So you’ve decided on a radio system for your car or truck, and you are ready to install it. While installation certainly doesn’t have to be a tricky process it has the potential to be. That being said, my goal isn’t to impress you with all the things I’ve forgotten since my test and then googled for this article, but to give you enough information, both technical and practical, to avoid some of the pitfalls that trip up so many people.
So let’s get started on everything you need to know to install your radio and get the most out of the system:
Wiring Your Radio
When you get your radio the chances are that it will have a power cable that has a fuse on both the red and black wires. There is a lot of debate on the subject of fusing the ground (black) cable or not, but my best advice is to follow the instructions given by the manufacturer. Whatever you do remember the golden rule for adding electronics of any type to your vehicle; Leave the factory stuff alone. In the case of mobile radios almost all manufacturers will recommend connecting your fused positive wire directly to the battery positive terminal to avoid electrical noise from your alternator. If you have more electrical needs you can add a sub panel which I will cover in a future article.
You can plug low power radios into a cigarette lighter with an adapter but you may get electrical noise. You can also try a fuse tap. Never do this with a radio with more than a 5 watts of transmit power as it will be an unsafe amount of current draw.
A note on power: A radio’s transmit power is not its power consumption. A “5 watt” radio will actually draw closer to 10-15 watts when keyed up (transmitting). Check the user manual for amp draw to properly size wires and fuses if you don’t plan on using the supplied ones. If the manual doesn’t say, assume at least 3x the transmit power for actual power consumption. If you need help choosing wiring or fuses check out this guide.
Reminder: You can find power in watts with the formula P(watts) = V (volts) x A (amps). To get amps from watts simply use P/V = A. Assume 13.8 volts for car voltage.
When looking for a suitable place for your radio you will also need to keep in mind a few things:
- Heat: A 50 watt radio consumers over 160 watts of power, most of that will be heat. Keep your radio away from hot areas and make sure it has adequate airflow. NEVER install a radio in the engine bay.
- Cable length: You want to keep the distance from your radio to your antenna as short as is practical.
- Functionality: Obviously you will need to reach the mic as well as read and use the controls. If your radio has GPS you will need to find out where the GPS antenna is and make sure it has sightlines to the sky.
- Interference: high powered radios can interfere with vehicle operation if the radio is placed too close to the engine or other computers that aren’t properly shielded. Most vehicle computers are near the passenger footwell or under a seat.
Cable Choice and Placement
An oft forgotten part of a radio system, and a very important one, is the cable. Cable tech can get convoluted surprisingly fast so I will try and keep this light.
There are many grades of cable, but the three most popular types for mobile use are RG-58, RG-8X, RG-8/U and a special case wire called RG-59:
What power your radio outputs, known as the Effective Radiated Power (ERP) will depend as much on the cable as the actual radio or antenna. All cable will introduce a loss, measured in dB, dependent on the type and length of cable and for a given frequency. Less loss is the goal. While it’s not as glamorous, spending money on good cable is as important as getting a good antenna. dB is logarithmic, power is doubled or halved every 3dB.
Other Important Installation Tips:
Avoid solid core cable for mobile installations as they aren’t as flexible.
Cable length can be calculated with this formula: Wavelength x Velocity factory of the cable. Close is good enough unless you need an impedance matched system, like a no ground plane system. Example 2 meter x .82 = 1.64 meters. Aim for close to a ¼, ½ or full length multiples. Multiples of 1.64 meters, .82 meters or .41 meters.
NEVER coil extra cable as that creates a choke which will dramatically increase the loss. Wind the cable around like a snake to use excess.
Your cable MUST be impedance matched to your radio and antenna systems. For most single antenna systems it’s 50 ohms (Ω). For dual antenna CB you will need 75Ω.
A Word on Connectors:
Most radios have a SO-239 connector (female) which mates with PL-259 (male, shown) also known as “M type (Motorola)” or UHF. Antennas tend to be either ⅜”-24” thread, M type (SO-239) or NMO (New Motorola). My CB is ⅜”-24” and my HAM is NMO and both have been reliable for me on rough trails though most off-road professionals recommend NMO for strength.
Antenna Placement and Tuning:
One of the most critical parts of your radio system is going to be your antenna. Antenna science is more than I want to get into here but there are a couple of basic things to keep an eye out for when shopping for an antenna.
