.
Now that you've made your list and checked it twice (or more), what I'd suggest you do next is go to the FCC's on-line "FM Radio Database Query" (a link is provided below) and do a search for licensed stations operating at the frequencies of interest in your state (and even in adjacent states if you live within about 90 miles of another state's border). When you get to the FCC page, simply select the state of interest from the drop-down menu you see near the middle of the screen. Next, select one of the frequencies that you are considering using in the field that says "Search for FM Stations in a Frequency Range". Select the same frequency for both the "Lower" and "Upper" box to look at just one specific frequency. Or, you could also try searching in a range from one channel below your desired frequency to one channel above. All other fields in the FCC Database Query can simply be left blank or at their default settings. Click "Submit Data" and soon a page will appear showing detailed information about all the stations in that state that are licensed to operate at the specified frequencies.
The somewhat subjective question to ask now is this:
Would any of these stations really be considered as "being received in your area"?
You should consider whether someone in the area where you want your signal to be heard might be a dedicated listener to one of those stations on the FCC Database list. Even though your own list showed there were no strong signals observed at that frequency, is it possible that someone using a more sensitive receiver and a better antenna could be pulling in their favorite station from a town many miles away? As unlikely as it might seem, even one of the weaker, more distant stations might have someone in your area who likes to listen to it, perhaps for a certain sports or religious program, regional news, their hometown station, something like that. Not likely you may think, but is it possible?
Well, FM radio waves need a "line-of-sight" pathway, so, due to the curvature of the earth, 90-120 miles is usually about the maximum distance that one can expect to reliably receive the signal from a high-powered (50,000 Watt) commercial FM station - and that's when using some pretty sensitive receiving gear. (Occasionally, unusual atmospheric conditions can cause FM signals to travel farther, but such effects are very transient.) In practical terms, 60-80 miles is probably a more realistic maximum range for a 50 KW station, and that's still a big stretch for the typical FM receiver/antenna combination. For stations of lesser power (and the vast majority of them operate at much less than 50 KW) the coverage area will be a lot lower. Often it's just a few tens of miles, sometimes even less.
By carefully studying and considering all the information available to you in the FCC Database Query, which includes the effective radiated power (ERP) and the location of all the stations in your region for the selected frequencies of interest, you should be able to make a very well-informed decision as to which frequency would be best suited for your station.
You should always be guided by common sense and good judgment when choosing the frequency for operating your transmitter, especially if you're planning to broadcast beyond the borders of your own house and property. My personal suggestion is that you be completely open and up-front concerning your broadcasting presence, even to go so far as to provide on-air announcements inviting anyone who might be listening to notify you directly (whether in person, by phone, or e-mail) if they feel that your transmission is affecting their reception of another station. Taking such an open and friendly approach should go a long way toward preventing any hassles down the road.
In the unlikely event that a neighbor does complain to the "authorities" that your transmissions are interfering with their reception of a licensed station, you could eventually receive a notice from the FCC informing you of the complaint and instructing you to comply with their regulations. Don't worry, you're not going to be thrown in jail or fined or anything like that - well, at least not as long as you comply with the FCC's request in a timely and respectful manner, which is exactly what you must do!
Here's a relevant quote taken directly from an FCC
bulletin:
FEDERAL COMMUNICATIONS COMMISSION /
Office of Engineering and Technology
OET Bulletin 63 / October, 1993 /Edited and reprinted Feb. 1996
If a Part 15 transmitter does cause interference to authorized radio communications,
Unquote.
Bottom line:
Here's the link to the FCC's Database Query Page. It's definitely worth checking out, lots of useful info can be obtained.
If you want a far more comprehensive discussion of the technical and
legal requirements associated with FCC Part 15 transmitters,
I recommend that you visit
this
page, hosted by Ramsey Electronics.
Here's another link to my
FM Frequency Survey Log
Sheets.
Scroll on down to see the next section or go back to the top.
So, once you've selected a frequency to use for your micro-broadcasting station, what's next? Well, if you've already obtained a transmitter and if you already know how to connect cables to ordinary audio equipment, you're well on your way. It's just a matter of selecting the right cables with the right type of connectors and/or adaptors to get all the parts of your studio equipment hooked up. Of course, there's a few things you need to know to get the most out of your system, but for now, let's take a look at the basic stuff.
In case you haven't already purchased a transmitter, I'll make a
few comments and suggestions about that.
Choosing A Transmitter:
Your first decision might be whether to choose an AM or FM unit. My own preference would be to go with an FM transmitter for all the same reasons that commercial FM broadcasters do: The audio quality will be so much better. Actually though, that's as much a function of the AM receiver as it is the transmitter, for reasons going back to the earliest days of radio.
You see folks, back in the early 20th Century, there was no such thing as hi-fidelity electronic audio equipment, and stereo hadn't even been thought about. When technical standards were being established to guide both broadcasters and receiver manufacturers, the standards were based on technologies available at that time. For all intents and purposes, those standards never changed much over the years. When FM technology was being developed, especially after WWII, the time was right to incorporate more modern standards into FM broadcasting and FM receiver manufacturing. Thus, the new medium of FM radio was able to take advantage of the many advancements that had been made in audio engineering - especially the advent of "hi-fi" and stereo. Meanwhile the already well-established world of AM radio was left unchanged and old fogies like me didn't have to throw out our sets. Which was good, since back in those days radio sets doubled as furniture.
