Frequently Asked Questions For
FM Transmitter Review

Definitions For Each Part Of The Review

Frequently Asked Questions

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Definitions For Each Part Of The Review:
Included below are definitions for each part of the the review to provide clarity and help avoid any confusion on what we mean by each section.

Audio Source Options:

Stereo Headphone Jack (1/8" or 3.5mm)
This is the standard size of headphone jack used by personal FM Transmitters. This is opposed to the 1/4" standard size use in stereo equipment. Go here to see a list of ways to search for a Stereo FM Transmitter.

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RCA Connector Cable (red & white plugs)
This type of audio connection was first introduced by the Radio Corporation of America (i.e. RCA) in the 1940's and has become a type of "universal" audio plug. It is commonly used for such audio equipment as Home Stereo, DVD Players and TV's.

The benefit of a FM Transmitter having the ability to use an RCA jack is it would allow you to broadcast audio from your home stereo, DVD Player, TV and other audio sources that don't offer a 1/8" stereo headphone jack audio out. Go here to see a list of ways to search for a Stereo FM Transmitter.

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Standard iPod 30 Pin/ Dock Connection
This was first introduced by Apple, Inc. for their iPod's. The first iPods to utilize this type of pin connection were the iPod classic 3rd generations. Until then you connected via FireWire.

This advancement in iPods opened the door for may accessories and devices to connect easily and sometimes seamlessly with an iPod. Go here to see a list of iPod FM Transmitters that use the pin connection.

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Power Source Options:

AC Wall Outlet Adapter
This type of power source lets you draw power directly from a standard 110V wall outlet. In Europe and other countries they use 220V.

Many FM Transmitters that offer an AC power source as an option will usually offer 110V or 220V. Most however, don't include the adaptors to fit the international types of plugs for 220V. Go here to see a list of AC Powered FM Transmitters.

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Internal Batteries
This power source option is pretty straight forward. These are for FM Transmitters that power themselves via batteries. Go here to see a list of Battery FM Transmitters.

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12V DC Car Cigarette Lighter Adapter
This type of power option allows an FM Transmitter to be powered via car's cigarette lighter. This avoids any need to use batteries. Cars in the U.S. use 12V while vehicles in other parts of the world use 24V. Most FM Transmitters that offer this type of power option usually include an adapter to work with either one. Go here to see a list of Car FM Transmitters.

This power options also offers the ability to charge certain MP3 Players (main iPods). Go here to see a list of iPod Car FM Transmitters.

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Computer USB Power Adapter
This power option allows a FM Transmitter to be powered via a standard USB port. Since most USB ports are used with computers it is labeled as such but they are not limited to just computers.

With the advent of internet radio, music downloads, podcasts, etc. there has been a growing need to get audio from the computer to a person's home stereo or other FM receivers. Go here to see a list of USB FM Transmitters.

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Powered Directly By An iPod
This type of power option is offered by FM Transmitters that connect directly to an iPod and use the batteries power of the iPod to power itself and broadcast.

This make for a seamless connection but can drain the battery life of the iPod quickly, possibly require software and upgrades for the iPod to recognize the FM Transmitter and become obsolete if the user were to upgrade or buy a new iPod. Go to Mobile FM Transmitters to see a list of transmitters that have this feature.

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Charging Capabilities:

Charges an iPod MP3 Player
The charging capability allows you to now only draw audio from the iPod but it keeps the iPod charged. Most of the FM Transmitters that have this capability are for inside the car. Go to iPod FM Transmitter Charger to see an entire list of these FM Transmitters.

Charges any other type of MP3 Player This charging capability is for MP3 Players other than iPod. Currently they are the Sansa, Zune, Zen and iRiver Clix. They also include FM Transmitters that have a USB port that a MP3 Player could connect to charge it. Go to Car FM Transmitter Charger to see an entire list of these types of FM Transmitters.

