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Micropower Broadcasting - A Technical Primer by Stephen Dunifer RPI:




               Micropower Broadcasting -  A Technical Primer
                             by Stephen Dunifer

   * FINDING A FREQUENCY
   * LOCATION OF STUDIO AND TRANSMITTER
   * FM TRANSMITTERS
   * AMPLIFIERS
   * POWER SUPPLIES
   * FILTERS
   * ANTENNAS
   * CONNECTORS AND CABLE
   * STUDIO SET UP
   * FINAL WORD

Many people still assume that an FM broadcast station consists of rooms
full of equipment costing tens of thousands of dollars.  The Micropower
Broadcasting, Free Radio Movement has shown this to be untrue.  Micropower
broadcasting uses FM transmitters whose power output is in the range of 1/2
to 40 watts.  Such
transmitters have a physical size that is not greater than that of your
average brick.  These transmitters combined with other equipment including
inexpensive audio mixers, consumer audio gear, a power supply, filter and
antenna enable any community to put its own voice on the air at an average
cost of $1000-$1500.  This is far more affordable than the tens or hundreds
of thousands required by the current FCC regulatory structure.

All of the technical aspects of putting together a micropower broadcasting
station are covered in the following material.  It is important to note
that the main argument the FCC uses against micropower broadcasting is the
issue of interference with other broadcast services.  Interference is a
valid concern.  By using equipment that is frequency stable and properly
fitted with harmonic suppression filters along with good operating
procedures
and standards, the FCC's argument can be effectively neutralized.

Further, the technical aspects of micropower broadcasting require some
basic knowledge in the areas of electronics and broadcast practices.
Hopefully, this primer will be able to convey some of this knowledge to
you.  If you are unsure of your abilities try to find someone who has the
technical experience to help you.  It
is hoped that as this movement grows a network of people with the required
technical skills will be formed to assist in the process of empowering
every community with its own voice.  If you are a person with engineering
or technical experience, please contact Free Radio Berkeley to become part
of this network.

                            FINDING A FREQUENCY

Before you can proceed any further you must determine if there are any
available frequencies in your area.  Due to frequency congestion in the
large urban metroplexes such as Chicago, Boston, LA, NYC, etc. this may be
a bit difficult.  You will need several items to do a frequency search: a
listing of the all the FM radio stations within a 50-70 mile radius of your
area; and a digitally tuned radio. There are several databases on the world
wide web which can be searched for FM radio stations in any given area.
Here is one:  www.airwaves.com/fccdb.html


Channel separation is the biggest problem.  FM broadcast frequencies are
assigned a frequency channel 200 kilohertz wide.  Good broadcasting
practice requires that at least one channel of separation must exist on
either side of the frequency you intend to use.  In other words, if you
have picked out 90.5 as a possible frequency then 90.3 and 90.7 should be
clear of any receivable signals.  This is why a digital receiver is an
important item for the frequency search.

Once you have a complete listing of all the FM radio stations look for
possible frequencies with the appropriate
channel spacing.  Depending on topography, distance and the output power of
the other stations certain "used" frequencies may in fact be open.  Compile
a list of the possible frequencies.  Then, using a digital FM receiver with
an external antenna, scan and check these frequencies.  Do this from a
number of locations and at varied times within the area you propose to
cover.  In most cases weak, intermittent, or static filled signals can be
ignored and counted as either usable or providing the necessary channel
separation.   Hopefully you will find at least one or two usable
frequencies.  If you live in a more rural area or some distance from a
large urban area, finding a usable frequency should not be very difficult.
87.9 can be used as a frequency under two conditions.  One, if there is not
an existing station on 88.1, and two if there is not a TV Channel 6 being
used in your area.

After compiling your list of possible frequencies have your friends check
them out on their receivers or radios as well.  It is helpful to do since a
variety of different receivers will more accurately reflect the listening
conditions in your area.  After all of this you should have a workable list
of frequencies to use.

                     LOCATION OF STUDIO AND TRANSMITTER

 Before you set up the station an adequate location must be found.  Since
the antenna will be there as well a site with adequate elevation is
required.  Ideally the top of a hill or a spot somewhere on the side of
hill overlooking the area of coverage is best.  FM transmission is "line of
sight" the transmitting antenna and receiving antenna must be able to "see"
each other.  Therefore, any large obstructions will have a tendency to
block the signal path.  Keep this in mind when choosing your location.  If
your site is a 1 to 3 story building,
a 30 foot push up style mast attached and guyed to the roof or a TV antenna
style tower bracketed to the side of the building will be needed to provide
adequate height for the antenna.  At the very least you need to have the
antenna at least 40-50 feet above the ground.  In some areas a building
permit may be needed to attach a mast or tower to a building.

