Posted-By: auto-faq 2.4ăArchive-name: radio/cb-faq/part4ăăRec.radio.cb Frequently Asked Questions (Part 4: Technical Trivia)ă------------------------------------------------------------------ă[last modified 11/21/92]ăăQuestions discussed in Part 4: (dates indicate last modification)ă* What is single sideband? (11/92)ă* Do I really get 120 channels on a radio with single sideband? (12/91)ă* What are "linear amplifiers" and can I use them with a CB? (4/92)ă* Who is "Skip" and why can I hear him all the way across the continent? (3/92)ă* How can I reduce engine noise on my mobile CB? (3/92)ă* What is GMRS? (6/92)ă* Where can I learn more about radio? (3/92)ăă--Rec.radio.cb Frequently-asked Questions-----------------------------Part 4--ăă* What is single sideband?ă--------------------------ăăTo understand single-sideband (SSB), one must first have a picture of what'săgoing on in a normal AM (amplitude modulation) transmission. For example, onăChannel 21 (in North America, Australia, and Europe), a "carrier" is transmit-ăted at 27.215 MHz. Your voice (or whatever you're transmitting) is used toăchange (modulate) the height (amplitude) of the signal so that it can beăreconstructed as your voice on the receiving end.ăăActually, the amplitude of the carrier does not change. The additionă(modulation) of another signal, like your voice, onto the carrier will increaseăthe amplitude at other frequencies adjacent to the the carrier. A 300 Hz tone,ăfor example, would add signals 300 Hz above and below the carrier. Everyăfrequency component of your voice has the same additive effect. These modula-ătion effects are the upper and lower sidebands of the transmitted signal.ăNormal AM transmissions include the carrier signal, the upper sideband (USB)ăand lower sideband (LSB). The difference in frequency from the bottom of theălower sideband to the top of the upper sideband is called the bandwidth of theăsignal; it will be twice as wide as the highest frequency modulated onto theăcarrier. (For poorly filtered radios, this can unintentionally interfere withăadjacent channels if the bandwidth is too wide. Poor filtering becomes easilyănoticeable at higher power levels.)ăăThere is enough information in either sideband to reconstruct the originalăsignal. Therefore, radios which can use single sideband are able to filterăout the carrier and the opposite sideband before transmitting them, leavingăeither the upper sideband or lower sideband, as selected by the CB operator.ăObviously, for a conversation to take place, both the transmitting andăreceiving radios have to be on the same channel and sideband setting (i.e.ăChannel 25 LSB.) Some additional tuning will be necessary with a "clarifier"ăcontrol.ăăAssuming proper filtering within the radios, it should be possible for sep-ăarate conversations to occur on the upper and lower sidebands of a givenăchannel without interfering with each other.ăăă* Do I really get 120 channels on a radio with single sideband?ă---------------------------------------------------------------ăă[This applies to the USA, Australia, Canada, and any other countries that useăAM and SSB on the same 40-channel band.]ăăYes and no. SSB does give you 120 different communications paths (40 AM, 40ăUSB, and 40 LSB) but they are not free of interference from each other. Theăupper and lower sidebands will interfere with the AM channel and vice versa.ăWith some radios, even the opposite sidebands can interfere with each other.ăăFor example, (and this works for any channel - just fill in a differentănumber from 1 to 40) Channel 17 has an AM channel that every radio can use.ăRadios with SSB will also be able to use the upper and lower sidebands.ăHowever, within a few miles of each other, you can't effectively use theăAM channel if people are talking on either sideband. Though not as strong,ăusers of sideband will know if someone is using the AM channel but the higherălegal power settings on sideband give them an advantage.ăăă* What are "linear amplifiers" and can I use them with a CB?ă------------------------------------------------------------ăăIn the USA, UK, Australia, Japan, France, and Germany, it is ILLEGAL to use aălinear amplifier on Citizen's Band radio. (Sorry, we don't yet have info onăCanada or other countries.) In the USA and Australia, the maximum amount ofăpower that is rated for CB is 4 watts on AM and 12 watts PEP (peak envelopeăpower) on sideband. In Japan, the Citizen's Radio band is limited to 1/2 wattăon AM and the Personal Radio band is limited to 5 watts on FM. In France andăGermany, CBs can transmit up to 1 watt on AM and 4 watts on FM. For otherăcountries, if there is a power limit on CB transmissions, any amplifier thatăboosts a radio's power beyond that point is obviously in violation.