Gain: Antenna gain is usually listed as either dBi or more commonly as dBd. dBi is gain compared to a theoretical single point radiator, like a bubble of energy coming off a single point. dBd is gain compared to a ½ wave dipole antenna which has a gain of 2.15 dBi and more of a donut shaped pattern. A radio is a system and totals in gains and attenuations (losses) need to be added up for your ERP. A few notes on gain
A gain of 3 dB, is a doubling of ERP. Increases in gain also translate to increases in reception range. An increase in gain is a reduction in angle of radiation. I.e. a more focused beam of energy that is closer to the horizon.
More gain isn’t necessarily better. A low gain antenna that has a higher angle of radiation would be more suitable in an urban environment as it can reach and see to the tops of buildings better. More gain, narrower beam. Less gain, wider beam.
SWR: Standing Wave Ratio is a comparison of energy being absorbed by your antenna to be radiated out compared to energy being backfed through the line. Ideal is 1:1 (not backfeed) and maximum for safe operation is 2:1 or about 10% backfeed. Backfeed generates enormous heat in your transmitter and can lead to radio failure. In order for you to get the most from your system and avoid excess heat you will need to tune your antenna for the lowest SWR possible. You can find cheap SWR meters to help you tune your system or find a professional that will do the job for you. Tuning for SWR is a CRITICAL step in your installation. A properly tuned antenna can be worth as much as twice as much transmit power compared to a poorly tuned antenna.
Band and length: You need to pick an antenna that is compatible with your radio. A CB antenna will not work for GMRS or HAM and vice versa. A GMRS antenna is not ideally compatible with 70cm HAM but can work. You will also want a long enough antenna so that as much of the antenna is above your vehicle as possible (or practical).
Ground plane: A ½ wave monopole antenna is actually only ¼ wave as it’s just one half of a dipole antenna. The ground plane solves for this by acting like a mirror for electromagnetic waves to sub in for the other half. The ground plane, therefore, needs to be electrically conductive and flat. Because a ¼ wave is being created from the antenna, another ¼ wave of area needs to exist as ground plane area as a reflector.
Example: GRMS is 65 cm wavelength. ¼ 65 is 16 cm (6 inches). You need a 6 inch diameter flat conductive area under your antenna.
If you are mounting where you will have limited ground plane area like a fiberglass bed, or camper, you should consider a no ground plane (NGP) antenna. Which either uses a special matched cable and antenna or tuned counterpoise (artificial ground plane) on the antenna to match the impedance of the radio. Electrical ground is not the same as ground plane, however an electrical ground is required for a ground plane to work.
Poor electrical ground and ground plane area are the #1 cause for poor radio performance. Worse case it can actually burn out your radio. Ignore grounding and SWR at your peril.
Note: Magnetic mount antennas are electrically grounded through inductance from the magnet to the metal in your car.
Mounting the Antenna
Mounting is arguably the hardest part of mobile radio installation. You have to balance theoreticals like ideal ground plane, object reflectivity (even something like a lightbar will affect your radiator pattern) and length of cable alongside other real world issues like physical interference with garages and trees, realistic cable run requirements, suitability for a hard mount, grounding options and practical interference with gear or sightlines.
My vehicle (pictured) is a perfect example of how to do it wrong. The CB has no real ground plane, though some ground plane exists as the antenna is far above the roof. The cable length isn’t a multiple of the wave, (though it’s close) and they are boxed in. My CB gain is actually negative like this, but it’s good enough for group comms within short range and my antenna is tuned for low SWR. My HAM antenna is a no ground plane antenna but it’s severely compromised projecting and receiving forward because most of the antenna is blocked.
Ideal? Hardly. This is a good example of the compromises we will make as we prioritise what’s most important to each of us. I did it like this because it allows me to leave my HAM antenna up full time and still fit in the garage at home and work and because the CB antenna chipped my glass lens where it was at before on my front bumper and I’m willing to take a pure performance hit for practicality.
The truth remains that the best place to put an antenna is smack dab in the middle of an electrically conductive roof, but that’s hardly practical. You will have to walk this balance yourself but keep in mind a few considerations:
Keep the radiating element (usually the tip) well above any obstructions like roof racks or tents.
Try and avoid boxing the antenna in like right behind the cab of a pickup or in the rear corner or an SUV. Glass does not count as ground plane area so be aware of sunroofs.
Make sure you can electrically ground your antenna. If you are mounting to a non-conductive surface you will need to find a mount that allows you to run a grounding strap. Remember that paint can also prevent a good ground connection.
And here we are at the end, I hope you feel a little more empowered to tackle a radio install and I hope I didn’t get carried away flying my nerd flag. I do this nerdy research so you don’t have to! See you on the trail.