It is good practice to keep the transmitter some distance from the audio
studio since the radio frequency emissions from the transmitter can get
into the audio equipment and cause noise and hum.  Your transmitter should
be set up in another room, attic space, etc. as close to the antenna as
possible.  Keep the distance from the transmitter to antenna as short as
possible to minimize signal loss in the coaxial cable feeding the antenna.

These are some of the basic issues regarding site selection.  Landlords,
room mates, leases etc. are your problem.

                              FM TRANSMITTERS

FM is an abbreviation for Frequency Modulation.  Modulation is how
information is imparted to a radio frequency signal.  In the case of FM the
audio signal modulates what is called the carrier frequency (which is the
frequency of the broadcast signal) by causing it to shift up and down ever
so slightly in response to the level of the audio signal.  An FM radio
receives this signal and extracts the audio information from the radio
frequency carrier by a process called demodulation.

Modulation of the signal takes place within the FM broadcast transmitter.
The transmitter consists of several different sections: the oscillator,
phase locked loop, and gain stages.  Generation of the broadcast carrier
frequency is the responsibility of the oscillator section.  Tuning (as
distinct from modulation) or changing the frequency of the oscillator
section is either done electronically or manually.  For a practical radio
station that will be operated for more than a few minutes, it is almost
essential to have the tuning done under electronic control since free
running or manually tuned oscillators will drift in frequency due to
temperature and inherent design limitations.  This is an important
consideration is selecting a transmitter.  Since one of the goals is to
deprive
the FCC of technical objections to micropower broadcasting it is critical
to have transmitters that stay on frequency and do not drift.  This, of
course, rules out using transmitters based on free running oscillators.

 Frequency control brings us to the next section.  Oscillator frequency
drift is corrected by a circuit known as a phase lock loop (PLL)
controller.  In essence, it compares the output frequency of the oscillator
to a reference frequency.  When the frequency starts to drift it applies a
correction voltage to the oscillator which is voltage tuned, keeping it
locked to the desired frequency.  In a PLL circuit the frequency is
selected by setting a series of small switches either on or off according
to the frequency setting chart that comes with the transmitter.  In
some cases the switch array may be replaced by 4 dial-up switches that show
a number for the FM frequency of transmission, i.e. 100.1 for 100.1 MHz.
Even simpler, some units have a display like a digital radio with up and
down buttons for changing frequency.

One part of the oscillator section, the voltage tuning circuit, serves a
dual purpose.  As described above it allows the oscillator to be
electronically tuned.  In addition, it is the means by which the broadcast
carrier frequency is modulated by an audio signal.  When the audio signal
is applied to this section the variations in the audio signal voltage will
cause the frequency of the oscillator to shift up and down.  Frequency
shifts brought about by audio modulation are ignored by the PLL controller
due to the inherent nature of the circuit design.  It
is important not to over modulate the transmitter by applying an audio
signal whose level is too great.  Many transmitters are equipped with an
input level control which allows one to adjust the degree of modulation.
Further control of the audio level is provided by a compressor/limiter
which is discussed in the studio section.

As the modulation level increases the amount of space occupied by the FM
signal grows as well.  It must be kept within a certain boundary or
interference with adjacent FM broadcast channels will result.  FCC
regulations stipulate a maximum spread of plus or minus 75,000 cycles
centered about the carrier frequency.  Each FM channel is 200,000 cycles
wide.  Over modulation- the spreading of the broadcast signal beyond these
boundaries- is known as splatter and must be avoided by controlling the
modulation level.  As a result the signal will be distorted and
interference with adjacent channels will take place.

Following the oscillator section are a series of gain stages which buffer
and amplify the signal, bringing it to a sufficient strength for FM
broadcast purposes.  In most cases this will be 1/2 to 1 watt of output
power.  This level is sufficient for a broadcast radius of 1-2 miles
depending on circumstances.  For increased power a separate amplifier or
series of amplifiers are used to raise the power level even higher.
Amplifiers are
covered in the next part of this primer.

Transmitters are available in kit form from a number of different sources
including Free Radio Berkeley, Progressive Concepts, Panaxis and Ramsey.
Assembly requires a fair degree of technical skill and knowledge in most
cases.  Free Radio Berkeley offers an almost fully assembled 1/2 watt PLL
transmitter kit
requiring a minimal amount of assembly.  Kits from Ramsey are rather
debatable in terms of broadcast quality.  An English firm Veronica makes
some rather nice kits as well.

                                 AMPLIFIERS

Although 1/2 to 1 watt may be perfectly adequate for very localized
neighborhood radio coverage, higher power will be required to cover larger
areas such as a town or a portion of a large urban area.  In order to
increase the output power of a low power FM exciter or transmitter an
amplifier or series of amplifiers are connected to the output of the
transmitter. Amplifiers are also referred to as amps, and should not be
confused with the unit of current also called amps.

