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Subject: rec.radio.cb Frequently Asked Questions (Part 4 of 4) Newsgroups: rec.radio.cb,rec.radio.info,rec.answers,news.answers From: ikluft@oes.amdahl.com (Ian Kluft) Date: Tue, 22 Nov 1994 06:11:03 GMT Posted-By: auto-faq 3.1.1.4 Archive-name: radio/cb-faq/part4 Revision: 1.1 08/21/94 21:58:25 Rec.radio.cb Frequently Asked Questions (Part 4: Technical Trivia) ------------------------------------------------------------------ THIS ARTICLE IS INTENDED TO BE A FREE RESOURCE FOR THE BENEFIT OF USENET READERS. YOU MAY COPY AND REDISTRIBUTE IT UNDER THE CONDITION THAT THIS MESSAGE AND CREDIT TO THE EDITORS AND CONTRIBUTORS ARE INCLUDED WITHOUT MODIFICATION. Material from the FAQ may be used to answer any questions. Corrections and updates are welcome. Questions discussed in Part 4: (dates indicate last modification) * What is single sideband? (11/92) * 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? (7/93) * Who is "Skip" and why can I hear him all the way across the continent? (7/93) * How can I reduce engine noise on my mobile CB? (3/92) * What is GMRS? (9/93) * 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 (amplitude modulation) transmission. For example, on Channel 21 (in North America, Australia, and Europe), a "carrier" is transmit- ted at 27.215 MHz. Your voice (or whatever you're transmitting) is used to change (modulate) the height (amplitude) of the signal so that it can be reconstructed as your voice on the receiving end. Actually, the amplitude of the carrier does not change. The addition (modulation) of another signal, like your voice, onto the carrier will increase the amplitude at other frequencies adjacent to the the carrier. A 300 Hz tone, for example, would add signals 300 Hz above and below the carrier. Every frequency component of your voice has the same additive effect. These modula- tion effects are the upper and lower sidebands of the transmitted signal. Normal AM transmissions include the carrier signal, the upper sideband (USB) and lower sideband (LSB). The difference in frequency from the bottom of the lower sideband to the top of the upper sideband is called the bandwidth of the signal; it will be twice as wide as the highest frequency modulated onto the carrier. (For poorly filtered radios, this can unintentionally interfere with adjacent channels if the bandwidth is too wide. Poor filtering becomes easily noticeable at higher power levels.) 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 or lower sideband, 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 USA, Australia, Canada, and any other countries that use AM and SSB on the same 40-channel band.] 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 USA, UK, Australia, Japan, France, Germany, Canada, New Zealand, and Russia it is ILLEGAL to use a linear amplifier on Citizen's Band radio. In the USA 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. And the list goes on... we have not yet heard of any country that allows more than 4 or 5 watts on AM/FM or 12 watts on SSB. 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 1000 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 the Sun's radiation 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 in the 11-meter band.) 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 often not a reliable means of communication. Besides, it may also be illegal to intentionally use skip for communications. For example, in the USA you may not use CB for communications beyond 150 miles. (150mi = 240km) CB is intended for local communications. However, in Russia, there is no distance restriction so skip is legal when conditions allow you to make a contact. 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 USA, 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 is a bigger problem with AM than with FM because FM has a "capture effect" where the stronger signal wins. 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. If you use AM, 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 "magnetic 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. * What is GMRS? --------------- [This applies only to the USA.] The General Mobile Radio Service is another personal radio band. Its frequency allocation is in the ultra-high frequency (UHF) spectrum, co-located with some other radio services in the range from 460-470 MHz. It requires a license but there is no examination. Though GMRS is not CB, it is listed here because it is intended for use by individuals. A single GMRS license also covers the members of the licensee's household. The license fee is US$35 for 5 years. frequencies (MHz) modulation description 460 - 470 FM General Mobile Radio Service (GMRS) regular channels (simplex or repeater outputs, repeater inputs are 5MHz higher) 462.550 462.575 462.600 462.625 462.650 462.675 462.700 462.725 simplex low-power channels 462.5625 462.5875 462.6125 462.6375 462.6625 462.6875 462.7125 31.0-31.3 GHz any mode microwave band now open for individuals to use, requires GMRS license GMRS is used for mobile-to-mobile or base-to-mobile communications but base-to-base communications are prohibited. One useful aspect of GMRS is that repeaters are allowed so that a mobile GMRS station can extend its range well beyond the limits of an ordinary 5-watt mobile transmitter. Only individuals may obtain GMRS licenses. Some businesses still have licenses from before the rules were changed in 1989 but no new licenses are being issued to "non-individuals." Recent changes in regulations have given GMRS licensees access to the micro- wave band at 31.0-31.3 GHz for both fixed and mobile use. The licensee must notify the FCC of each transmitter with a standard FCC form. For more information, you can obtain compressed text files via anonymous FTP at ftp.amdahl.com in the /pub/radio/other directory in the files called gmrs.intro.Z and gmrs.microwave.Z. If you don't have access to the Internet or need help with FTP, see the rec.radio.amateur.misc FAQ, Part 3. Another source of information is the Personal Radio Steering Group. A reader said they have lots of good information and are enthusiastic about GMRS. Personal Radio Steering Group PO Box 2851 Ann Arbor MI 48016 Phone: (313) MOBILE 3 BBS: (313) 995-2100 Compuserve: 73016.163 * 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. Japan, Sweden the USA and many other countries have authorized a new amateur radio "no-code" license which requires only a written test (no morse code). However, the test is challenging enough to necessitate several weeks of study (or a few days if you already have some familiarity with the subject) so don't go unprepared. More information, see the FAQ articles in the rec.radio.amateur.misc newsgroup. 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.