ăăLinear amplifiers allow a radio to transmit using much greater power thanăis legal. They may range from 30 watts to above 500 watts. The problem isăthat, in reality, you can't communicate with anyone you can't hear. But youăcan easily interfere with people you can't hear if you use more than legalăpower. So a good antenna is the best solution for getting better range out ofăyour CB.ăăThe reaction to questions about linear amplifiers on this newsgroup will varyăwith the country you are from. In countries where they are illegal, you willăprobably find yourself severely flamed... In other countries, to avoid aănegative response, you may want to limit the distribution of the article toăyour country or continent, as appropriate.ăăBesides, most black-market linears are low quality and will distort yourăsignal and interfere with consumer electronics or other radio services.ă(Stray signals from a linear outside of the CB frequencies can bring a swiftăresponse from whoever it interferes with.) It is far better to run a cleanăradio with a clean signal into a high-quality coaxial feedline and antennaătuned for low SWR.ăăMost radios are deliberately undertuned from the factory (to make sure that\ăthey are well within type-acceptance limits). Take it down to a 2-way radioăshop and get it a "full-legal maxout" to 4 Watts AM, 12 Watts SSB at 100%ămodulation.ăăă* Who is "Skip" and why can I hear him all the way across the continent?ă------------------------------------------------------------------------ăăIf you've heard the term "skip" before, it refers to a condition where a radioăsignal bounces (actually, refracts, but that's getting technical) off the Iono-ăsphere, a higher layer in the Earth's atmosphere. For all practical purposes,ăthis only happens during daytime hours because it's the Sun's radiation thatăcharges up the Ionosphere and allows the signals to be bounced back to theăsurface. (Some residual "charge" remains in the upper atmosphere during theănight but it is normally not as noticeable.)ăăIt's called "skip" because, in terms of where a given signal can be heard, ităskips over a long distance before reaching the surface again. The distancesăvary but they can easily be over 1000 miles (1600km). For example, in Califor-ănia, under skip conditions it's possible to hear signals from Alabama or otherăeastern states.ăăHowever, skip is not a reliable means of communication. Besides, it may alsoăbe illegal to intentionally use skip for communications. For example, in theăUSA you may not use CB for communications beyond 150 miles. (150mi = 240km)ăCB is intended for local communications.ăăSkip signals are normally weak, though enough of them from different placesăcan add up to a lot of noise. If you hear a strong signal, the person is prob-ăably using a linear amplifier which, as discussed above, is illegal in at leastăthe USA, UK, Japan, France, and Germany and frowned-upon almost everywhere.ăThough there are some conditions when a legally-powered station with a specialăantenna can create a strong signal via skip, these are rare.ăăMost CB users' experiences with skip is to turn their squelch higher to cut outăthe heavy background noise during the daytime. This means that anyone youătalk to (locally) will have to be close by to send a strong enough signal toăovercome the background noise and your squelch setting. There's really no wayăaround this so get used to it.ăăă* How can I reduce engine noise on my mobile CB?ă------------------------------------------------ăăIf you have problems with engine noise in a mobile installation, there areăseveral things you can do to reduce or eliminate the problem.ă- Make sure you've completely installed your CB and the antenna is groundedă properly. Check the amount of noise with and without the antenna connected.ă If the noise is greater when the antenna is connected, the source is probablyă the ignition system. If the noise remains unchanged after disconnecting theă antenna, the source is probably in the CB's power supply.