Amplifiers are much simpler in design and construction than a transmitter.
Most of the amplifiers used in micropower broadcasting employ only one
active device, an RF power transistor, per stage of amplification.  By
convention most broadcast amplifiers have an input and output impedance of
50 ohms.   This is similar to audio speakers having an impedance between  4
and 8 ohms.  When an RF amplifier with a 50 ohm input
impedance is attached to the 50 ohm output impedance of a transmitter this
matching of impedances assures a maximum flow of electrical energy or power
between the two units.

A mismatch between any elements in the chain from transmitter to amplifier
to filter to antenna will reduce the
efficiency of the entire system and may result in damage if the difference
is rather large.  Imagine the results if a high pressure water pipe 4
inches in diameter is forced to feed into a 1/2" water pipe with no
decrease in the action of the pump feeding the 4 inch pipe.  In an RF
amplifier the RF power transistor will heat up and self-destruct under
analogous conditions.

An RF power amplifier consists of an RF power transistor and a handful of
passive components, usually capacitors and inductors which are connected in
a particular topology that transforms the 50 ohm input and output
impedances of the amplifier to the much lower input and output impedances
of the RF power transistor.
Detailed circuit theory of this interaction between the components is not
covered in this primer.

Amplifiers can be categorized as either narrow band or broad band. Narrow
band amplifiers are tuned to one specific frequency.  Broad band amplifiers
are able to work over a specified range of frequencies. without tuning.
Most of the amplifiers that have been used in micropower broadcasting are
of the first type.  A tunable amplifier can be a bit of a problem for those
without much experience.  In a typical tuned stage amplifier there will be
two tuning capacitors in the input stage and two more in the output stage.
If not correctly adjusted the transistor can produce unwanted sideband
spurs at other frequencies both within and outside of the FM band.

To make set up easier for the average micropower broadcaster a broad band
amplifier is preferable or one with a minimal amount of tuning stages.
Several designs are available.  One rather popular one is a 20-24 watt
amplifier using a Phillips BGY33 broad band power amplifier module.  It is
a rather rugged device that requires no tuning and produces a full 20-24
watts output for 250 milliwatts of drive from the transmitter.  Free Radio
Berkeley has a kit based on this device.  This kit includes an output
filter as well which other vendors may not include in their kits.
Regardless of the source, the BGY33 is not the most efficient device and
requires a good sized heat sink for proper dissipation of heat, and the use
of a cooling fan is strongly suggested as well.

If you buy a kit or transmitter package based on this device be certain to
determine from the manufacturer that the BGY33 is mounted directly to the
heat sink, not to a chassis panel with a heat sink on the other side of the
chassis panel.  It must directly contact the heat sink with a layer of heat
sink heat compound between the module mounting flange and the heat sink
surface.

Broad band designs are not as a common due to the degree of design
experience required to create a functional unit.  It seems a number of kit
providers are content not to optimize and improve their amplifier designs.
Free Radio Berkeley is now offering amplifiers that are either no tune or
minimal tune designs in
several different ranges of power.  Certain broad band designs may be too
wide in their range of frequency coverage and will amplify the harmonics
equally well.  For FM broadcast purposes the width of frequency coverage
should be  for only the FM band, about 20-25 Megahertz wide.

Selecting the right amount of power is rather important since you should
only use enough power to cover the desired area.  Unfortunately there is
not an easy answer to the question of how much area a certain amount of
power cover.  Antenna height is very critical, 5 watts at 50 feet will not
go as far as 5 watts at 500 feet.  Assuming you do not have a 10 story
building or a convenient 500 foot hill to site your antenna and transmitter
on,  experience in urban environments has yielded the following rough
guidelines.  With based an antenna approximately 50 feet above the ground.
1/2 to 1 watt will yield an effective range of 1 to 3 miles, 5-6 watts will
cover out to about 1-5 miles, 10-15 watts will cover up to 8 miles, 20-24
watts will cover up to 10-12 miles and 30-40 watts will cover up to 15
miles.  Coverage  will vary depending on terrain, obstructions, type of
antenna, etc.  If your antenna is very high above average terrain you will
be able to go much further that the figures given above.  Quality of the
radios receiving your signal will be a determining factor as well.  Since
the power levels are rather low in comparison to other stations an external
antenna on the receiver is highly suggested, especially an outdoor one.

It is very important to provide adequate cooling for RF amplifiers.  This
means using a properly sized heat sink and an external cooling fan.  Heat
sinks have heat dissipating fins which must be placed in an upward pointing
direction.  Overheating will cause premature failure of the transistor.  A
cooling fan, usually a 4 to 5 inch square box fan, will offer extra
insurance.  It should be placed so that the air flows over the fins of the
heat sink.

