Version 0.9, Preliminary
(This document is under development and as yet incomplete, however I think the need
for expediency in disseminating this information outweighs the concern about completeness
at this time.)
"To sound legitimate -- even if you aren't."
I've prepared this document for public distribution for the public well-being and in
the interest of promoting responsible, professional operations of LPFM (micropower) radio
stations. There are a great many people who feel that radio is a toy and that
"getting on the air" is a game to be taken lightly. It is not. Many
"pirate" broadcasters have made a bad name for the micro community broadcasters
because of reckless, irresponsible operation and cause of much interference as well. This
document will help all concerned to better understand the issues, the equipment and the
procedures required to build and maintain a small FM station.
The airwaves are a community property. One must always treat it as such, respecting the
space of other stations, both commercial and micro. In addition, there are many other
services that use radio frequencies which may be disturbed unintentionally by micropower
stations if care is not taken to evaluate each step of the process of constructing your
station. A "Do not do unto others..." attitude is a good start. Acquiring
knowledge needed to operate responsibly and with awareness of what is going on with the
signals in your area is the next step.
Admittedly, some parts of the country have no empty channels. Places like south
Florida, California, New York and Chicago are virtually crammed full of stations. For the
rest of us, if we look hard, we can locate one or more unused channels. There is MUCH more
to this than simply turning on a cheap radio in ONE LOCATION and tuning around the dial. I
recommend a 4-pronged approach:
Use a high-quality receiver connected to a high-gain beam antenna mounted on a tall
mast. If you're on a mountaintop location, this works well at ferreting out the weakest
signals that may exist on the prospect channel. Find a channel where you hear nothing.
Rotate the antenna 360 degrees and see if it remains empty from all directions.
Remember, the next adjacent channel should be at least 75 miles away for a 3000-watt
station, and the second adjacent should be at least 50 miles away. Once the prospect
passes phase one of your testing, move on to the next step:
Choose an automobile with a very sensitive FM receiver (if you don't have one, ask a
friend to help you and offer to buy him lunch for his time). Next, monitor the empty
channel prospect and drive out to the farthest extents of your station's proposed coverage
contour. Better yet, drive another 15-20 miles out and make a huge arc around the station
site so you get a feeling for any signals that are on the channel that were too weak to
pulling at your home site. If you hear a consistent, weak signal on that channel for any
substantial leg of the journey, do not use that channel.
Elliot Broadcasting Services
provides a good online reference which will help you determine what stations are using
your frequency and where they're located. The FCC publishes the M Street Directory, a
printed publication which lists the current stations that are on the air, but it costs
more to obtain.
Take a few weeks and recheck the prospect channel periodically. Conditions may change due
to weather, varied operations of co-channel stations, etc.
It may not be obvious, but it's a very good idea to find out what service is using
TWICE the frequency of your broadcast channel. Usually, this will be a TV station. The
Radio Amateur's Handbook will have data on this. Get a book that lists all the frequencies
from AM to microwaves and multiply your channel by 2, 3, 4, 5 and 6 times the frequency
and then look up what service is using that frequency. You may need to buy a scanner radio
to monitor those frequencies to determine if you are causing any interference.
You've located a channel that's clear and has no strong nearby adjacents broadcasting.
Fine. Now it's time to look into what you'll need to do this project right.
I should point out that most pirates work completely blind. Not a good idea. Get the right
lab equipment or get out of broadcasting. The fines are too great if you screw up some
vital service and you make a nuisance of yourself regardless this way. Let's do it right.
Here's what you'll need:
Educate yourself about radio theory. Buy the Radio Amateur's Handbook and study it.
Learn the theories and some of the formulae. Read every book you can find on the subject
of radio and if you can obtain books on commercial FM theory of operation, read those
especially.
You'll need some essential tools to avoid working blind.
No circuit that operates above 20Mhz operates independently of its environment.
Shielding and all-metal chassis construction is a necessary part of circuit design. Forget
about that audio amp kit you built on a wooden box. Different physical laws are at work in
the VHF spectrum. Every RF unit MUST be encased in metal boxes. AC mains MUST be RFI/EMI
filtered. These are two essential keys to hum-free unmodulated carriers and to spurious
signal elimination.
Build all RF circuits on PC boards. Don't use perf board or project breadboard circuit
evaluators. You'll need to etch your own circuit layouts on double-sided copper-clad PC
board material, for reliable operation. A lot of groundplane is necessary in these
designs. A well-constructed transmitter uses "airline" inductors and
"stripline" interconnection methods, keeping distances short between components.
All DC voltage supply leads should have .001uf bypass capacitors shunting RF to ground so
it doesn't enter the power supply. Mica capacitors work well for this. Do not use
electrolytic! They will overheat and explode in some cases.
Keep all leads as short as possible and install components close to the PC board to reduce
stray coupling effects.
