Comms Primer Part 2

 Continued from Part 1

More from Jim- first, I hate to break this to you, but THE EARTH IS ROUND. No, I’m
not kidding -- it really is spherical despite what they told you in school or at church.
Ok, now that you are past that, you should visualize in your head that radio waves
travel in a straight line. Since the earth curves, it is not possible to talk over distances
of over about 20 miles without "help". this "help" can come in a multitude of ways,
and is somewhat dependent on the height of the transmitter and receiver, the gain of
their respective antennae, the frequency of transmission, the weather and other
atmospheric conditions, the sun cycle, hams around you erecting things called
repeaters, and a few dozen other things. Nevertheless, the key point here is that the
further you are away from each other, the more likely it is that the curvature of the
6 of 14 earth is going to be the limiting factor. Always remember that without "help", radio
communications are "line of sight". HF, or high frequency (roughly defined as everything below about 30MHz [10 meters wavelength]) signals can bounce off of a charged belt (called the ionosphere) which
completely envelopes the earth. HF thus can communicate over long distances by
using one or more "bounces" -- you may have heard the CB term "skip". With just a
few dozen watts, and a proper antenna, it's possible for you to talk (or more likely
communicate using mores) with a station 2000 miles away. Interestingly, it is
sometimes difficult to talk to nearby stations that are "under the skip", that is they are
too close to you to hear the reflected wave. One primary disadvantage of HF
communications is that the antenna has to be physically long. Nevertheless, HF can be
a valuable asset in emergency communications -- like it was during hurricane Katrina
when everything else was tits up. In general, when you think of HF you should think of
long distance comms. With some exceptions, most HF rigs are designed for desktop
use and the supporting equipment (power supplies, antenna tuner, etc) is heavy and
not so portable. But there is a class of mobile and low power HF rigs which allow you
to talk over great distances with just a few pounds of equipment. …[DWB States: Updated equipment is now much smaller and lighter.  Mobile and Mobile/Base rigs are now the size of some older CB radios, and come with the same power and efficency as the desktop base stations, and a lot of the same whistles and bells as their big brothers]… the primary limitation of VHF/UHF is the short range imposed by the curvature of
the earth combined with the fact that VHF/UHF signals do not bounce off of the
ionosphere (in case you were wondering, they pass right through it). Of course it helps
greatly with VHF/UHF to be up as high as possible, as this gives more "line of sight"
distance -- the same way you can see much farther when atop a tall building. But
that's not always practical. e.g., I live right at sea level -- no kidding. How can I, the
low lying ham, communicate with any distance using VHF or UHF? Am I stuck trying to
use HF for comms more than a few miles?

Enter the "repeater". Simply put, a repeater is an unattended radio advantageously
located on a hill or with the antenna high up on a tower. The purpose of the repeater
is to retransmit your signal in real time. It does this by listening on one frequency,
called the input, and simultaneously transmitting the input audio on a second
frequency, called the output. Accordingly, my radio would be set to transmit on the
repeater's input frequency, and listen on the repeater's output frequency. (The
difference is known as the "offset".) All that is required from a radio implementation
standpoint is a little bit of frequency agility -- when you press the transmit button,
your radio tunes it's transmitter to the required new frequency. When you unkey, it
changes back. All this happens in milliseconds and without your involvement save for
some initial settings. The beauty of this set up is that with a low power HT (typ, 0.5W
to 5W) you can talk for perhaps hundreds of miles! The repeater provides the "help"
for VHF and UHF, just like the ionosphere provided the "help" for HF.
There are 3 basic ham licenses and one sort of sub group (at this time). There is
Technician, General and Extra. The sub group is this: if you have passed the 5 word per
minute Morse code test, then as a Technician, you have some limited access to the HF
bands.

A technician license allows the operator to use any UHF or VHF frequency with any mode.
A general licensee is allowed to use almost all of the HF bands (all modes) as well as all of
the UHF and VHF. Some small sections of each HF band are reserved for extra and
advanced licensees.

An extra class license allows the full use of all amateur bands with all modes.
For the actual US code regulations that govern amateur radio, see
http://www.access.gpo.gov/nara/cfr/waisidx_05/47cfr97_05.html which lists ‘PART 97--
AMATEUR RADIO SERVICE’.