Under no circumstances should an amplifier/transmitter be operated without
a proper load attached to the output.  Failure to do so can destroy the
output transistor.  When testing and tuning a dummy load is used to present
a load of 50 ohms to the transmitter/amplifier.  It is very bad practice to
tune a unit with an antenna attached  Use a dummy load of proper wattage
rating to match the transmitter output wattage.

An output filter must be used between the transmitter/amplifier and the
antenna.  Some amplifier kits come with a filter included, such as the 20
Watt FRB amplifier.  These do not need an additional filter.  More on this
in the filter section.

Heavy gauge (12-16 AWG) insulated stranded wire is used to connect the
amplifier to the power supply.  Observe correct polarity when making the
connection.  Reversing the polarity will result in catastrophic failure of
the transmitter.  Red is positive and black is negative or ground.

                               POWER SUPPLIES

Most of the transmitters and amplifiers used in micro broadcasting require
an input voltage of 12 to 14 volts DC. Higher power amplifiers (above 40
watts) require 24-28 volts DC. In a fixed location the voltage is provided
by a power supply which transforms the house voltage of 110 volts AC to the
proper DC voltage.

Power supplies are not only measured in terms of their voltage but current
as well.  A higher power amplifier is going to require a greater amount of
input power as compared to a lower power amplifier.  Output current is
measured and specified as amps..  A power supply is selected on the basis
of its continuos current output which should be higher than the actual
requirements of the amplifier.  Power supplies operated at their
fully rated output will have a tendency to overheat under continuos
operation.  An amplifier which requires 8 amps will need a power supply
with a 10 to 12 amp continuos capacity.  In most cases the following
ratings are suggested for transmitters requiring 13.8 volts.

1-5 Watt Transmitter  2-3 Amps
10-15 Watt Transmitter  5-6 Amps
20-24 BGY33 Based Unit  10 Amps
40 Watt Transmitter   12 Amps

Any power supply you use must have a regulated voltage output along with
protection circuitry.  Some reasonably priced brands include Pyramid,
Triplite and Astron.  Do not use any of the wall transformer type of power
supplies.  Such units are not adequate for this application.  Higher power
transmitters require power
supplies with an output voltage of 28 volts.  Astron is the best
manufacturer of this type of power supply.  A 75 watt transmitter will
require a power supply with a current rating of 6-8 amps and 28 volts.

For mobile applications voltage can be fed from the cigarette lighter
socket of a car with the correct plug and heavy gauge wiring.  This may not
work well in some newer vehicles with are reported to have some sort of
current limit protection on the lighter socket.  Check with an auto
mechanic about this if you are in doubt.  Electrical systems on newer
vehicles are rather sensitive and can be damaged if not properly
understood.

Another problem with mobile operation is battery drain.  A 20-40 watt
transmitter running for 4-5 hours can deplete the battery to the point
where the vehicle may not start.  It is better to have separate battery
running parallel to the charging system with an isolator.  Isolators are
available from Recreational Vehicle
accessory suppliers.  Use a high capacity deep discharge type of battery.

Lead acid batteries are not very benign.  Acid can leak and spill on
people, clothing and equipment.  It best to keep the battery in a plastic
battery box.  Vapors from the battery are explosive in confined areas.
Keep this in mind for mobile vehicle operations.  You might consider using
a gell cell type of battery which is sealed and can not leak.  These are a
bit pricey but have far fewer problems.  A good quality gel charger must be
used to ensure battery longevity.

Smaller gel cell batteries work really well for setting up a low power (6
watts or less) transmitter on a street corner as a public demonstration of
micropower radio.  In Berkeley a 6 watt micropower station is set up at the
local flea market as a community demonstration on weekends.  It is called
Flea Radio Berkeley.  Transmitters can be set up at demonstrations and
rallies so motorists can tune their radios to the frequency which is
displayed on large banners near the streets and listen in on what is
happening.  This has worked very well.  Use your imagination to show how
micropower broadcasting can be brought into the community.



                                  FILTERS

Although it is rather simple in design and construction a filter is one of
the most important elements in broadcasting.  No matter what, a proper
filter must be used between the transmitter and antenna.  Use of a filter
will help deprive the FCC of one of its main arguments against micropower
broadcasting - interference with other broadcast services.