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FCC Compliance Details For Part 15 Regulations:

Obtained FCC Equipment Certification [15.201(b)]
This regulation reads as follows:

"all intentional radiators operating under the provisions of this Part shall be certificated by the Commission pursuant to the procedures in Subpart J of Part 2 of this Chapter prior to marketing."

This means any FM Transmitter seeking to qualify under part 15 must first obtain an FCC ID before any importing, marketing, selling or use of the FM Transmitter can begin.

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FCC ID Permanently Labeled On Unit [15.19(a)5]
According to this regulation every FM Transmitter under part 15 must have:

• The FCC ID "permanently attached" to the FM Transmitter directly regardless of the FM Transmitter's size

It also goes on to state the following:

• The size of the font used for the FCC ID must be readable but not required to be over eight (8) point font [15.19(b)2]
• "Permanently Attached" means that the label is etched, engraved, stamped, silk screened, indelibly printed, or otherwise permanently marked on a permanently attached part of the equipment or on a nameplate of metal, plastic, or other material fastened to the equipment by welding, riveting, or permanent adhesive. [15.19(b)4]

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FCC Compliance Label On FM Transmitter [15.19(a)3]
Under this regulation all FM Transmitters "shall bear the following statement in a conspicuous location on the device"

If the FM Transmitter is too small it "shall be placed in a prominent location in the instruction manual or pamphlet supplied to the user or, alternatively, shall be placed on the container in which the device is marketed." [15.19(a)5]

The compliance label is to read as follows:

This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) this device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

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Unique Antenna Connection [15.203]
The FCC says that a "unique antenna connection" is "one that is not of a standard type found in electronic supply stores."

The official legal text is written as follows:

"An intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator shall be considered sufficient to comply with the provisions of this Section. The manufacturer may design the unit so that a broken antenna can be replaced by the user, but the use of a standard antenna jack or electrical connector is prohibited."

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Resides Wholly Within Public FM Band [15.239(a)]
The frequency rage of a compliant part 15 FM Transmitter is as follows:

"Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88-108 MHz."

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Meets Field Strength (i.e. Power) Limits of 250 μV/m
@ 3 m [15.239(b)]
The maximum power a Part 15 FM Transmitter can have is as follows:

The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector.

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Technical Details Provided by Manufacturer:

FM Transmitter Manual
To offer a convenient location for consumers looking for a FM Transmitter manual please go to our FM Transmitter Manual page.

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Frequency Response
According to Wikipedia "Frequency Response" is:

"the measure of any system's spectrum response at the output to a signal of varying frequency (but constant amplitude) at its input. In the audible range it is usually referred to in connection with electronic amplifiers, microphones and loudspeakers...

Frequency response curves are often used to indicate the accuracy of amplifiers and speakers for reproducing audio. As an example, a high fidelity amplifier may be said to have a frequency response of 20 Hz - 20,000 Hz ±1 dB. This means that the system amplifies all frequencies within that range within the limits quoted. 'Good frequency response' therefore does not guarantee a specific fidelity, but only indicates that a piece of equipment meets the basic frequency response requirements."

Generally, the larger the range the better.

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Signal to Noise Ratio
According to Wikipedia it is defined as the following:

Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering measurement, also used in other fields (such as scientific measurements, biological cell signaling), defined as the ratio of a signal power to the noise power corrupting the signal.

In less technical terms, signal-to-noise ratio compares the level of a desired signal (such as music) to the level of background noise.

The higher the ratio, the less obtrusive the background noise is.

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Stereo Separation
(definition not completed)

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Harmonic Distortion (or Total Harmonic Distortion)
According to Wikipedia it is defined as the following:

The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the Fundamental frequency. Lesser THD allows the components in a loudspeaker, amplifier or microphone or other equipment to produce a more accurate reproduction by reducing harmonics added by electronics and audio media.

Generally, the lower the harmonic distortion the better.

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PLL (Phase Lock Loop) Tuning
According to Wikipedia it is defined as the following:

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. A phase-locked loop is an example of a control system using negative feedback.

