The penguin's motorcycling and Jeep blog

Friday, March 13, 2009


Communications are important when you're far from civilization. So my Jeep has four antennas on it. The satellite radio antenna is hidden under the padding for my roll bar. The AM/FM antenna is on the right front fender. Then here, on the back of my Jeep, are a CB (11 meter) antenna, and an amateur radio 2m/70cm antenna. Not visible is the FRS/GMSR antenna, which is fixed onto the "walkie talkie" that it came on and clips to the headrest in my Jeep, or the internal antenna of the SPOT satellite messenger, or of my iPhone (which, however, is mostly an inert lump if I'm away from pavement).

The AM/FM antenna and satellite antenna came with the Jeep, so let's talk about the CB and ham radio antennas. The CB antenna is a Firestik wire-wound fiberglass antenna that someone gave me. I mounted it on the center brake light housing on a Firestik spring through-hole mount, by drilling a hole in the brake light housing for the stud of the spring mount. I then ran the coax alongside the brake light wiring to inside the rear tailgate of the Jeep, made a semi-loop inside to allow opening and closing the tailgate, and from there along the seam of the tailgate doorsill, protected by a lot of duct tape. Once I reached the driver's side of the Jeep I then ran it alongside the existing wiring until it got to under the dash, where it hooks into the junction box of my CB radio. From the junction box I then ran the CB handset's wiring across the interior bottom of the dashboard (on the frame structure for the bar that keeps you from sliding under the dash if you're not belted and the airbag goes off, part of the federally-required airbag system), then inside the center console on top of the driveshaft tunnel, then it comes out under my seat, from whence it can be stashed alongside my seat or hooked on the dashboard microphone hook depending on whether I want that big heavy handset and coil wire hanging around (if I'm not on the trail and thus don't need the CB, it lives alongside my seat).

So anyhow, that's my CB setup. It shouldn't work, the textbooks say. Up there on the plastic brake light housing it doesn't have a good enough ground, they say, and it's too far above the tub of the Jeep for the tub of the Jeep to serve as a good ground plane (remember, the roof of my Jeep is fiberglass -- it's basically transparent to radio waves). But the 10 gauge wire running down to a ring connector slipped over one of the brake light housing mounting bolts appears to give enough ground for it to work, I hear fine on the trail, and talk good enough, and has fairly low SWR. So it works as good as the cheesy freebie antenna will ever work, I suppose, and good enough for the communicating I do with it.

Which brings us to the ham radio antenna. Now, one thing a lot of people have a problem with is the idea of putting holes into their nice purty vehicle. I've had the mount that I use for the ham radio antenna for, like, forever, but I had that same problem back when the Jeep was new. Not anymore. I put a Rock-It Parts CB Antenna Mounting Plate between the driver's side tail light and the body of the Jeep. This required drilling two holes into the body of the Jeep for supporting the top part of the bracket (the template for doing this came with the bracket), which raises the antenna up to above the level of the body of the Jeep to prevent reflected waves from messing things up and allows using the tub of the Jeep as an (asymmetric) ground plane (remember, my roof is either fiberglass or cloth depending on whether I have the hard-top or convertible top on, both will attenuate radio waves but do not reflect them). I then spaced the tail light out from the bracket a bit to give more room to get the antenna wire down into the hole for the tail light wires.

The next thing to add was the actual mount. I used a Diamond C101. I had to drill out the hole in the CB mount to be big enough for the UHF connectors and I did not have a big enough drill bit, so I went over to Cheap Chinese Tool Place and bought 2 Piece Titanium Nitride Coated M2 High Speed Steel Step Drill Bit Set for my drill, which did the job of reaming that hole out nicely. I just kept going up one step until the mount fit through it! I could then put the antenna onto the mount. I chose a Diamond NR77HA antenna. I wanted a 2m/70cm antenna because those are the two frequencies most used by repeaters (the 2m for local repeaters, the 70cm for interconnected repeaters), and the longest antenna that could still be protected by the roof of my Jeep from being whacked by brush, and the vibration of offroad driving also was a factor limiting me to about 40 inches of total length. Size matters with antennas (unlike with, err, other pointy things), but like with all good things too big is just too big. This was the best size I found for my Jeep.

Once I had the mount mounted in the hole, there came the job of running the supplied coax into the Jeep. I just looped it on the *inside* of the tail light and ran it through the existing hole behind the tail light. I protected it with plastic wire loom to protect it from anything that might abrade the coax. There is a rubber grommet where the existing wiring passes into the tub of the Jeep, I poked a new hole through it and pulled the coax through the new hole into the Jeep, then ran it alongside the existing wiring to the front of the Jeep, adding a mini-UHF to UHF converter and coupler to extend the wiring enough to get to the antenna location. Then I tested the antenna system by using an SMA-to-UHF converter to attach it to my VX-8 portable radio, used a local repeater's signal test function late at night (when nobody is using that repeater, which is 20 miles from my apartment), and found that it gives approximately twice the transmitted power as even the long Diamond SHR940 HT antenna on my VX-8.

The next thing to add will be a mobile 2M/70cm radio. I am looking at the Kenwood TM-D710A, which has some nice features. It will output 50 watts, unlike the 5 watts of my little Yaesu VX-8, and will make long-distance communications within line-of-sight much more reliable in the desert areas where I travel. The big issue is money, but that should be resolved shortly for reasons I won't say more about here. Once I am finished with that, my Jeep's communication systems will be pretty much finished, and I can then move on to other mods to the Jeep. More as that unfolds...

-- Badtux the Radio Penguin

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