A proper filter reduces or eliminates harmonics from your broadcast
signal.  Harmonics are produced by the transmitter and are multiples of the
fundamental frequency you are tuned for. For example, if you broadcast at
104.1, you may produce a harmonic at 208.2, and (less likely) 312.6 and so
on.  Most filter designs are of the low pass type.  They let frequencies
below a certain frequency pass through unaffected.  As the frequency
increases and goes beyond that point the filter begins to attenuate any
frequency that is higher than the set point. The degree of attenuation
increases with the frequency.  By the time the frequency of the first
harmonic is reached it will be severely attenuated.  This is very important
since the first harmonic from an FM transmitter falls in the high VHF TV
band. Failure to reduce this harmonic will cause interference to
neighboring TV sets.

You do not want to generate complaints from folks who engage in the odious
habit of watching TV.  Noble sentiments, such as telling them to smash
their TV if they have a problem will not suffice.  Use a filter.
Complaints increase the possibility of the FCC showing up at your door.
One needs to be good broadcast
neighbor and an asset to the community

Harmonics further up the scale can cause interference to other mobile and
emergency radio services.  Not desirable either.

Transmitters with output power ratings of less than 25 watts will need at
least a 7 pole design.  Higher power units will need a 9 pole design.  An
increase in number of poles increase the degree of attenuation.
Representative designs are shown.  If you build one of these put it in a
metal, well shielded enclosure.

Not really related to filters but an important side issue is the use of FM
frequencies at the bottom and top ends of the band.  Do not use 87.9 to
88.3 or so if their is a channel 6 TV frequency being used in your local
area.  Television sets have notoriously poor selectivity and your signal
might end up coming in on the sound carrier of the TV if channel six is
being used.  At the top end of the band do not go any higher than 106 MHz
if the
transmitter is near an airport.  In fact, do everything possible not be too
close - at least several miles and away from the flight path(s).  Even
though interference possibilities are minimal there is not any point in
taking chances since the FCC has claimed airplanes will fall from the sky
if micropower broadcasting is given free reign.  Corner cutting corporate
airline maintenance polices most likely pose a greater danger to public
safety than micropower broadcasting, however


                                  ANTENNAS

An antenna's primary purpose is to radiate the FM broadcast signal from the
transmitter to surrounding FM radio receivers. In order to do this several
conditions must be met.  First, the antenna must be tuned to the frequency
being transmitted. Secondly, it must be sited and oriented properly.

At FM frequencies the radio waves travel in a straight line until an
obstacle is met.  This is known as line of sight transmission. If the
receiving antenna and transmitting antenna can "see" each other and the
path distance is not too great to attenuate the signal, then the broadcast
signal can be received.  Radio signal strength is based on the inverse
square law.  Double the distance and the signal strength will be 1/4 of
what it was.

Since FM broadcast transmissions are line of sight, the height of the
antenna is very important.  Increasing the height is more effective than
doubling or tripling the power.  Due to the curvature of the earth the
higher the antenna the greater the distance to the horizon.  Increased
height will place the antenna above obstructions which otherwise would
block the signal.  Your antenna should be at least 40-50 feet above the
ground.  Count
yourself lucky if you can site the antenna on a  hill or a ten story
building.

An antenna is rough tuned by adjusting the length of the radiating
element(s).  Many antenna designs are based on or derived from what is
called a dipole, two radiating elements whose length is roughly equivalent
to 1/4 of the wavelength of the desired frequency of transmission.
Wavelength in inches is determined by dividing 11811 by the frequency in
megahertz.  The result is either divided by 4 or multiplied by .25 to yield
the 1/4 wavelength.  A correction factor of .9 to .95, depending on the
diameter of the element, is multiplied times the 1/4 wavelength resulting
in the approximate length of each element.

Fine tuning the antenna requires the use of an SWR power meter. SWR is an
abbreviation for standing wave ratio which is the ratio between power going
into the antenna and the power being reflected back by the antenna.  A
properly tuned antenna is going to reflect very little power back.  Correct
use of an SWR meter
is described a bit further down in this section.  IF you can afford $100.
get a dual needle meter which shows both reflected and forward power at the
same time.  A good brand is Daiwa.

A dipole with tuning stubs is one of the easiest antennas to make and
tune.  Two dipoles can be combined on a 10 foot mast if they are spaced 3/4
of a wavelength from center to center with the elements vertical and fed
with a phasing harness.  A phasing harness consists of two 1.25 wavelength
pieces of 75 ohm coaxial cable (RG11) cut to a length that is the product
of the 1.25 wavelength times the velocity factor (supplied by the
manufacturer) of the cable   A PL259 plug is attached to the end of each
cable.  These are connected to a 259 T adapter with the center socket being
the connection for the feed cable coming from the transmitter.  The other
ends go respectively to each dipole. Such an arrangement will increase the
power going into the antenna by a factor of 2.