Essentially PLL keeps the signal stable so you it doesn't drift and cause you to hear static or lose the signal while listening to it.

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Channel Balance
(definition not completed)

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Signal Format
(definition not completed)

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Stereo or Mono Transmission
This is relating to the type of transmission the FM Transmitter is broadcasting:

Mono Transmission means: designating sound transmission or recording or reproduction over a single channel

Stereo Transmission means: the reproduction of sound, using two or more independent audio channels

A stereo transmission sounds better but a mono will broadcast further or better said, you can hear it further away on a FM receiver.

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Frequently Asked Questions

• What is FCC Part 15 and why should I care?
• How Far Can a Low Power FM Transmitter Go... legally?
• What is a Non-Licensed, Low Power FM Transmitter?
• Can I build my own Low Power FM Transmitter?
• Can I sell my home built Low Power FM Transmitter or FM Transmitter Kit?
• How many home built or kit FM Transmitters can I own at one time?
• What does "Field Strength" mean for an FM Transmitter?
• Can I use a FM Transmitter to spy on someone?
• What type of FM Transmitter antenna can I use?
• What happens if one sells, imports or uses non-compliant low-power FM transmitter?
• What changes can be made to an FCC-authorized FM Transmitter device without requiring a new FCC authorization?
• What is the relationship between "microvolts per meter" and Watts of a FM Transmitter?
• So what exactly is the maximum watts a Non-Licensed, Low Power FM Transmitter can have?
• What is the relationship between "microvolts per meter" and distance the FM Transmitter will go?

What is FCC Part 15 and why should I care?

Part 15 is the section of code (law) that gives the FCC the power to regulate low power FM Transmitters (and other transmitters too). Consequently it also makes low power FM Transmitters (and other transmitters too) available for public use. It is a two edged sword it allows the public to benefit from low power FM Transmitters and it also makes sure they stay regulated.

How Far Can a Low Power FM Transmitter Go... legally?

Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice. On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221.

These devices must accept any interference caused by any other operation, which may further limit the effective service range. For more information on Part 15 devices, please see OET Bulletin No. 63 ("Understanding the FCC Regulations for Low-Power, Non-Licensed Transmitters").

What is a Non-Licensed, Low Power FM Transmitter?

Throughout this site the terms "low-power FM Transmitter," "low-power, non-licensed FM Transmitter," and "Part 15 FM transmitter" all refer to the same thing: a low-power, non-licensed FM Transmitter that complies with the regulations in Part 15 of the FCC rules. Part 15 FM Transmitters use very little power, most of them less than a milliwatt. They are "non-licensed" because their operators are not required to obtain a license from the FCC to use them.

Can I build my own Low Power FM Transmitter?

Yes, Hobbyists, inventors and other parties that design and build Part 15 transmitters with no intention of ever marketing them may construct and operate up to five such transmitters for their own personal use without having to obtain FCC equipment authorization. If possible, these transmitters should be tested for compliance with the Commission's rules. If such testing is not practicable, their designers and builders are required to employ good engineering practices in order to ensure compliance with the Part 15 standards. (Section 15.23)

Can I sell my home built Low Power FM Transmitter or FM Transmitter Kit?

No, You can not build a FM Transmitter from scratch or a kit and then sell, lease or rent it.

How many home built or kit FM Transmitters can I own at one time?

You can own up to five (5) FM Transmitters and be within the law. (Section 15.23)

What does "Field Strength" mean for an FM Transmitter?

Field Strength means: The intensity of a radio signal measured at a certain distance from the transmitting antenna. Field strength is usually expressed in volts per meter (how many volts the signal will induce into a one-meter-long receiving antenna), or millivolts per meter. For example a FCC Part 15 FM Transmitter broadcasting on the public band (88 to 108 Mhz) is 250 microvolts with a 1 meter antenna at 3 meters distance.

Can I use a FM Transmitter to spy on someone?

