In-Vehicle Communications: Everything You Need to Know

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Don’t rely on your cell phone. If you venture into the backcountry it’s one of the best pieces of advice I can give you. Like so many of my life lessons, I’ve had to learn this one the hard way, from dead batteries on a cliff at night with no map, to less threatening situations of not being able to find where the group is camping.

So, no phone. Now what? Well, for the overland adventure seeker there are, more or less, 4 main technologies that you can use to keep talking where the service ends:

  • FRS – Family Radio Service
  • GMRS – General Mobile Radio Service
  • CB – Citizen Band
  • HAM – Amateur radio

This chart gives you the basic rundown on each option:

Radio chart
Click for full size

Which one you chose is a matter of preference, need and compatibility. FRS and GMRS are cross-compatible but CB only talks to CB and FRS/GMRS can’t legally talk to HAM and vice versa.

Of course the best radio is worthless if you have no one to talk to, and what the rest of your group is using is ultimately going to be your biggest consideration. Most casual gatherings rely on FRS/GMRS simply because they are so easy to get and use, though be advised that you legally can’t use GMRS without a license. CB is very common in organized groups because it’s simple and cheap. Many clubs and groups have traditionally required CB and most still do.


If you want to go beyond basic convoy comms and expand your range then you have 2 choices – GMRS or HAM.

Let’s start where these two are similar. Both use FM technology for clearer voice quality, both allow for high power operation (up to 50 watts for GMRS, up to 200 watts for HAM, though 50 is more common in mobile) and both have access to repeaters for greater range, more on that later. Both can operate in the UHF or Ultra High Frequency spectrum. UHF is a good balance between long waves which travel well and short waves that penetrate and carry more information.


Where they differ is in ease of use, cross compatibility and features. GMRS is FAR easier to learn, offering the same 22 channels as FRS also making them cross compatible so long as you are following the rules for power output for FRS. GRMS, however, is far less flexible than HAM which not only operates in the UHF spectrum, but also the HF and VHF (Very High Frequency) spectrums and by frequency selection instead of channels. The most common HAM bands are the 2 meter VHF and 70 cm UHF bands. The 2 meter VHF band tends to travel further and be less bothered by terrain than the 70cm band and is more suitable for open undulating terrain. Mobile HAM radios that transmit in the HF band (80-10 meters) allow for very long over the horizon comms but that’s more rare and less suitable for off-road use simply because of antenna requirements.


Both GMRS and HAM are limited in direct radio to radio, or simplex communication by how far the antenna can “see” to the horizon. This distance, called the radio horizon, is mostly a function of antenna height and can be calculated here.

The way around this line of sight limit is a repeater. A repeater is a radio station installation that listens for a signal and rebroadcasts it on a slightly different frequency called the offset. This type of communication is called duplex as it allows the radio to listen and talk simultaneously. By using repeaters you can relay your signal much further than the radio horizon.



For example, if my antenna was a total of 10 feet off the ground then my radio horizon is around 4 miles. If another antenna I want to talk to is also 10 feet tall, we could communicate up to roughly 8 miles apart. However if I send my signal to a repeater high on a mountain, say 9000 feet, my new radio horizon is 130+ miles to that peak. Which means I can reach the repeater up to 130 miles away and the repeater can rebroadcast my signal another 130 miles. A single repeater can extend my line of sight from 8 miles to 260 miles. In addition, repeaters can talk to other repeaters.

So if you placed 2 repeaters each on 9000 foot peaks they could send your signal 260 or more miles and then on again so long as there are linked repeaters in range. It’s common for me to be having conversations 500 or 600 miles away even with a handheld with a small antenna and low power output via the magic of linked repeater networks. The bonus here is that big networks like this usually have someone listening, so if you need help you are more likely to find it, taking your cry for help state or even region wide to any radio tuned in.

Both GMRS and HAM have this duplex capability, though the feature will still be radio dependent. You can find repeaters for HAM here and repeaters for GMRS here. There are generally far more HAM repeaters than there are GMRS, so you’ll want to take that into consideration.

One area where HAM creams GMRS is in advanced features. While both systems have provisions for data transmission, it’s currently only found in HAM. This opens the door wide to some very cool and handy feature.

One of the more useful features for the adventure traveler is APRS, or Automatic Packet Reporting System, which automatically sends out packet data like position and other telemetry to be bounced around through repeaters that also connect to the internet. Using a simple web interface, loved ones can keep track of your location and path. It also allows other HAM users to locate you and navigate to your signal so you can, for example, meet up when cell phones are useless and without having to be in direct communication via radio. You can even send messages via the packets to other radios or even to SMS text.

GMRS has provisions for data and location sharing, but to a much lesser extent and far less open across brands. If it’s features you want, it’s HAM all the way.

So What’s Right for You?

CB – My main radio is a CB so I don’t have to worry about batteries and because it’s easier to use on the trail. It’s also the defacto standard for most groups and clubs.

FRS – For casual or sporadic use, it’s hard to argue against simple handheld FRS or GMRS radios. They are cheap, common and easy to use plus they can be used for so much more than vehicle radios. I still carry an FRS/GMRS radio to stay compatible with folks who don’t have CB or to use for spotting purposes.

GMRS – This is the sweet spot for many users, with good mobile units that can be vehicle powered and transmit at or near the power limit of the plan. If you aren’t locked into a system yet, such as CB for a club gathering, consider GMRS for its power, ease of use and access to repeaters. However you must pay for a license to use this legally.

HAM – This is what you want if you want more than basic communication. A huge repeater network, large user base with lots of active users and advanced features and modes to keep you growing with your radio for years. The main downside to HAM is that it requires some skill to earn your license and use properly, which will turn off most users so the chance that you will get your whole group onboard is smaller.

A Few Other Things to Keep in Mind:

Mount cable quality matters, don’t cheap out, especially with HAM.
Most antennas need to be tuned to work properly. In some cases you can actually ruin your radio if you don’t.

Most antennas are ground-plane meaning you will need to ground the antenna for it to work. Keep this in mind when mounting to non ferrous materials like fiberglass even when grounding through painted metal. A good ground is key to good performance.

Many people and companies will happily sell you a radio that is illegal to operate without a license. Please be aware of what you are buying and be prepared to do what’s needed to operate them legally and safely, it often won’t be stated expressly.

Hope this helps. Happy trails, 10-4, and 73. Check back with us soon to dive into installation tips tricks.

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We're all nerds at heart, some are sports nerds, some are sci-fi nerds and so on. Pat is a bonafide car nerd and proud of it. Pat loves absorbing some obscure technical car info and dispensing it to anyone willing to listen. He actually reads his owners manual.

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