Jim,
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
Jim,
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
