Two Letters Re: Some Transceiver Antenna Questions

A few pieces of additional information about antennas: Quarter wave antennas are traditionally used for portable radios as they match the radio’s output impedance of 50 Ohms. By matching this impedance with an antenna of 50 Ohms you get maximum energy transfer. A 1/2 wavelength long antenna (twice as long as a 1/4 wave) has an impedance of several thousand Ohms and is not a suitable radiator unless you add some sort of impedance matching between the radio or the antenna. This can be in the form of a coil and capacitor or an antenna tuner. One advantage a 1/2 wave antenna (with a matching section) has is that it is less reliant upon a metal ground plane to function efficiently. VHF marine antennas are almost always 1/2 wave antennas.
Now, this is not to be confused with a 1/2 wave dipole which is really two 1/4 wave elements attached to each side of the coax (impedance = 75 Ohms and usually close enough to 50 Ohm to not create a problem). If mounted vertically, the lower element attached to the shield of the coax acts as the ground plane portion of the antenna. In fact, by adding a couple of additional elements to the shield side and positioning them at a 45 degree angle, this creates a 1/4 wave ground plane that has an impedance of 50 Ohms.
Sometimes you can get over an impedance mismatch by simply using a longer antenna with more capture area thereby delivering better reception. When transmitting though you should keep the antenna close to 50 Ohms to avoid damage to the radio’s transistor final amplifiers.
For more information that you could digest at one sitting, see the links at this site. — Rob at MURS radios


Concerning the letter from “SF in Hawaii” on the topic of “Some Transceiver Antenna Questions”: Another factor in antenna selection is the impedance of the antenna at it’s resonate frequency. Almost all 2-way radios are designed for use with a 50-Ohm antenna system. Select the correct 50-Ohm impedance coaxial cable to connect your radio to an external antenna. Typical 50-Ohm coaxial cables are RG-58, RG-8, and RG-213. Do not use 75-Ohm coaxial cables designed for regular TV or cable television (CATV) systems! Typical 75-Ohm coaxial cables to avoid for 2-way radio use are RG-59 and RG-6. Special co-phasing harnesses made with RG-59 coaxial cable are used in special applications, such as dual CB antennas on a tractor-trailer truck, so that the resulting impedance between the two antennas is 50-Ohms at the connection to the CB radio. But for a single CB antenna, you need to stick with 50-Ohm coaxial cable.

Then there is the antenna itself. The 5/8-wavelength, 1/2-wavelength, and 1/4-wavelength antenna design tends to have an impedance close to 50-Ohms. A full-wavelength or other fraction thereof antenna is no where near 50-Ohms. That is why you never hear any other type of antenna mentioned. Because the radio is designed for a 50-Ohm antenna system, 50-Ohm coaxial cable is used along with either a 5/8, 1/2, or 1/4-wavelength antenna. For the maximum radiated signal, all three components (radio, coaxial cable, and antenna) must be near the same 50-Ohms of impedance. An impedance miss-match results in wasted power and possible damage to the transmitter section of the 2-way radio due to reflected transmitter power coming back down the coax from the antenna and going back into the radio.
Your correspondent in Israel may not be familiar with the American Citizen’s Band (CB) radio service. Various countries have created similar “CB” radio services, but the allocated frequencies can vary depending on each country. The CB radio band in the US is a range of High Frequency (HF) radio frequencies that lie between one of the shortwave broadcast bands and the Amateur Radio Service 10-Meter band. These frequencies are divided into 40 channels, separated by 10 kHz steps, from 26.965 to 27.405 MHz. The term “Meters” is a measurement of the wavelength of a radio signal at a given frequency. The 27 MHz CB band is 11-meters. The 28 MHz Amateur Radio Service (ham) band is 10-meters.

The length of an antenna is directly proportional to the radio wavelength used. The shorter the wavelength, the shorter the antenna. Convert 11-meters to feet and you end up with one wavelength being about 36-feet long at the 27 MHz CB frequencies. This is why a 1?2-wave CB base station antenna is typically 18-feet long, and a 1?4-wave mobile whip antenna is 9-feet long. The measurements of frequency, wavelength, and antenna length are all interrelated.

A general rule of thumb for mobile 2-Way radios is not to use ridiculously short antennas and not to mount them right next to the car body! A transmitting antenna needs to be free and clear of obstructions in order to radiate a signal effectively. As previously mentioned, a traditional 1?4-wave whip antenna for the CB band is 9-feet long (102-inches, plus a 6-inch shock spring). It is naturally resonant on the 27 MHz frequencies used by CB radios and contains no loading coils. But at highway speeds the long whip antenna tends to lean far back due to wind resistance, reducing the effective range of the signal being transmitted. Various CB antenna designs utilize “loading coils” to reduce the physical length of the antenna, while maintaining the equivalent electrical length of the 1/4-wave 9-foot whip. These designs are a compromise, since it is the whip portion of the antenna that radiates most of the signal – not the loading coil. The shorter antennas do a better job of staying vertically upright at highway speeds. The performance of a 60-inch CB antenna with a base loading coil is usually an acceptable compromise from the far more awkward 9-foot whip antenna. A variety of 4 to 7-foot long 5/8-wave fiberglass antennas with a wire “wrap” are also available and provide another good compromise over the full size whip antenna. Their thick fiberglass core does a better job of keeping the antenna vertical at highway speeds. The better ones have a “tunable tip” feature that allows the antenna to be fine tuned without having to use a hacksaw to cut off excess length. (Plus, once you cut it off – you can’t put it back!)

Whatever you do, don’t buy a cheap 19-inch long magnetic-mount CB antenna and expect it to equal the performance of the full size 9-foot whip! Those antennas have so much of the antenna length replaced by a loading coil that their effective range is usually measured in yards instead of miles. Those who prefer the performance of the 9-foot whip antenna for CB radio sometimes use fishing line tied between the antenna and the vehicle body to hold the antenna in the vertical position when driving down the highway. The appropriate strength or “test” of fishing line will keep the whip upright while going down the road, but still break if the antenna strikes an overhead obstruction such as a low tree branch. Quality CB antenna brands include K-40, Wilson, and Firestik.

A base station directional antenna (or “beam” antenna) such as the Yagi or the Log Periodic design not only needs to be correctly mounted with the correct polarization – it also has to be pointed at the direction you want to talk. This requires an electric antenna rotator with sufficient rating to handle the size of your antenna (TV antenna rotators are usually unable to handle a large directional “beam” antenna). If you only need to communicate between two specific locations, a properly oriented (polarized) beam antenna will be very effective. But for general purpose use, an omni-directional antenna, such as a 1/2-wave [vertical] base station antenna would be your best choice. – Sarge