Part 18
Welcome to part 18 of our series.
Back to DC Power in the shack. This month we'll take a look at the "shack-on-wheels". The basics of powering the radio don't change when you install a rig in your car, but there are some additional considerations that you need to take into account. First and foremost, the three most important rules take on added importance in a mobile installation, so I'll repeat them here for you:
1 Make sure you put a fuse as close as possible to the power source.
2. Make sure you put a fuse as close as possible to the power source.
3. Make sure you put a fuse as close as possible to the power source.
Wiring in the car is subject to chafing, vibration, and possible stresses that are not typically encountered at home.
A "Car-B-Que" can really ruin your day! Don't take chances, don't take shortcuts.
It's convenient, and tempting, to power the radio from the lighter socket on the dashboard. But lighter sockets, and their associated circuitry, are generally not designed to provide the high current required by a radio transmitter. And there's always the possibility of RF traveling down the power leads and into other circuits and devices in the car. Do people get away with doing it? Yes, especially with low power HTs. Will YOU get away with it? Maybe. Are you feeling lucky?
Far better to do it right. Run your power directly from the battery, referring to rules 1-3 above. You can review parts 16 and 17 below for a discussion of needed wire sizes. Since nearly all mobile installations require some effort to be done right, I wouldn't consider using anything smaller than #10 wire. You never know what you're going to want to power in the future. Don't wind up down the road wishing you had spent just a few more bucks on heavier wire.
Back in the "good old days", it was usually a simple task to run a cable from the battery, through the firewall, and into the cabin. Have you looked under the hood of your modern automobile? Yikes!! Nowadays, it can be a real challenge. If you want to do it yourself, here are a few tips. Safety First! Make sure that the wire under the hood will not chafe against other wiring, ducts, or hoses, or other metal. And that it can't interfere with fans, belts, or other moving parts and linkages. Similarly, make sure that the wire inside the cabin cannot interfere with proper operation of pedals, controls, seat belts, etc. If your wire runs under the seats, make sure it's clear of any adjustment tracks and mechanisms. Never EVER drill through the firewall (or anything else) without being double darn sure there's nothing in harm's way on the other side.
If you do have to pass your wire through a hole, make sure to use a grommet to protect it. It's sometimes possible to pass the wire through an existing grommet that carries a wiring harness into the cabin. Here's a trick an experienced ham passed on to me. With the car in the garage, or at night, put a drop-light on the floor of the cabin. Then look under the hood for the light shining through the firewall.
In truth though, I think the best solution is simpler (although it will cost you a few dollars). Those annoying little rice-boxes running around the neighborhood blasting rap music at mega-decibels are using considerable current. Probably on a par with, or exceeding, what you need for a transceiver. Go to a place that installs car stereos or alarm systems. A reputable one will know the best place to run your wire, and should be able to do a neat and clean job for you. If your skills, equipment, or available time are not quite up to it, they may even be willing to do the entire installation for you. It's well worth the small investment for at least the wiring job itself, in my opinion.
Most new HF and FM mobile transceivers come with fused power cords. Typically, there are fuses in both the positive and negative leads. Most aftermarket replacement cables (including the ones I sell) have the same arrangement. Why fuse both lines? Well, the thinking is that should the heavy wire that connects the engine block (and thus the starter) to the body of the car (and thus the battery's negative terminal) somehow fail, all of the starting current would try to flow through the radio cable's negative wire. That's way too much current for that relatively light wire, hence the fuse.
But this is based on some false assumptions. Next time, we'll look at why fusing the negative line is a poor idea, and how to properly power your radio in a vehicle
That's it for this month. Next time, we'll continue looking at DC Power in the shack.
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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