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Posted- By: auto- faq script Archive- name: cb- radio- faq/part
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Posted-By: auto-faq script
Archive-name: cb-radio-faq/part4
Rec.radio.cb Frequently Asked Questions (Part 4: Technical Trivia)
------------------------------------------------------------------
[last modified 4/15/92]
Questions discussed in Part 4: (dates indicate last modification)
* What is single sideband? (1/92)
* Do I really get 120 channels on a radio with single sideband? (12/91)
* What are "linear amplifiers" and can I use them with a CB? (4/92)
* Who is "Skip" and why can I hear him all the way across the continent? (3/92)
* How can I reduce engine noise on my mobile CB? (3/92)
* Where can I learn more about radio? (3/92)
--Rec.radio.cb Frequently-asked Questions-----------------------------Part 4--
* What is single sideband?
--------------------------
To understand single-sideband (SSB), one must first have a picture of what's
going on in a normal AM transmission. For example, on Channel 21 (in the U.S.
and Australia), a "carrier" is transmitted at 27.215 MHz. Your voice (or
whatever you're transmitting) is used to change (modulate) the height (amp-
litude) of the signal so that it can be reconstructed as your voice on the
receiving end.
A side effect of this modulation process is present on some of the closest
frequencies adjacent to the carrier, above and below it. These are called
sidebands. Normal AM transmissions include the modulated carrier and the
upper and lower sidebands.
However, there is enough information in either sideband to reconstruct the
original signal. Therefore, radios which can use single sideband are able
to filter out the carrier and the opposite sideband before transmitting them,
leaving either the upper sideband (USB) or lower sideband (LSB), as selected
by the CB operator. Obviously, for a conversation to take place, both the
transmitting and receiving radios have to be on the same channel and sideband
setting (i.e. Channel 25 LSB.) Some additional tuning will be necessary with
a "clarifier" control.
Assuming proper filtering within the radios, it should be possible for sep-
arate conversations to occur on the upper and lower sidebands of a given
channel without interfering with each other.
* Do I really get 120 channels on a radio with single sideband?
---------------------------------------------------------------
[This applies to the U.S., Australia, and any other countries that use AM and
SSB.]
Yes and no. SSB does give you 120 different communications paths (40 AM, 40
USB, and 40 LSB) but they are not free of interference from each other. The
upper and lower sidebands will interfere with the AM channel and vice versa.
With some radios, even the opposite sidebands can interfere with each other.
For example, (and this works for any channel - just fill in a different
number from 1 to 40) Channel 17 has an AM channel that every radio can use.
Radios with SSB will also be able to use the upper and lower sidebands.
However, within a few miles of each other, you can't effectively use the
AM channel if people are talking on either sideband. Though not as strong,
users of sideband will know if someone is using the AM channel but the higher
legal power settings on sideband give them an advantage.
* What are "linear amplifiers" and can I use them with a CB?
------------------------------------------------------------
In the US, UK, Australia, Japan, France, and Germany, it is ILLEGAL to use a
linear amplifier on Citizen's Band radio. (Sorry, we don't yet have info on
Canada or other countries.) In the US and Australia, the maximum amount of
power that is rated for CB is 4 watts on AM and 12 watts PEP (peak envelope
power) on sideband. In Japan, the Citizen's Radio band is limited to 1/2 watt
on AM and the Personal Radio band is limited to 5 watts on FM. In France and
Germany, CBs can transmit up to 1 watt on AM and 4 watts on FM. For other
countries, if there is a power limit on CB transmissions, any amplifier that
boosts a radio's power beyond that point is obviously in violation.
Linear amplifiers allow a radio to transmit using much greater power than
is legal. They may range from 30 watts to above 500 watts. The problem is
that, in reality, you can't communicate with anyone you can't hear. But you
can easily interfere with people you can't hear if you use more than legal
power. So a good antenna is the best solution for getting better range out of
your CB.
The reaction to questions about linear amplifiers on this newsgroup will vary
with the country you are from. In countries where they are illegal, you will
probably find yourself severely flamed... In other countries, to avoid a
negative response, you may want to limit the distribution of the article to
your country or continent, as appropriate.
Besides, most black-market linears are low quality and will distort your
signal and interfere with consumer electronics or other radio services.
(Stray signals from a linear outside of the CB frequencies can bring a swift
response from whoever it interferes with.) It is far better to run a clean
radio with a clean signal into a high-quality coaxial feedline and antenna
tuned for low SWR.
