Mobile radio installation tips

If you’re planning to operate your ham radio from a mobile installation, it’s important to understand that your vehicle is packed full of sensitive electronics, and some of them, like the ECM and TCM, are vital to your cars operations. The very last thing you want to do is pay for an expensive repair because you got RF into the vehicles electronics and killed something important, or made a sloppy wiring mistake. So, with that in mind, how do you proceed?

Here are a few points:

  1. If you have a mobile radio, the owners manual will almost certainly recommend connecting it directly to the battery. This is essential to maintain a safe supply of power. Make sure that the leads are of proper gauge wire, and fused at both the positive and negative leads, as close to the battery as possible, using fuses properly rated for your load. It’s been recommended that instead of connecting the negative lead directly to the battery, to instead connect it to where the ground cable from the battery meets the chassis.
  2. Do not use a fuse tap or add your radio to any existing fused circuit. You will overload the circuit.
  3. Do not use a cigarette lighter adapter if you will exceed half of the outlet’s rated value, or the outlet’s rated value is unknown. You can overheat the wiring and overload the circuit. (Remember: DC watts is roughly double your RF watt output.)
  4. Attach the radio’s mounting bracket to a location that has a good chassis ground. Use an ohmmeter to test. If you cannot ground the bracket, then run a grounding strap from your mounting location to the nearest chassis ground point. Avoid Velcro mounting if at all possible. It will not stay attached in the case of a severe accident. Reference your vehicle’s manual to ensure that no components such as microphones or head units are in the airbag deployment area.
  5. Attach any accessories, such as duplexers or amplifiers, securely. Consider putting all your radio equipment in your trunk and only having the headunit and microphone in your passenger area. Although this involves running more wire, you’ll have plenty of room to secure your components.
  6. Avoid magnetic mounts on any frequency lower than the 2-meter band. Use an antenna mounting method that provides a good ground, such as a trunk-lip or through-hole. Ensure that whatever mounting method you use supports your antenna well, and check it periodically. Use thread-lock compound on any adjustment screws but NOT on screws that provide a ground, as it will interfere with the ground. If using a trunk mount, check for an included metal plate (stability plate) and ensure that two screws bite into the metal plate (use thread-lock on these) and two cut through to the metal in your trunk lid (do not use thread-lock on these).
  7. Mount the antenna as high as practically possible, keeping metal under the antenna instead of up against it. Mounting on roofs and trunk lips will work, but not so well on bumpers or against a metal truck bed cover.
  8. Remember the legal limit in the US is 13’6″ high, and parking garages and drive-through burger joints are often much lower than that (around 10′). If you are going to exceed 9’6″ from the street to the tip of your antenna, be alert when driving and parking, and be sure your antenna is removable without too much hassle.
  9. Check your SWR after installation. If you are having a difficult time getting a good SWR reading, check the location of your antenna and the ground connection. If you’re having trouble getting a good SWR on 6-meters or below, check to see if your trunk lip (or other body part) has a good ground path to the rest of the vehicle chassis metal. Run flat ground strap as appropriate. The lower the band, the more continuous ground you need. Remember, your ground is your counterpoise.
  10. Remember the elements, and weatherproof and seal connections as necessary. If you’re using a trunk lip mount, or you’re going through any vehicle weather seal, consider a mount that uses a very thin type of coax (such as RG174) to keep the displacement of weather seals to a minimum. The inside of your vehicle will thank you.
  11. Don’t operate using more than 100 watts. I’ve heard stories about people operating a kilowatt from their vehicles. Once you consider RF exposure limits, there’s no justification for this. Save it for the base station where you can get your antenna further than 10 feet from your body.
  12. NMO is the only connector that I am aware of that is waterproof when removed. While “UHF” (PL259/SO239) is a popular connector, and performs well, be aware to avoid moisture leaking into the connector when installing or removing your antenna, and keep a rain cap on the connector when you don’t have an antenna installed. It’s a good idea to seal your connections using coax tape or similar to avoid moisture contamination, even when your antenna is installed.

Remember your physical safety as well as the electrical safety in your vehicle are of the utmost importance. It’s difficult to operate from a hospital bed, and your car insurance won’t cover a vehicle fire from sloppy wiring. Ruining a vehicle over a few hundred bucks of radio equipment is just not worth it.