What are these modes?
Modes are basically the way we modify a signal to carry information. The most common
modes that everyone knows about are AM (amplitude modulation) and FM (frequency
modulation). Others common modes include SSB, CW, and digital.
SSB is a method of amplitude modulation (AM) where the carrier wave and one of the side
bands (there are 2 – upper and lower) is suppressed. This means that the signal you are
sending out takes up smaller bandwidth and has higher power (relatively) than a
traditional AM signal.
CW is short for continuous wave or the method for sending Morse code. This is the
traditional method of communication with a very small bandwidth and is useful when
communications conditions are poor.
Digital modes are various and include RTTY (radio teletype), PSK31, PSK63, Hellschreiber,
packet modes, sstv, fstv and many others. There are new digital modes being invented
everyday. In general these are text based modes that use some sort of computer
interface to a radio. This is not at all internet based. It is simply a computer interfacing
with a radio to send a signal. SSTV and FSTV are video modes with allow the
transmission of pictures. There is also D-STAR which is a new digital mode based on the
Japanese protocal. It is designed for simplex and repeater use on 2m, 70cm, 1.2 GHz.
There is also a hybrid mode of communication which does interface with the internet. The
main types are IRLP, echolink and Wires. These are simply interfaces that allow radio
users to send their signals to other users, radios or repeaters in other parts of the world.
However, this relies on an internet connection and should not be relied on in emergency
situations.
What sort of equipment is available? (just a small sampling)
HT (handi-talkies/handheld transceivers) – available for VHF/UHF, single band/dual
band/triband/quad band. Transmit powers range from 300milliwatts (.3 watts) to 6 watts
in general. Pros - Extremely portable, light weight, low power consumption, available with
extremely wide receive frequencies (to pick up broadcast, shortwave, public service,
aircraft and other services). Cons – relatively low transmit power with respect to other
types of equipment. Stock rubber duck antennas are generally very poor radiators.
Mobile/Portable rigs – These can be used in an automobile, out in the field or in a
backpack or on a bench as a base station. They are smaller an lighter than dedicated
base stations, but are usually more suited for emergency communications because they
may be more robust and easier to transport in different disaster situations. They may be
low power (QRP) with only 5, 10 or 20 watts or up to about 100 watts full power. These
are mainly available for HF, HF+ VHF/UHF, VHF only, UHF only or VHF/UHF.
Base stations – These typically have the most features of all radios including digital
processing, filters to improve signals as well as other enhancements. Power on these rigs
may go up to about 250 watts without the use of an external amplifier. These are mainly
available for HF, or HF+ VHF/UHF.
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Antenna – Basically the most important part of the radio system is the antenna. Without
a good antenna system, your transmitter is just going to heat up the ‘antenna’ like a
resistor.
Antenna length, is inversely proportional to the frequency being transmitted:
CB= 27 MHz
MURS= 151 MHz
GMRS= 462 MHz
So the CB antenna will be longest and the GMRS will be the shortest.
Antennas can come in almost any form and with a good antenna tuner, almost anything
can be made to radiate (including a barbed wire fence or your aluminum house gutters).
Some of the basic forms are wire dipoles,verticals and beams. Antennas are usually
defined by the wavelength at which the antenna is designed to operate. The most basic is
a ½ wavelength dipole fed with ladderline or coax at the center. If it is mounted low to
the ground it is good for NVIS. Up higher at ¼ wavelength up, then better for DX
(distance comms). Another basic form is a ¼ wavelength vertical. This is basically ½ of a
½ wave dipole. It is fed at the end and relies on a reflection from a ground plane or
grounding radials. An ht antenna is sometimes a ¼ wave vertical without a good ground
plane. Beams are directional antennas which allow the main power from the antenna to
be directed in one directions. This lowers the amount of noise coming from other
directions and increases the power toward your intended target. Beams can be in the
form of a Yagi or a quad or several other types. A promising option (that I haven’t tried)
for emergency comms is a ‘hamstick dipole’. See:
http://www.varaces.org/techrefs/HamstickDipoleFactSheet.pdf
Antenna tuners – these are devices that allow antennas that are not designed for a
specified frequency to operate on that frequency. There are automatic and manual and
they work by matching the impedance (by altering the capacitance and inductance) of the
antenna and feedline to the transceiver.
Repeaters – not something that most hams will ‘buy’ or build, but may help pay for the
upkeep and use of the repeaters. Most are open to use by any licensed ham with priority
given to any emergency traffic. Most repeaters are owned and maintained by local ham
clubs.
Jim- the disadvantage of repeaters is simple: in order for the repeater to work, you
need power and the antenna has to be upright. These are not likely conditions in areas
overcome by, for example, a category 4 or 5 hurricane. while tons of lead acid
batteries may delay the inevitable, there is a finite amount of no-AC-power operation
time for any repeater unless expensive measures have been taken (e.g. a diesel
genset and good sized fuel tank, as you would find mounted behind a police station).
Nevertheless, hams are resourceful people and generally fixing the repeaters is an
immediate priority in disaster areas. Moreover, all the equipment necessary for a
repeater can be carried in the back of a Tacoma, with room to spare. So if a makeshift
antenna can be erected on the mountainside, a substitute repeater can be up and
running in a few hours to replace the one crushed by the flying oak tree.
One way to look at a ham repeater is as an analog of a cell site. Having many cell sites
make it possible for your low power cell phone to communicate anywhere the global
phone network reaches. Similarly, hams link repeaters using point-to-point RF, the
phone network, or these days using the internet. all of these methods allow greater
"reach" from your low power HT. with the exception of point-to-point RF, the other
methods require public infrastructure that may or may not be available when the
SHTF..

Survival Communications Primer
By Vector_Joe
ModernSurvivalOnline.com