Hello,

I have a portable music player (MP3, etc) with a built-in FM
transmitter. The original transmission antenna is a thin foil thing
shaped like a very elongated letter 'E'. It has terrible range;
aftermarket transmitters will nearly always do better.

Could somebody point me in the right direction for making a suitable
replacement that will still fit inside the very tight (2"x3"x almost
nothing) available space? Is it possible, for example, to make a
simple (1/48th wave?) dipole antenna of some sort that will be an
effective transmitter?

Many thanks for your help to one entirely ignorant in this field.

I have seen fractal antennas on printed circuit boards that might be of
some use. I think the ARRL handbook included a photo of a 2m fractal
antenna in the shape of a Koch snowflake; it was only a few inches long.

Terry C. wrote:
"It has terrible range."

There are severe limitations on unlicensed radiation. There are no
regulatory restrictions on receiving antenna gain. Use a high-gain
receiving antenna to extend range.

Best regards, Richard Harrison, KB5WZI

Whether you're violating the FCC's rules depends on what you mean by
"terrible," and how "effective" you plan on making your transmitter. If
you've got a transmitter that is designed to work in a car, and you
want it to work at home, it's very likely that you can extend the
transmit range to cover the area of your house without breaking any
rules. If you are talking about setting up a little broadcast station
for your neighborhood, well, that's a different story.

I don't know if the FCC requires type acceptance (in other words,
prohibits you from tinkering with the transmitter by principle) for
these fm wireless gadgets. This is the case with FRS; in particular,
you're not allowed to crack open your radio and attach a quarter wave
vertical. You might want to check on that, too.

You can mire through all of the FCC's rules on their web site. But I
would still love to know if those fractal antennas work.

wrote:
Whether you're violating the FCC's rules depends on what you mean by
"terrible," and how "effective" you plan on making your transmitter. If
you've got a transmitter that is designed to work in a car, and you
want it to work at home, it's very likely that you can extend the
transmit range to cover the area of your house without breaking any
rules. If you are talking about setting up a little broadcast station
for your neighborhood, well, that's a different story.

I don't know if the FCC requires type acceptance (in other words,
prohibits you from tinkering with the transmitter by principle) for
these fm wireless gadgets. This is the case with FRS; in particular,
you're not allowed to crack open your radio and attach a quarter wave
vertical. You might want to check on that, too.

You can mire through all of the FCC's rules on their web site. But I
would still love to know if those fractal antennas work.


Making the assumption that the "built-in FM transmitter" is the FM broadcast
band and the device falls under the Part 15 unlicensed emitter rules, the
FCC does not allow tampering with the antenna. To wit:


[Code of Federal Regulations]
[Title 47, Volume 1]
[Revised as of October 1, 2004]
From the U.S. Government Printing Office via GPO Access
[CITE: 47CFR15.203]

[Page 805]

TITLE 47--TELECOMMUNICATION

CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION

PART 15_RADIO FREQUENCY DEVICES--Table of Contents

Subpart C_Intentional Radiators

Sec. 15.203 Antenna requirement.

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. This requirement does not apply to
carrier current devices or to devices operated under the provisions of
Sec. 15.211, Sec. 15.213, Sec. 15.217, Sec. 15.219, or Sec. 15.221.
Further, this requirement does not apply to intentional radiators that
must be professionally installed, such as perimeter protection systems
and some field disturbance sensors, or to other intentional radiators
which, in accordance with Sec. 15.31(d), must be measured at the
installation site. However, the installer shall be responsible for
ensuring that the proper antenna is employed so that the limits in this
part are not exceeded.

[54 FR 17714, Apr. 25, 1989, as amended at 55 FR 28762, July 13, 1990]