Besides the dipole a number of other antenna designs are employed in
micropower broadcasting.  Each one has a characteristic pattern of
coverage.  Antennas can be broken down into two basic types -
omnidirectional and directional.  Under most circumstances the omni is the
antenna of choice for micropower broadcasting.  Polarization is another
aspect to consider but does not play that big of a role in most cases.
Antennas can be
vertically, horizontally or circular in polarization.  Most micro broadcast
antennas are vertically polarized.   In theory a vertically oriented
receiving antenna will receive better if the transmitting antenna is
vertically oriented as well. Obstructions in the receiving environment will
have a tendency to bounce the signal around so that the signal will be not
be exactly vertically polarized when it hits the receiving antenna,
particularly in a car that is moving.  Commercial broadcasters employ
circular polarization which yields both vertical and horizontal components
to the signal.  It is said that this is best for car radios.  This may be
true given the dependence of
commercial broadcasters on "drive time" as a peak listening period.

A single radiating element vertically oriented will have a rather high
angle of radiation where a good portion of the signal is going up to the
sky at angle of around 35 degrees or more.  When you combine two vertical
elements such as two dipoles you reduce the angle of radiation to a point
where the signal is more
concentrated in the horizontal plane.  This is what accounts for the
apparent doubling of radiated power when you use two dipoles phased
together.  Power output from the antenna or antenna array is known as
effective radiated power (ERP) and is usually equal to or greater than the
input power.

Several vertical element antenna designs have a lower angle of radiation
even though they only use one element.  These are the J-Pole and the Slim
Jim designs.  Having a signal pattern that is more compressed into the
horizontal plane makes the Slim Jim ideal for urban environments.  Both can
be easily constructed from 1/2" copper pipe and fittings.  Plans are
available from FRB directly or the FRB web site

Another class of antennas are the 1/4 and 5/8 wave ground plane antennas.
A commercially manufactured 5/8 ground plane for FM broadcast purposes is
available for around $100.  It is an ideal antenna for those want an easy
to tune and assemble antenna.  Set up time is less than 15 minutes.  Plans
for these antennas are
available from FRB.

Directional antennas are not usually required for micropower broadcasting.
If the area you wish to cover lies in one particular direction you might
consider the use of such an antenna.  An easy way to do this is to put a
reflecting screen 1/4 of a wavelength behind a vertical dipole.  The screen
will need to be bit taller than the total length of the elements and about
2-3 feet wide.  This will yield a nice directional pattern with a fair
amount of power gain   Your pattern will be about 60-70 degrees wide.
Another type of directional antenna is the yagi
which has a basic dipole as the radiating element but additional elements
as reflectors and directors.  A yagi can be a bit difficult to build for
those not well versed in antenna design and construction.  Your best choice
is a dipole with a reflector.

For those who wish for a practical design that can be built and put to use
the following is a basic dipole antenna which can be constructed from
common hardware store items.  It uses 1/2 inch copper water pipe and
fittings along with aluminum tubing.  A half inch plastic threaded T is
used with a copper 1/2 inch threaded to 1/2 inch slip adapters at all three
points.  An aluminum tube 9/16 of inch or so in diameter will fit into this
slip adapter and is attached with two #6 self tapping sheet metal screws.
This tubing is 20 inches long.  Another piece of
aluminum tubing 15 inches long with a diameter small enough to slip inside
the other tubing is used as the adjustable tuning element.  Four slots 90
degrees apart and 1 1/2 inches long are cut into in one end of the larger
tubing.  A small diameter hose clamp is slipped over that end.  With the
smaller tubing inserted
inside the hose clamp is tightened to hold it in place.  This is repeated
for the second element.  A copper half inch thread to slip adapter is
soldered to one end of a 36 inch piece of 1/2 copper tubing which is the
support arm for the dipole.  A copper T is soldered to the other end.
Then, two 3 inch pieces of 1/2 inch copper tubing are soldered to the T
fitting.  This allows easy clamping to a mast.  A solder lug is attached to
each element using one of the self tapping screws holding the elements to
the slip fittings.  Your coaxial cable will be attached to these solder
lugs.  Center conductor to one, braid or shield to the other.  You can get
a little fancier and make an aluminum bracket to hold an SO239 socket and
attach this to the T connector.

Once you have it all put together as shown in the diagram it is time to
tune it. Adjust the element lengths to the 1/4 wave length you arrived at
with the above formula.  Tighten the clamps so the tuning stubs can barely
slide back and forth.  Mark each stub where it enters the larger tubing.
Using either hose clamps or U clamps attach the antenna to the end of a
mast piece 10 feet long.  The element to which the braid or shield of the
coax is
attached must be pointing down  Support the mast so that is stands straight
up with the antenna at the top.  It is best to do this outside.