NO, Except for the operations of law enforcement officers conducted under lawful authority, no person shall use, either directly or indirectly, a device operated pursuant to the provisions of Part 15 for the purpose of overhearing or recording the private conversations of others unless such use is authorized by all of the parties engaging in the conversation.(Section 15.9)

What type of FM Transmitter antenna can I use?

Only the one provided by the seller of the FM Transmitter.

A FM Transmitter should be designed to ensure that only the antenna it is furnished with the FM Transmitter will work with the unit. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the FM Transmitter will be considered sufficient to comply with the law.

The manufacturer may design the unit so that a broken antenna can be replaced by the user, but the use of a standard antenna jack or electrical connector is prohibited. (Section 15.203)

What happens if one sells, imports or uses non-compliant low-power FM transmitter?

The FCC rules are designed to control the marketing of low-power transmitters and, to a lesser extent, their use. If the operation of a non-compliant transmitter causes interference to authorized radio communications, the user should stop operating the transmitter or correct the problem causing the interference. However, the person (or company) that sold this non-compliant transmitter to the user has violated the FCC marketing rules in Part 2 as well as federal law. The act of selling or leasing, offering to sell or lease, or importing a low-power transmitter that has not gone through the appropriate FCC equipment authorization procedure is a violation of the Commission's rules and federal law. Violators may be subject to an enforcement action by the Commission's Field Operations Bureau that could result in: (Section 15.1, Section 15.5, Section 2.803, Section 2.805 & Section 2.1203)

(Section 1.80, 47 U.S.C. 302, 47 U.S.C. 501, 47 U.S.C. 502, 47 U.S.C. 503, 47 U.S.C. 510 & 18 U.S.C. 3571)

What changes can be made to an FCC-authorized FM Transmitter device without requiring a new FCC authorization?

The person or company that obtained FCC authorization for a Part 15 transmitter is permitted to make the following types of changes:

For certified equipment, the holder of the grant of certification, or the holder's agent, can make minor modifications to the circuitry, appearance or other design aspects of the transmitter. Minor modifications are divided into two categories: Class I permissive changes and Class II permissive changes. Major changes are not permitted. (Section 2.929 & Section 2.1043)

Minor changes that do not increase the radio frequency emissions from the transmitter do not require the grantee to file any information with the FCC. These are called Class I permissive changes. (Note: if a Class I permissive change results in a product that looks different than the one that was certified it is strongly suggested that photos of the modified transmitter be filed with the FCC.)

Minor changes that increase the radio frequency emissions from the transmitter require the grantee to file complete information about the change along with results of tests showing that the equipment continues to comply with FCC technical standards. In this case, the modified equipment may not be marketed under the existing grant of certification prior to acknowledgement by the Commission that the change is acceptable. These are called Class II permissive changes.

Major changes require that a new grant be obtained by submitting a new application with complete test results. Some examples of major changes include: changes to the basic frequency determining and stabilizing circuitry; changes to the frequency multiplication stages or basic modulator circuit; and, major changes to the size, shape or shielding properties of the case.

No changes are permitted to certified equipment by anyone other than the grantee or the grantee's designated agent; except, however, that changes to the FCC ID without any other changes to the equipment may be performed by anyone by filing an abbreviated application. (Section 2.1043 & Section 2.933)

For verified equipment, any changes may be made to the circuitry, appearance or other design aspects of the device as long as the manufacturer (importer, if the equipment is imported) has on file updated circuit drawings and test data showing that the equipment continues to comply with the FCC rules. (Section 2.952, Section 2.953 & Section 2.955)

What is the relationship between "microvolts per meter" and Watts?

Watts are the units used to describe the amount of power generated by a transmitter. Microvolts per meter (μV/m) are the units used to describe the strength of an electric field created by the operation of a transmitter.

