In a previous article we covered UHF, VHF, and HF portions of the electromagnetic spectrum, and briefly touching on antennas. In this article, we will cover antennas and wavelength, in more detail. Understanding the properties of antennas and learning some basics of theory, will help you to find the right equipment to meet your individual needs. You may have heard people refer to different frequency parts of the spectrum as “meter” instead of “megahertz.” Neither of these references are incorrect, and this article will help you understand some of the vernacular fellow hams use.
Have you ever taken a rope, electrical cord, or garden hose and shaken one end to create a visible wave that extends down its length? If so, have you noticed that exaggerated arm movements create larger waves, and short but fast movements create different wave forms? Creating different wave forms can be equated to changes in frequency. When we talk about radio frequency in length of measurement, we are talking about one wave measured from tip to tip.
The formula for wavelength is λ = c / f , where wave length (λ) is measured in meters, c is the speed of light (a constant of 299,792,458 meters per second), and f is the frequency, measured in cycles per second or Hertz.
Using this formula, we can find the distance from the tip of one RF wave to the next:
(c = 299,792,458) / (f = 144,000,000Hz) = 2.08m
The 2m band is 144Mhz.
You can also convert for the frequency, if you know the wavelength being discussed, using f = c/λ
(c = 299,792,458) / λ = 2m) = 149Mhz
One thing you probably noticed is the difference in the frequency in the equations above, 144Mhz vs 149Mhz. That is because the bands cover a pretty wide range. The intent of these calculations is to help you get familiar with the terminology. You can commit this formula to memory or simplify as noted below (which is what we often like to do!):
Round up the speed of light to 300 and divide by the frequency in Mhz, by removing all the zeros 000,000 (144,000,000 hertz is equal to 144Mhz).
It’s quick,easy, and gets the job done without a calculator.
Antenna length goes hand in hand with the frequency you desire to transmit on. The higher the frequency, the shorter the wavelength. Most commercially available antennas come in length equivalent from 1/4 to 5/8’s of a full wavelength. This means that if you want to run a 2m rig (144’ish Mhz),a half wavelength antenna would be 3.3” long, and 3.3 ft is very manageable for vehiclular operations. This length is one of the reasons (among the others) that CB radio is not the best option for off road usage. To get this half wavelength equivalent antenna on CB, as what we discussed for the 2m band, lets do some quick math. We know that CB operates around 27Mhz. Lets use the easy formula to plug in 300 divided by 27Mhz = 11.1m. This tells us the CB band operates around 11 meters wavelength, which equals 36.66 ft. Ok, now for the wow factor - in order to have the relative antenna gain (the term used to describe an antenna’s performance and efficiency) for 11m like we had on the 2m, we would need an antenna that is a little over 18ft long. That’s a little ridiculous to run as a vehicle, mobile mounted antenna, so often on CB a 1/4 wave or less antenna would be employed. To find 1/4 wave length, just halve the 18ft, down to 9’ish feet. Those obnoxious 8-9ft stainless steel whip antennas actually work pretty good, but don’t have any place on the trails. Bystanders will be highly agitated, if not injured by these flexy whips.
Most folks on the trails or general overloading rigs run 3-5ft fiberglass CB antennas. This means they’re running the equivalent of 1/6-1/9th wavelength antennas. Knowing this, now consider how efficient the 8” rubber duckie antenna the handheld CB radios use. In the scheme of things, it’s not much better than no antenna.
On the opposite end of the spectrum, if we decided we wanted to run something in the 900Mhz range, we could use a half wave antenna for the 33cm band and have a total antenna length of 6.5”. Eventually though, there does come a point of diminishing returns on antenna length, as capture area goes diminishes as the length is dropped.
When choosing an antenna, some of the things you should look for are:
Gain. The higher the better. This will allow you to talk farther and be able to pick out signals that would otherwise not be any louder than the white noise you would hear in the background. When talking about a vertical antenna the longer the antenna, the higher your gain will be. This can be a spot where your function can take over form. Not everyone loves a big honking antenna towering over their vehicle, but it's a personal preference preference. Personally, we dig antennas. A 1/4 wave antenna will work ok, but we would recommend trying to stick with antennas of 1/2 wavelength if possible. There will not be many times as a ham operator you wish you had less antenna.
Location. Try to mount the antenna where nearby structures will have less effects on its properties. Installing it
as high as possible will offer better line of sight, and will have less likelihood of something acting as a reflector and redirecting the transmitted signals. Also, by mounting it near structure such as a roll bar, you could alter the point at which the antenna is resonant at and change the area in which the antenna is most efficient and performs the best.
Mounting. There are 3 main options when it comes to installing your antenna. The first one is using a magnetic mount. The pros for this are that it's not permanent, and can be moved from vehicle to vehicle. Another option is drilling a hole in one of the horizontal surfaces of the vehicle and installing a type "NMO" base. This antenna base will sandwich the metal on both sides of the drilled hole. This type of antenna is typically the best performing but the major drawback is drilling an permanent hole in the body of the vehicle. The last of the most
common mounting techniques is using a commercially available "lip" mount. It can go at the edge of a hood, or edge of a lift gate and offer adjustability of antenna orientation. This is a great compromise between the very permanent drilling method, and the temporary magnetic options.
Antenna selection is not something to take lightly. It should be considered at least as important as the radio itself. It doesn’t matter how much power the radio is putting out, if you’re running a lossy antenna, both your transmitted power, and your receiving signals will be marginalized. A good antenna can often times make up for a weak radio. As mentioned in a previous article, we at Team RCO, enjoy seeing how far we can talk on as little power as possible. We do this by making sure the antenna choices and positions allow them to be as efficient as possible. We could get by with running 10x the power with less efficient antennas, but as part of the overlanding lifestyle we have to keep in mind that when traveling off the beaten path power in the form of electrical energy is a precious thing that we can't afford to waste.
The two radios shown in the title picture are just a couple of the handhelds we own. The one on the right is an Icom ID-51a Plus, it's shown equipped with a Diamond SRH320a antenna which is a 1/4 wave 2m or 220Mhz, and 1/2 wave on the 70cm band . The radio is a UHF/VHF that will operate on FM, Digital Voice, and DSTAR modes. It has built in GPS , is waterproof, and even has a slot for and SD memory card. The radio on the left is a Baofeng BF-F8HP. It also transmits, and receives on UHF/VHF. They are a great way for a new ham operator to get into the hobby. When combined with a higher gain antenna can be a decent little radio. It doesn't have all the bells and whistles of the Icom, but it's much less expensive, and makes a god radio to loan to people as you upgrade, and can introduce them to the hobby.