292 Radio Shop FireCommunications Network Frequently Asked Questions ---------------------------------------------------------------------------------------------- what is a good height for your base/fixed antenna? best results will be had when the feedpoint (the point where the feedline connects to the antenna) is any number of wavelengths above ground. 37', 74' and 111' are all good numbers for both local AND skywave (skip....SKywave Ionospheric Proragation) conditions. maximum radiated power always takes place when the reflected wave from the ground immediately underneath the antenna arrives back at the feedpoint to re-inforce (in-phase with) the incident signal. feedpoint heights between these figures will be less than optimal. ---------------------------------------------------------------------------------------------- Choosing a CB antenna is fairly easy once you have sorted through a few basic rules. This information should help. Basic Rules: 1 Top loaded (helical wire-wounds) antennas perform better than center loaded antennas, center loaded antennas perform better than base loaded antennas. 2 Taller antennas perform better than shorter antennas. 3 Antennas mounted higher perform better than if mounted lower. 4 Single antennas perform better when mounted as close to center as possible on metal vehicles. 5 Base loaded antennas cannot be co-phased with satisfactory results. 6 Fiberglass and aluminum vehicles, or those with little available ground plane, need co-phased or "no ground plane" antennas systems. 7 The physical length of the antenna should be determined by the likelihood of repetitive hitting of overhead objects and/or the type and strength of the antenna mount itself. 8 At least two-thirds of the antenna should be above a vehicles roof line for optimum performance. 9 If the antenna/mount combination is available in a kit form, you will normally save money by purchasing it as a kit. 10 The bandwidth of the antenna should always meet or exceed the needs of a 40 channel CB. ---------------------------------------------------------------------------------------------- 63 Things Every CB'er Should Know....http://www.firestik.com/Tech_Docs/63Things.htm ---------------------------------------------------------------------------------------------- Several times a year we get tech calls from installers who place dual antennas on their vehicle and run RG-58 or RG-8X from each antenna to a T-connector at the back of their radio only to find that the system "doesn't get out". You should not use 50 ohm coax on a ground plane dependent dual antenna installation ... it MUST be 75 ohm RG-59 type coax. These flawed installs can be misleading because SWR tests can show exceptionally low SWR, making the installer think that all is well. However, the impedence of the antenna system does not match the requirements of the radio and therefore, the output power of the radio is greatly reduced. In several tests, we found that a 4 watt radio would only generate 1.75 watts of output power which is the equivalent of having an SWR reading that exceeds 6.0:1. Q The antenna I bought claims to be pre-tuned. Do I need to do anything? A Yes. You must tune your antenna to your vehicle. The antenna is pre-tuned on a test range to make certain it is within the general frequency specifications. It will be somewhat different on your vehicle because of the difference in the ground plane and surroundings. Always check your antenna ... even if you move it from one location on your vehicle to another. Q Can I use any kind of wire to hook my antenna to my radio? A No! For single antenna installations we recommend RG-8M/X type coaxial cable. If you are running dual antennas (co-phased) you must use RG-59 A/U type coax. Make sure you buy the best cable too. We see numerous problems caused by low grade coax. Don't cut corners when it comes to coax. Q Is the length of the coax cable important? A We find that it is very important ... especially with high performance top-loaded antennas. Your safest bet is to use 18 feet (5.5 meters) coaxial leads on all of your CB installations. in any situation where the antenna swr is anything other than 1:1 at resonance changing the length of the feedline will affect the swr. Q I only needed 9 feet of coax to go from my radio to my antenna. How should I handle the excess? A What ever you do, do not roll it into a small convenient coil. It will become an RF choke. If you cannot let it lie loose under a seat or in a headliner, wrap it into a yarn-like skein of about 12 to 16 inches, put a wire tie in the center and tuck it under your dash, seat, etc. Q My system has very high SWR, but I only talk very short distances. Since distance isn't important, should I be concerned about high SWR? A Absolutely! High SWR will limit distance and may cause serious damage to your transmitter. The time spent tuning your antenna is time well spent. Don't take the chance. Q What is are the most common errors you find on CB installations? A In order of most to least common, 1) antenna not tuned to vehicle, 2) mounting locations chosen for convenience or appearance versus effectiveness, 3) coax cable ... low quality, worn out, wrong length, or severely pinched, 4) standard antennas used on vehicle with no ground plane instead of special no-ground-plane system. Q There are a lot of different antennas available for CB. Are some better than others? A Without a doubt, top loaded antennas are better than center loaded antennas, and center loaded antenna are better than base loaded antennas. Also, within each style, the taller the antenna the better it will generally perform. Q How important is it to have the antenna mount grounded to the vehicle? A Unless you are using a no-ground plane system, it is extremely important. Ungrounded mounts will usually cause SWR to be high across all channels. Q I am using a Firestik no-ground plane (NGP) antenna system and the mirror arm I mounted it to is grounded to the vehicle. Do I need to insulate it? A No! We already took care of that with the design. The NGP systems will operate the same regardless of whether or not the mount is grounded. Q I have a couple of different antennas and I've noticed that there are times when one works better over long distances than the other. However, it isn't consistent, Why is this happening? A Every antenna design has a different angle of radiation and it stays pretty constant unless the antenna is moved. What keeps changing is the ionosphere height. Your signal is bouncing off of the ionosphere and the varying height of it changes the angle at which the signal bounces back to earth. You have no control over this. An antenna might work long distances on one day, and not the next. ----------------------------------------------------------------------------------------------