A particular transmitter that generates a constant level of power (Watts) can produce electric fields of different strengths (μV/m) depending on, among other things, the type of transmission line and antenna connected to it. Because it is the electric field that causes interference to authorized radio communications, and since a particular electric field strength does not directly correspond to a particular level of transmitter power, most of the Part 15 emission limits are specified in field strength. Although the precise relationship between power and field strength can depend on a number of additional factors, a commonly-used equation to approximate their relationship is:

where:

4(pie) x D(squared) is the surface area of the sphere centered at the radiating source whose surface is D meters from the radiating source. 120(pie) is the characteristic impedance of free space in ohms.

Using this equation, and assuming a unity gain antenna (G = 1) and a measurement distance of 3 meters (D = 3), a formula for determining power given field strength can be developed:

where:

• P = the transmitter power (EIRP) in watts and
• E = the field strength in volts/meter.

So what exactly is the maximum watts a Non-Licensed, Low Power FM Transmitter can have?

Based on the answer above and using the formula:

A 1 Watt FM Transmitter with a 1 meter antenna at a distance of 3 meters would put off 18,750 mirovolts. The FCC Part 15 rule is 250 microvolts when using the same antenna size and distance.

So it would be: 250/ 18,750 = 0.01333 watts, or 13.333 milliwatts of power is the maximum a Low Power, Non-Licensed FM Transmitter can have.

If the antenna length is shortened or lengthened then the maximum power allowed would increase or decrease. This is why the FCC chose to use microvolts instead of watts.

A person could have a 13 milliwatt transmitter put it 20 feet in the air and use a full-wave commercial grade antenna and it go much further than a FM Transmitter that is more powerful (milliwatts) but with an antenna that is only a couple of inches long.

(please check our work and email us, we welcome the math and engineering gurus of the world to confirm or refute our calculations,)

What is the relationship between "microvolts per meter" and distance the FM Transmitter will go?

The best source we've found to help explain that is at Ramsey Electronics.com. Below is a copy of what they say:

The new FCC Part 15 Rules specify a maximum "Field Strength" of your transmitted signal. Since it is unlikely that you have the equipment to carry out accurate field strength measurements in microvolts, it is useful to understand at least the theory of field strength so that you can understand both what you can expect from such transmitters, and what limits the FCC intends. Previous limits on non-licensed FM-broadcast band devices were defined as a maximum field strength of 40µV per meter measured at a distance of 15 meters.

The June 1989 revised rule specifies a maximum of 250 µV per meter, but measured at 3 meters from your antenna. The term, "250µV per meter" means that an accurate field-strength meter with a calibrated and scaled 1-meter antenna may indicate a maximum signal field strength of 250µV (In contrast, non-licensed operation from 26.96 to 27.28 MHz, your standard CB walkie-talkie, is limited to a field strength of 10,000 µV per meter at 3 meters).

In all cases, the field strength of a signal decreases in direct proportion to the distance away from the antenna. Power decreases by the square of distance: for every doubling in distance, the signal power is quartered, but the field strength voltage is only halved. Using this theory, we can construct a simple chart to show the maximum permitted performance of a non-licensed FM band transmitter. The theoretical figures assume a simple 1 meter receiving antenna in all cases and do not take into consideration that reception can be greatly enhanced with larger, multi-element antennas and preamplifiers on the receiver. In the following chart, the field strength (theoretical minimum) gets stronger as you move from the edge of these circular boundaries toward the antenna:

This "exercise in meters and microvolts" demonstrates that the FCC clearly intends to limit the theoretical range of non-licensed devices operating in this band. It also shows the potential for causing interference at a home down the street from you. But it also shows that you can legally put out quite a good signal over wider areas than you might have imagined.

For other kinds of radio services, the FCC restricts such factors as transmitter power or antenna height, which cannot really limit the possible "range" of a transmission under good conditions. By restricting the maximum field strength at a specific distance from your antenna, the FCC clearly plans for your signal to "die out" at a specific distance from your antenna, no matter what kind of transmitter power or antenna you are using. On the other hand, the FCC standards do make it legal and possible for you to broadcast on a school campus, campground or local neighborhood, as long as you remain within the field strength limitations and do not cause interference to broadcast reception.

Some Common Ways Others Have Found A FM Transmitter:

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