All radios need an antenna and the type of antenna chosen will determine important performance characteristics. Let us start with a radio most people are familiar with. The citizens band
radio is typically supplied with some form of mobile vertical whip style antenna. This antenna is usually mounted vertically on the vehicle and it radiates radio frequency energy with mostly vertical polarization. Most CB users have vertical antennas so they are most sensitive to similar signals having vertical polarization. If a person using CB radio wanted to make his signal heard by his comrades more and less by people outside his group, he could change his antenna to a horizontally polarized antenna style and have his comrades do the same. There are other tricks to making a radio signal less likely to be intercepted by an unfriendly force. Antennas can be built to have directional characteristics so that the majority of the radio frequency energy is directed toward your comrades but not towards the opposing force. One cheap and user friendly wire beam antenna is named the Moxon beam. The Moxon beam is built to operate on a narrow band of frequencies and it typically doubles your effective radiated power twice and reduces radiation to the rear by a large degree. The Moxon beam antenna also can be set up for vertical or horizontal polarization. A very important feature of the Moxon beam antenna is it’s suitability for matching the impedance of the radio transceiver. Most radio transceivers have a 50 ohm nominal impedance and that just means that in order to transfer the most energy to the antenna, you need the connecting cable and the antenna to both have 50 ohm ratings. Coaxial cable is often used to connect the transceiver to the antenna and this cable for low power would typically be RG-58 (available from Radio Shack and many others).
Antennas radiate differently depending how high off the ground they are mounted. Let us take our CB radio and connect it to a dipole antenna. The frequency of CB radios is about 27 Mhz(megahertz) and another way to express the frequency is by the mention of the wavelength which is approximately 11 meters. If we mount our dipole antenna at ¼ wavelength above ground, it will radiate energy efficiently perpendicular to the wire and practically no energy in line with the wire. The ¼ wavelength at 11 meters is close to 108 inches. If we bring the antenna closer to the ground at 50 inches the antenna radiates most of it’s energy straight up towards the clouds(which is the reason this type of antenna is called a cloud burner). A point of safety: any antenna radiating radio energy must be isolated so that people are not able to physically touch the wire or an RF burn may result. An RF burn can burn to the bone and take many months to heal. Also, ionizing radiation from antennas can be hazardous if power levels are elevated and/or a power gain antenna is used and directed where humans may be.
Why would you want to direct your radio frequency energy towards the sky? A skywave coming from a near vertical incidence skywave (NVIS) antenna at the appropriate frequency will cover an area that would be described as regional as opposed to global. An NVIS generated signal is much harder to direction find by the opposing force.
Another wire antenna type for point-to-point communications is the Rhombic antenna. This antenna can make a pin point beam width of about 30 degrees and is useful for a wide range of frequencies but may take up a large area to erect. The Rhombic antenna is diamond shaped and each side of the diamond must be 3 or 4 wave lengths. For CB a rhombic antenna would be a little less than 6×11 meters long or about 200 feet overall. On the 75 meter ham band the length would be just under 1400 feet. On the 2 meter ham band the overall length would be under 36 feet. The big plus of the Rhombic antenna is pin point aiming which give higher effective radiated power and much better hearing for the receiver. The antenna gives an amplifying effect without using extra power. The power gain is estimated at 10 dB (Decibels) or greater. The power is doubled effectively for every 3 dB of gain. If the transmitter output power started with 5 watts, this style antenna could boost the directional power to over 30 watts of effective radiated power.
My personal favorite antenna for ham radio is a relatively discreet loop of wire that circles my yard at an average height of less than 15 feet and it has about a 550 foot circumference. It is inexpensive and proven to work on all high frequency ham bands (1.8 to 28 Mhz) with the use of an antenna tuner. This antenna is not for talking reliably for more than about 400 to 600 miles using 100 watts of power on the 75 meter ham band.
For more in depth antenna study, consult the American Radio Relay League (ARRL) and their publications. The ARRL Antenna Book has a wealth of information as does the ARRL Radio Handbook. The ARRL web page has information about where to test for your ham radio license and QRZ.com has sample test questions to prepare for the test to become an amateur radio operator.
The ham tests are written and all the answers are at QRZ.com.There is no longer a code test but Morse code is a great mode to operate with under low power conditions or less favorable propagation and it makes your communications more secure from the average eavesdropper.
Disclaimer: Check with your Radio Authority before transmitting to be sure you are operating according to rules of the Authority having Jurisdiction over your geographic area. In the US that authority is the Federal Communications Commission (FCC). – Uniform Delta