ă- For ignition noise, there are several parts of the vehicle that can act likeă RF noise transmitters if not properly grounded. Check (and connect toă ground on the chassis/frame, if necessary) the engine block, hood, muffler,ă and exhaust pipe.ă- Ignition noise can be further reduced by installing "magnetic suppression"ă spark plug cables. (These are significantly more expensive than normală spark plug cables.)ă- For power supply noise, check if you have a capacitor attached to youră ignition coil, alternator, and distributor. These can act as filters rightă at the source of the noise.ă- Ensure that your antenna feedline and power lines are as far as possible fromă any of the noise sources mentioned above, especially the engine block.ă- A shielded power cable can help as well. Make sure it is of sufficientă guage to handle your radio's power consumption and that the shielding has aă good, strong connection to ground on the vehicle chassis or frame.ă- Electronics stores such as Radio Shack sell cheap power filters for noise-ă reduction purposes. You get what you pay for, though - most comments on thisă newsgroup are that they are not as effective as the other measures above.ăOf course, the best performance can be obtained from a more expensive radioăwith better noise suppression circuits. These measures, however, will improveăthe engine noise levels for nearly any mobile radio.ăăă* What is GMRS?ă---------------ăă[This applies only to the USA.]ăăThe General Mobile Radio Service is another personal radio band. Its frequencyăallocation is in the ultra-high frequency (UHF) spectrum, co-located with someăother radio services in the range from 460-470 MHz. It requires a license butăthere is no examination. Though GMRS is not CB, it is listed here because ităis intended for use by individuals. A single GMRS license also covers theămembers of the licensee's household.ăăGMRS is used for mobile-to-mobile or base-to-mobile communications butăbase-to-base communications are prohibited. One useful aspect of GMRS is thatărepeaters are allowed so that a mobile GMRS station can extend its range wellăbeyond the limits of an ordinary 5-watt mobile transmitter. ăăOnly individuals may obtain GMRS licenses. Some businesses still have licensesăfrom before the rules were changed in 1989 but no new licenses are being issuedăto "non-individuals."ăăRecent changes in regulations have given GMRS licensees access to the micro-ăwave band at 31.0-31.3 GHz for both fixed and mobile use. The licensee mustănotify the FCC of each transmitter with a standard FCC form.ăăFor more information, you can obtain compressed text files via anonymous FTPăat charon.amdahl.com in the /pub/radio/other directory in the files calledăgmrs.intro.Z and gmrs.microwave.Z. If you don't have access to the Internetăor need help with FTP, see the rec.radio.amateur.misc FAQ, Part 3.ăăă* Where can I learn more about radio?ă-------------------------------------ăăIf you like what you see here and want to learn more about the technicalăaspects of radio, you may be interested in getting an amateur ("ham") radioălicense. These licenses are available in almost every country in the world,ăincluding all of the countries that have CB.ăăAmateur radio is very different from CB. This is neither good nor bad - theyăhave different purposes. With the training that a licensed amateur radioăoperator is expected to have, much more powerful transmitters are allowed -ăbut the operator should know when that power is not necessary and refrain fromăit. Amateurs have no limits on the range of their communications (at least,ăby law or treaty) so it is possible to make distant "DX" contacts using atmos-ăpheric skip, satellites, or even bouncing signals off the Moon. Again,ătraining and the willingness to cooperate (to avoid interference with others)ăare necessary for obtaining and using these privileges.ăăIn what may become a trend in other countries, Sweden and the USA have author-ăized a new amateur radio "no-code" license which requires only a written testă(no morse code). However, the test is challenging enough to necessitateăseveral weeks of study so don't go unprepared. More information, see the FAQăarticles in the rec.radio.amateur.misc newsgroup.ăăEven if you don't want the license, books intended for amateur radio operatorsămake available a wealth of information on understanding transmitters, recei-ăvers, antennas, modulation, signal propagation, electronics, and many otherătopics which also apply to CB.ă