Also, when you throw in the fact that frequencies in the AM broadcast band are more subject to atmospheric irregularities (like static from lightning discharges) as well as man-made RF noise (like power line hum), you begin to understand the inescapable fact that the AM band is simply not as good as the FM band for transmitting top quality audio.
In case you ever wondered, all this explains why AM radio has become best known as a talk show medium, while FM is the music lover's choice. Of course, if your intended programming format happens to be geared to "talk radio", maybe you should give AM some serious consideration. For one thing, the FCC regulations are a bit more lenient when it comes to AM transmitters. Another possible advantage is that the AM band is generally less crowded, it might be easier to find an open spot on your AM radio dial, and less likely that your transmitter will cause interference to someone else's station. Also, since AM radio wavelengths propagate with different characteristics than the higher frequency FM waves, you might be able to squeak out a little better coverage area with an AM signal.
Bottom line though, if high-quality stereo music is mainly what
you want to broadcast, FM is the slam-dunk case-closed choice.
Well, at least until digital AM stereo becomes a reality. At any rate, I think
the rest of my comments should apply to either type of transmitter you choose,
so please read on ...
There's a lot of low power transmitters on the market today. At the bottom end of the price range are the toys that come in a hand-held plastic case with a built-in microphone for the kids to use in playing Radio Idol. For a few dollars more, many electronic supply houses sell transmitter kits that really aren't much (if any) of an advancement over the toys. All the transmitters I know of in this lower price range feature frequency tuning via a small variable capacitor. To set the frequency, you generally use a tiny screwdriver to twist and tweak the variable capacitor until you hear yourself coming through at the desired frequency on a nearby radio. This kind of tuning method is inexact at best, and then, even after you think you've got the frequency set correctly, the tuning will always tend to drift over a period of time due to internal variations with the tuning capacitor. You either have to keep retuning the receiver or tweaking the transmitter itself.
This is obviously not a very satisfactory arrangement for the serious micro-broadcaster. And keep in mind that for listeners using receivers with digital tuning (as opposed to the older style radios with analog tuning dials) your transmitter needs to be spot-on the intended frequency or it might not be heard at all!
Something else to worry about are "harmonics" and "spurious" signals. Harmonics refers to radiation being generated at multiples of the fundamental frequency. In other words, if your transmitter is set to 104.3 MHz, it will also be putting out some amount of RF at 208.6 MHz (Fx2), 312.9 MHz (Fx3), etc. As it happens, the strongest harmonics from an FM band transmitter will fall within the TV channel 7-13 range (174-216 MHz). Hence, you should be aware of the possibility that your FM transmitter could be causing interference to a neighbor's television set. (And remember, we've already discussed other possible problems with TV channel 6.)
BTW, the next order of FM band harmonics gets into the frequencies used for satellite and space communication. Your harmonic signals probably won't be traveling too far into outer space, but if you happen to live near a NASA or space communications related facility, that might be something to think about.
In addition to producing RF harmonics outside the FM broadcast band, less sophisticated transmitters are also notorious for producing spurious signals at unintended frequencies within the band. Either scenario is something you should want to avoid.
Here's an experiment you can try with your transmitter:
If you have a portable scanner type receiver or one of those multi-band radios that tune in the short-wave, TV, aircraft and emergency bands, try searching to see if your transmitter is producing harmonics and spurious signals. Start by tuning the radio to the frequency that's exactly twice the operating frequency of your transmitter, then three times the frequency etc. While you're at it, try tuning across each band to see if you can hear your transmitted signal coming in somewhere it shouldn't be. If you do pick up your signal at any unwanted spots, check to see how far away you can get from the transmitter before it drops off. If the harmonics or spurious signals carry for more than about 50-75 feet or so, you could be causing unexpected problems - especially if those unwanted signals happen to fall in the TV, aircraft, police, or fire and rescue bands.
Yes, some of the less expensive units will be better than others at minimizing frequency drift, harmonics, and spurious radiation, but still, the serious micro-broadcaster would be well-advised to obtain a transmitter that features what's called Phase-Locked Loop (PLL) tuning. If you want to get technical about it, here's what the Wikipedia says about Phase-Locked Loop circuitry:
A phase-locked loop or phase lock loop (PLL) is a control system that generates a signal that has a fixed relation to the phase of a "reference" signal. A phase-locked loop circuit responds to both the frequency and the phase of the input signals, automatically raising or lowering the frequency of a controlled oscillator until it is matched to the reference in both frequency and phase.
Got it? Well, it's the last part of that second sentence that's important, and basically what you should understand is that a transmitter with PLL circuitry is continually monitoring its own signal, making internal voltage adjustments automatically and instantaneously so as to maintain steady operation at the intended frequency. Hence, no frequency drift, and no worries.
Well, fewer worries anyway. Since we're getting a little technical here let me say yes, it's true that any RF transmitter, including a PLL type, will generate harmonics. Again though, the more sophisticated (hence higher priced) transmitters will incorporate extra levels of filter circuitry to keep these unwanted signals to a minimum.
I personally have built and operated several of the non-PLL type transmitters before working my way up to the Ramsey FM25B and FM100B. I've subjected all my transmitters to the "harmonics and spurious signal search experiment" described above, and I can tell you it is a slam-dunk for the Ramsey team. The FM25B and FM100B really do offer a combination of quality and reliability that no other products in their price range even approach, much less beat.
My Personal Recommendations for the Well-Tempered Micro-Broadcaster:
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FM25B PLL FM Stereo Transmitter |
The "Radio Station in a Box" |
A Great AM Band PLL Transmitter
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More info coming soon . . .