Most radios are deliberately undertuned from the factory (to make sure that\
they are well within type-acceptance limits). Take it down to a 2-way radio
shop and get it a "full-legal maxout" to 4 Watts AM, 12 Watts SSB at 100%
modulation.
* Who is "Skip" and why can I hear him all the way across the continent?
------------------------------------------------------------------------
If you've heard the term "skip" before, it refers to a condition where a radio
signal bounces (actually, refracts, but that's getting technical) off the Iono-
sphere, a higher layer in the Earth's atmosphere. For all practical purposes,
this only happens during daytime hours because it's the Sun's radiation that
charges up the Ionosphere and allows the signals to be bounced back to the
surface. (Some residual "charge" remains in the upper atmosphere during the
night but it is normally not as noticeable.)
It's called "skip" because, in terms of where a given signal can be heard, it
skips over a long distance before reaching the surface again. The distances
vary but they can easily be over 1000 miles (1600km). For example, in Califor-
nia, under skip conditions it's possible to hear signals from Alabama or other
eastern states.
However, skip is not a reliable means of communication. Besides, it may also
be illegal to intentionally use skip for communications. For example, in the
US you may not use CB for communications beyond 150 miles. (150mi = 240km)
CB is intended for local communications.
Skip signals are normally weak, though enough of them from different places
can add up to a lot of noise. If you hear a strong signal, the person is prob-
ably using a linear amplifier which, as discussed above, is illegal in at least
the US, UK, Japan, France, and Germany and frowned-upon almost everywhere.
Though there are some conditions when a legally-powered station with a special
antenna can create a strong signal via skip, these are rare.
Most CB users' experiences with skip is to turn their squelch higher to cut out
the heavy background noise during the daytime. This means that anyone you
talk to (locally) will have to be close by to send a strong enough signal to
overcome the background noise and your squelch setting. There's really no way
around this so get used to it.
* How can I reduce engine noise on my mobile CB?
------------------------------------------------
If you have problems with engine noise in a mobile installation, there are
several things you can do to reduce or eliminate the problem.
- Make sure you've completely installed your CB and the antenna is grounded
properly. Check the amount of noise with and without the antenna connected.
If the noise is greater when the antenna is connected, the source is probably
the ignition system. If the noise remains unchanged after disconnecting the
antenna, the source is probably in the CB's power supply.
- For ignition noise, there are several parts of the vehicle that can act like
RF noise transmitters if not properly grounded. Check (and connect to
ground on the chassis/frame, if necessary) the engine block, hood, muffler,
and exhaust pipe.
- Ignition noise can be further reduced by installing "magenetic suppression"
spark plug cables. (These are significantly more expensive than normal
spark plug cables.)
- For power supply noise, check if you have a capacitor attached to your
ignition coil, alternator, and distributor. These can act as filters right
at the source of the noise.
- Ensure that your antenna feedline and power lines are as far as possible from
any of the noise sources mentioned above, especially the engine block.
- A shielded power cable can help as well. Make sure it is of sufficient
guage to handle your radio's power consumption and that the shielding has a
good, strong connection to ground on the vehicle chassis or frame.
- Electronics stores such as Radio Shack sell cheap power filters for noise-
reduction purposes. You get what you pay for, though - most comments on this
newsgroup are that they are not as effective as the other measures above.
Of course, the best performance can be obtained from a more expensive radio
with better noise suppression circuits. These measures, however, will improve
the engine noise levels for nearly any mobile radio.
* Where can I learn more about radio?
-------------------------------------
If you like what you see here and want to learn more about the technical
aspects of radio, you may be interested in getting an amateur ("ham") radio
license. These licenses are available in almost every country in the world,
including all of the countries that have CB.
Amateur radio is very different from CB. This is neither good nor bad - they
have different purposes. With the training that a licensed amateur radio
operator is expected to have, much more powerful transmitters are allowed -
but the operator should know when that power is not necessary and refrain from
it. Amateurs have no limits on the range of their communications (at least,
by law or treaty) so it is possible to make distant "DX" contacts using atmos-
pheric skip, satellites, or even bouncing signals off the Moon. Again,
training and the willingness to cooperate (to avoid interference with others)
are necessary for obtaining and using these privileges.
In what may become a trend in other countries, the US has authorized a new
amateur radio "no-code" technician license which requires only a written
test (no morse code). However, the test is challenging enough to necessitate
several weeks of study so don't go unprepared. More information can be found
in other newsgroups such as rec.radio.amateur.misc.
Even if you don't want the license, books intended for amateur radio operators
make available a wealth of information on understanding transmitters, recei-
vers, antennas, modulation, signal propagation, electronics, and many other
topics which also apply to CB.
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