156 lines
8.2 KiB
Plaintext
156 lines
8.2 KiB
Plaintext
PIRATE RADIO SURVIVAL GUIDE
|
|
|
|
Note: this chapter is from the book "Pirate Radio Survival Guide" written by; Nemesis of
|
|
Radio Doomsday, and Captain Eddy of The Radio Airplane. If you like this book and would
|
|
like to support their efforts, you may send a donation of your choice to either Nemesis or
|
|
Capt. Eddy at PO Box 452, Wellsville NY 14895.
|
|
|
|
Please note that some chapters refer to illistrations or drawings, these could not be included in
|
|
this BBS version of the book. If you would like the illistrations or have other questions you
|
|
may inquire at the above adddress.
|
|
|
|
ANTENNAS
|
|
|
|
Antennas are probably one of the most debated, most studied and
|
|
cause for the most misinformation of any field in radio. The most
|
|
important part of your station is a properly constructed antenna!
|
|
For example: Transmitter A runs 100 watts to a improper antenna
|
|
and gets heard only marginally. Transmitter B runs 10 watts to a
|
|
properly constructed, resonant antenna and gets heard much better
|
|
and louder than Transmitter A. I cannot stress enough the
|
|
importanceof a good antenna! It makes all the difference in the
|
|
world!
|
|
|
|
For the sake of simplicity, only three antenna designs are
|
|
going to be presented here. If you are a beginner, I suggest that
|
|
you erect the dipole and worry about the other designs later. For
|
|
the intermediate and advanced, by all means try out the Vertical
|
|
and Loop! There are many books available on Antennas, which go
|
|
into much greater depth, detail and variety.
|
|
|
|
Perhaps the easiest and most popular antenna is a Dipole. The
|
|
dipole antenna is easily constructed, almost impossible to mess
|
|
up and works well at almost any height above ground. For the
|
|
beginner, this is the antenna to use. For your antenna to work
|
|
well, you need to determine the frequency you are going to
|
|
operate on. For example, we'll say 7445 Khz. To determine the
|
|
length of wire our dipole antenna will need, we use the following
|
|
formula: 468 divided by Frequency in Megahertz = Length in Feet.
|
|
So, working the math, 468 / 7.445 =
|
|
62.86 Feet. Round that off and we come to 62 Feet 10 Inches. This
|
|
is TOTAL length of the antenna. To make a dipole, cut two wires,
|
|
each one 31 Feet 5 Inches long. See Figure 1.
|
|
|
|
Already I see the plethora of questions spinning around your
|
|
head. You're probably wondering what kind of wire to use for the
|
|
antenna. Small speaker wire can be quite serviceable and it's
|
|
easily found. You will also need Three Egg insulators, One for
|
|
the center and each end of the dipole. These are getting to be
|
|
difficult to find but I think Radio Shack still has them. If you
|
|
cannot find insulators anywhere, small, rectangular scraps of
|
|
phenolic perfboard will work just as well (at low power levels).
|
|
You are also going to need some string or high test fishing line
|
|
to support the ends of your dipole. Also, electrical tape or
|
|
silicon sealer to be used on the center connection of your dipole
|
|
to keep moisture out of the coax. A soldering Iron is also a must
|
|
have item for any station; a 100 watt iron should cover just
|
|
about any job. They say a picture is worth a 1000 words, so I
|
|
refer you to Figure 2 for a typical dipole installation.
|
|
|
|
While not absolutely necessary, a Balun is recommended. For
|
|
Dipole antennas that are fed with Coax line, a 1 to 1 Balun is
|
|
suggested. A Balun matches a BALanced Line (Our Dipole) to an
|
|
UNbalanced Line (Our Coax). This makes for an even greater
|
|
transfer of power from the feedline to the antenna and will also
|
|
prevent the ground shield of your Coax from becoming a radiator
|
|
of RF! Baluns are a complex and difficult subject to fathom but
|
|
there are books out that explain the How To's better than I
|
|
could. Just remember, A Balun is optional but is worth the
|
|
trouble and $25 to install one.
|
|
|
|
Another question you might be asking "How high should I try to
|
|
get my dipole ?". My answer: as high as possible. If 10 feet is
|
|
all you can manage, then that will have to do. The higher, the
|
|
better. Dipoles typically have the most favorable radiation
|
|
patterns when they are 1 wavelength above ground. In the case of
|
|
our 40 Meter Dipole, that comes to a whopping 125 Feet! I think
|
|
it's safe to say that 99.99% of all 40 Meter Dipoles erected DO
|
|
NOT reach these lofty heights!