Set up your transmitter and connect an SWR/Power meter between the
transmitter and the antenna.  Adjust your meter to read SWR according to
the directions that came with it.  SWR is the ratio of power coming from
the transmitter and the power reflected back from the antenna.  A properly
tuned antenna will reflect very
little power back, resulting in a very low SWR ratio.  Too much reflected
power can damage the transmitter.

Turn on the transmitter and observe the SWR or amount of reflected power.
Shut the transmitter off if the level is very high and check your
connections.  Rough tuning the antenna by measurements should have brought
the readings down to a fairly low level.  Turn off the transmitter and
adjust each tubing stub up or down about 1/4 of an inch.  Turn the
transmitter back on and note the readings.  If the reflected power and SWR
ratio went
lower you went the right direction in either increasing or decreasing the
length of the stubs.  Turn off the transmitter and continue another 1/4
inch in the same direction or the opposite direction if the SWR ratio and
reflected power increased.  Turn the transmitter on again.  If  the reading
is lower continue to go in the same direction in 1/4 inch increments being
sure to turn off the transmitter to make the adjustments.  Continue to do
this cycle until you have reached the lowest possible reading. At some
point the readings will start to increase again.  Stop there.

You can do this with two dipoles as mentioned earlier in this section.
Each dipole is tuned by itself and then both are connected with a phasing
harness when mounted to the mast section.

                            CONNECTORS AND CABLE

Radio frequency cables are referred to as coax as a generic term. It is
short for coaxial.  A coaxial cable consists of an inner conductor inside
an insulating core  .This is surrounded on the outside by a metal braid or
foil, called the shield  .This shield is in turn covered by an insulating
jacket of plastic material. Coaxial cables are specified in terms of
impedance which for most micropower broadcasting purposes is 50 ohms except
for dipole phasing harnesses.

In the 50 ohm category there are a number of choices when selecting coaxial
cable.  The most important characteristic of coax is it's level of signal
attenuation.  This depends on the length of the cable and its particular
frequency response.  RG58 coaxial cable has a high degree of attenuation
and should only be used for short connections.  RG8X or mini 8 works well
for lengths under 50 feet and is suited for portable and mobile set
ups since it is rather flexible.  RG8 and its higher performance cousins
such as 213 and Belden 9913 are the best for fixed installations.  Belden
9913 has the lowest loss for any given length as compared to other
variations of RG8.  In fact, it has a loss figure at 100 MHz that compares
well with commercial broadcast hard-line coax.  It is rather stiff cable
and must be installed correctly.

Coaxial cables do not take rough treatment very well, especially 9913.
They must be carefully rolled up by hand, not wrapped between palm of hand
and elbow like a rope.  Kinks are to be avoided at all costs.  When routing
a cable keep the bends from being sharp and keep it away from circumstances
where it can be pinched or slammed.

Three types of connectors are in general use - BNC, PL259 and N. Most
micropower broadcasting equipment uses PL259 and its mating socket known as
the SO239.  Any connector will introduce some small degree of signal loss.
N connectors are used where high performance and reliability are of most
importance.

                               STUDIO SET UP

A typical broadcast studio consists of an audio mixer (DJ style works
best), one or more CD players, one or more cassette tape decks, a turntable
or two, several microphones, and a compressor/limiter.  Optional items can
include a cart machine and a phone patch.

Reasonable quality mixers start at $200 and go up in price from there.  DJ
styles are best since they have a large number of inputs available and
support turntables without the need of external phono preamps. Any mixer
you select should have least 2 or more microphone input channels.  These
should be low impedance inputs.  Other features to look for include high
visibility VU (level) meters, slide faders for each channel, switchable
inputs for each channel, stereo or mono selection for the output signal,
and an auxiliary output for an air check tape deck.

CD players and tape decks can be your average higher quality consumer audio
gear.  Day in and day out usage will eventually take their toll so pay for
the extra warranty period when it is offered.  When one wears out in 6
months or so just take it back under warranty for either repair or
replacement.

DJ style turntables are the best choice for playing vinyl. Cheaper units
just will not stand up to the wear and tear of daily usage.  Select a heavy
duty stylus as well.

Microphones should be fairy good quality vocal types.  They can be either
directional or omnidirectional.  Directional microphones will pick up less
ambient noise but need to be on axis with the person's mouth for best pick
up.  Since some folks do not pay attention to where the microphone is in
relation to their mouth, an omnidirectional might be considered a better
choice if this is the case.  A distance of about 4 inches should
be maintained between the microphone and mouth.  Place a wind screen foam
piece over each microphone.  Some microphones have built-in shock and
vibration isolation to keep bumps to the microphone from being audible.  It
is a good idea to use some sort of isolated holder for the DJ microphone.
An old swing arm
lamp can be adapted to hold a microphone.