Use components that are designed for the application. Don't use an audio transistor for
your RF oscillator, for instance. Motorola makes many suitable transistors for this kind
of application. Consult their RF Data Manual for specifics.
Mount sensitive circuits like VCO modules in sub-chassis within your main chassis for
extra shielding and isolation from other circuits.
Good design practice will pay off. Instead of a sloppy, interference "hog" with
25db of signal-to-noise ratio, you could achieve 80-90db of signal-to-noise and spectral
purity so good you might not need a filter to suppress harmonic radiation. Such a
transmitter will sound like any other commercial transmitter, if you've paid the same
attention to other aspects of circuit design as well as packaging.
Harmonic interference is a major problem for any FM station that operates on a
sub-multiple of a TV channel that is important to the community. This second harmonic
interference can destroy television reception over several blocks radius of the
transmitter in the case of a micro station and over several miles in the case of a
full-power FM station. I'm going to share with you an effective, yet simple solution to
second harmonic interference that will make your transmitter so clean that TV reception in
the same house as the transmitter can be unharmed. It's a 1/4-wave tuning stub. (I've
actually seen this type of stub in use at an NPR station.) Here's how it works:
Transmission line has a particular characteristic at multiples of a wavelength. At
1/2-wave multiples, if a short is at the far end of the line, a short appears at the
opposite end too. If it's open, then the other end is nearly infinite resistance too.
However, at 1/4-wave intervals, the OPPOSITE is true. A short at halfwave interval appears
like an infinite resistance at half the frequency where that distance is only a quarter
wavelength. For example, a second harmonic filter stub for a station operating at 100Mhz
would be approximately two feet long. At 100Mhz the shorted far end would be only 1/4
wavelength away from the generator, hence appear unshorted. But at the second harmonic,
200Mhz, that distance is 1/2 wavelength and characteristically, the short at the far end
appears at the generator end too. It's a frequency selective short, in other words.
This stub gets inserted into your antenna transmission line by adding a Tee connector in
series with the line and hanging the stub off the tee. It doesn't look like it can do
anything, but it's darned effective and when two or more are cascaded 1 wavelength apart,
even the most severe second harmonic interference can be banished.
This is one good reason why you'll need an SWR Analyzer / Resistance Bridge, because
you'll need to fine-trim the stub by measuring its resistance at fundamental and second
harmonic frequencies. Tuning is critical and cannot be guessed at.
Optionally, if you broadcast in stereo, you'll need to add the following:
Once the engineering work is done, and the field testing is done and you're satisfied
with your coverage, the sound of your transmitted signal, etc., it's time to focus on
professionalism as a key part of your broadcast operation.
Before you go on the air, practice reading the news and weather into a taperecorder. Study
professional announcers by listening to how they use the pitch and intensity of their
voices -- how they add emphasis and excitement to a phrase or sentence -- and try to
emulate a style you find that fits your way of speaking. Practice and listen to the
recordings, noting the things that need work, yet recognizing the things you did well.
After a few months of practice, you'll find that your reading skills have improved and
your voice is becoming trained.
Plan ahead what your programming is going to focus on and develop a schedule. Decide what
services you want to provide, such as news, weather, community calendar, local events
coverage and formulate a plan for carrying out these activities.
Work on your station's "theme". This could include station ID, a jingle, etc.
Try to produce it as professionally as you can manage. You'll need good audio equipment to
insure the quality of everything you produce.
Think about the equipment you'll need. Here's a list of suggestions:
IMPORTANT: an EXCELLENT quality microphone with a
cardioid directional pattern and a wide frequency response. This will make or break a
station, because "pirates" usually give themselves away just by the tonal
quality of the audio pickup. A broadcast-quality mic will make you sound like a pro
station, even if you don't sound perfect.
Every day before going on the air, prepare your program schedule. Obtain the materials and
information you will need for that day's broadcast BEFORE you turn on the Big Switch. Line
up your music selections, if you run a music program, think about what you're going to
say. Try to be creative and knowledgeable about the program content. If you're announcing
a song title, sometimes it makes for more interesting listening if you can mention some
interesting fact about the song or the band that performed it. Learn to improvise and
think on your feet.
Theme is up to you, but I make this statement: I have noticed a definite pattern between
FCC busts and politically-dangerous commentary. I suspect that such content is dimly
looked upon by the feds and hence made a priority second only to interference complaints.
If this is a fun hobby, and political change is not your main reason for broadcasting, you
will be safer leaving the political talk to Rush and others who have to defense budget to
back them.
When you make a conscious effort to follow these suggestions and operate more like a pro
than a "pirate," you'll enjoy the possibility of increased positive notoriety
and less chance of a negative experience such as a complaint, or a visit from the FCC.
Authored by your friendly "Peg-legged" Bovine One