|
|
|
|
The last consideration you need to think about is that of
|
|
antenna orientation. A dipole will radiate the majority of power
|
|
in lobes that are perpendicular to the axis of the dipole. What
|
|
this means is, if you run your dipole North to South, then the
|
|
majority of your RF signal will be radiated in a East to West
|
|
pattern. So depending on your geographical location of your
|
|
transmitter and the location of your listeners will depend on how
|
|
you orient your antenna. You may also find that there is only one
|
|
or two possible ways to place your dipole on your property, don't
|
|
sweat it. Just hang it up and see what happens!
|
|
|
|
TIME PASSES ...
|
|
|
|
Now that you have your dipole antenna erected AND know how to
|
|
tune your transmitter, it's time to test out the new antenna! You
|
|
SHOULD have read the chapter on SWR, if not, NOW is a good time
|
|
to read it! Find out where your dipole is resonant and determine
|
|
if you need to add or subtract wire, if you're lucky you won't
|
|
have to change anything!
|
|
|
|
That's it! By now you should have a good working knowledge of
|
|
radio transmitting and all that's left is to put on a good show!
|
|
|
|
ADVANCED ANTENNAS
|
|
|
|
Going beyond our simple dipole, the next most popular and
|
|
efficient antenna for LOW BAND work is the Vertical. Vertical
|
|
antenna's produce what is known as a "Low Radiation Angle" given
|
|
a decent height above ground. This is very important for Long
|
|
Distance or DX type work. Verticals are also Omni-Directional,
|
|
which means they radiate EQUALLY in ALL directions. To construct
|
|
a vertical, use the formula for figuring half wave antenna
|
|
lengths
|
|
and refer to Figure 3. Vertical antennas can be fed with 50 Ohm
|
|
coax. Vertical antennas work best with as many radials as you can
|
|
manage. Just keep in mind, for any improvement you must double
|
|
the number of radials. Eight, typically is a manageable number of
|
|
radials to use.
|
|
|
|
The final antenna presented here is the Full Wave Loop. These
|
|
are a little more advanced than a dipole and will possibly
|
|
exhibit some gain over one. They can also perform as verticals if
|
|
fed properly. The only downside to using full wave loops is that
|
|
they require more room and supports than a dipole or vertical. I
|
|
refer you to Figure 4.
|
|
|
|
A loop will radiate the majority of it's power at right angles
|
|
to the plane of the loop, similar to the radiation pattern of our
|
|
dipole, so it's important to keep this in mind when orienting
|
|
your loop. To figure the length of wire needed for our loop
|
|
antenna, we must use another formula. Divide 1005 by Frequency in
|
|
Megahertz will give you total length in feet. For example, if we
|
|
wanted our loop to operate on 7415 Kilohertz we would 1005 /
|
|
7.415 = 135.53 or 135 Feet 6 Inches of wire would be required.
|
|
|
|
We also must use what is known as a Matching Section in order
|
|
to feed this
|
|
antenna with 50 Ohm coax. To make the matching section, a quarter
|
|
wavelength of 75 Ohm coax is required. To figure out the length
|
|
of the matching section, use the following formula: 246 Divided
|
|
by Frequency in Megahertz Equals Length in Feet. Thusly, 246 /
|
|
7.415 = 33.17 Feet or 33 Feet 3 Inches. BUT, DON'T START CUTTING
|
|
COAX JUST YET, We also need multiply our result by the Velocity
|
|
Factor of the particular 75 Ohm coax we are using! Don't worry
|
|
about the what's and how's of velocity factors in Coax cable.
|
|
Right now all you need are the proper numbers to plug into the
|
|
formula. The insulation used in Coax cable plays the
|
|
biggest part in determining a cables velocity factor. Solid
|
|
Polyethylene insulation has a V.F. of 0.66 and Foam Polyethylene
|
|
has a V.F. of 0.80. Remember to ask when you purchase your Coax,
|
|
they should be able to tell you this. RG-59/U is the recommended
|
|
type and it has a V.F. of 0.66. So, to determine the final length
|
|
of our matching section we Multiply 33.17 by 0.66 and come up
|
|
with 21.9 Feet or 21 Feet 10 Inches. Now that you have all the
|
|
math,
|
|
building your very own loop should be no problem.
|