For programmers who do a lot of reading on material on the air a headphone
microphone is something to consider since it will maintain a uniform
distance from mouth to microphone no matter where the head moves to.  One
drawback is that they tend to be a bit fragile in rough hands.

Headphones are essential for monitoring and curing up program material.
You can either opt for high quality rugged units that are a bit costly or
plan on replacing an inexpensive set every few months.

A limiter/compressor is an essential part of the audio chain.  It is used
to keep the audio signal from exceeding a preset level.  Without this the
transmitter will be overmodulated resulting in signal splatter and
distortion.  Signal splatter will cause interference with adjacent stations
and distortion will send your listeners elsewhere.

Common to most limiter/compressors are a set of controls - input level,
output level, ratio, threshold, attack and decay. To properly set up the
mixer, limiter/compressor and transmitter you start with a steady audio
source (a signal generator plugged into the board or a test tone CD, tape
or record).  You adjust the input level and master output level controls so
that the meters are reading zero dB.  Master level should be at mid
position.  Audio output goes from the mixer to the limiter/compressor and
from there to the transmitter.  Do not turn the transmitter on at this
time.

Most limiter/compressors have indicator lights or meters to show how much
gain reduction is being applied and the output level.  Set the ratio
control to the infinity setting, this enables hard limit function.  Attack
and decay can be set around mid position.  Adjust the threshold and the
input level until the gain reduction shows activity.  Adjust the output
level so that the indicator lights or meters show a 0 dB output level.

Turn the level input on the transmitter all the way down and power up the
transmitter.  Monitor the signal on good quality radio.  Slowly turn the
level control until you can hear the test tone.  Compare the signal level
to that of other stations.  Your level should be slightly less since most
other operations are using quite a bit of audio processing on their
signal.  You may have to make fine adjustments to the limiter/compressor to
get things exactly right.

When everything is set up correctly any audio signals that exceed 0 dB on
the board will be kept at that level by the compressor/limiter.  You will
need to listen carefully to the signal to make sure when a "hot" audio
source exceeds this that the transmitted signal keeps an even level and
does not distort or splatter.  There will be some interplay between the
output level and the threshold setting.  Nor do you want a signal that is
too low in level either since that will produce a weak sounding broadcast.

A very important consideration is to keep as much distance between the
studio gear and the transmitter as possible.  RF (radio frequency signals)
will find their way into audio equipment and produce a hum or other types
of noise.  You can separate the two areas by using a low impedance cable
between the limiter/compressor and the transmitter.  This can be a long
microphone cable with XLR connectors or a made up shielded 2 conductor
cable with XLR connectors.  You can have about 150 feet of cable maximum.
A high impedance to low impedance transformer will be needed at one end or
both depending on whether the
limiter/compressor and transmitter have low or high impedance connections.
These transformers usually have an XLR female connector on the low
impedance side and a 1/4" phone plug on the high impedance side.  If your
transmitter has an RCA style input you will need the proper adapter to go
from 1/4" phone plug to the RCA plug.

Your studio should be arranged to provide easy access to all controls and
equipment with plenty of table space.  An L or horseshoe shape works well
for the studio bench.  An open area within the sight line of the operator
should be provided so their will be a place for extra microphones and
guests.



                                 FINAL WORD

Although it seems like there is a lot to deal with in setting up a
micropower station, it can be broken down into three areas- studio,
transmitter and antenna.  It should not be difficult to find someone with
studio set-up experience to help with the project.  Transmitters,
particularly their construction and tuning, should be left to an
experienced person.  If such a person is not available there are a number
of people who will assemble, test and tune your transmitter for whatever
fee they have set.  Stick to a commercial, easy to tune antenna such as the
Comet if your skills are minimal.  These can be purchased pre-tuned for an
additional fee from FRB and L. D. Brewer.  It best to put most of the
energy into organizing and setting up the station.

Experience has shown that once the technical operation is in place and
running, it will require very little in the way of intervention except for
routine maintenance (cleaning tape heads, dusting, etc.) and occasional
replacement of a tape or CD player.

What requires most attention and "maintenance" is the human element,
however.  More time will be spent on this than any equipment.  As a
survival strategy it is best to involve as much of the community as
possible in the radio station.  The more diverse and greater number of
voices the better.  It is much easier for the FCC to shut down a "one man
band" operation than something serving an entire community.  Our focus is
on empowering communities with their own collective voice, not creating
vanity stations.  Why imitate commercial radio ?

Before you commit to your first broadcast it would be advisable to have an
attorney available who is sympathetic to the cause.  Even though they may
not be familiar with this aspect of the law there is a legal web site which
offers all of the material used in the Free Radio Berkeley case.  There are
enough briefs and other materials available to bring an attorney up to
speed.




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