A Mobile Coax Extension for External Antennas

An External Feed System

For Temporary Antennas

Geoff Haines, N1GY

In a recent article about the construction of a temporary mast to elevate an antenna for greater range, an extendable handle for changing light bulbs where the light fixture is mounted on a tall ceiling was converted to a 16 foot telescopic mast that mounted on the rear bumper of a vehicle. The next part of the system I looked at was how to conveniently switch the coax feeds from the normal mobile antennas (HF and VHF/UHF) on the vehicle to the external temporary antennas mounted on the external masts.

Since recently downsizing the antenna farm at my QTH, I had several two position coax switches available to use. The plan was pretty obvious at first. Insert the coax switches into the feed line between the radio and the mobile antennas. Then an extra connection would be available to connect the coax runs to the temporary mast mounted antennas. There was just enough space available on the mounting board for the IC-706MkIIG and the RT-11 automatic tuner residing in the rear compartment of my SUV to mount the coax switches. Short coax jumpers were built to connect these to the input side of each switch. The feed line runs to the mobile antennas were then connected to one of the output connectors on each coax switch. The other output connector would then be free to, when necessary, connect the coax runs from the mast mounted antennas. Then with a simple flip of each coax switch the external antennas would be placed into the circuit and the mobile antennas removed.

Further reflection on the procedures involved in converting the system from one set of antennas to the other raised a few flaws. First the temporary mast cannot be erected until the tailgate of the SUV is closed. Even the rear window can only be opened a few inches with the mast in place on the rear bumper. I needed a way to get the connecting points for the external antennas away from the inside of the vehicle and outside. Then the mast could be erected and the coax connected without having to get at the inside of the SUV.

I measured the distances involved and built two coax runs of about 5 feet in length. This would allow me to toss them out the open rear window of the tailgate and then close the window on the coax without crushing them, as might happen if I ran the coax runs out under the tailgate door itself. At one end of each of these two coax runs, a PL-259 connector joins the coax to the second output connector of each switch. At the other end of the five foot coax jumpers, the PL-259's were connected to feed-through barrel connectors. I mounted the barrel connectors into a small metal box. This way the ends of the jumpers will be less likely to wind up in the mud. Each pass-through barrel connector is protected by a metal cover tethered to the box. Originally, I was going to use soft plastic covers, but these seemed to be a better way to go. The covers were made by soldering a small coin to the back of a PL-259 shell along with a small screw-eye so that the covers can be tethered to the box. I used fishing line swivel clips to attach the tethers to the shields and a ring crimp connector to attach the tethers to the enclosure. A rubber O-ring at the base of each barrel connector makes the cover water-resistant if not water proof. Added after the pictures were taken was a length of flexible corrugated plastic sheath placed around the coax jumpers to neaten up the appearance of the extension.

Obviously, the easiest way to accomplish the desired effect of having external connectors for the temporary antennas would have been to mount the pass through connectors through the steel body of the vehicle. Easy, except for the two holes that would have to be drilled in the car body, the interior fittings that would have to be disassembled to get at the back side of the body sheet metal, and the obvious problems when the time came to trade the car in on a new one. I have been through that scenario before, and it is not one I want to repeat.

The box containing the external connectors for the temporary antennas can be placed on the rear bumper and secured if necessary. To do that, I modified a Stanley (tm) Bar Clamp with a mounting point for the box. The bar clamp clamps to the rear bumper and the box is secured to the bar clamp with a ball bungee that holds it on the mounting "shelf". It is possible to permanently fasten the box to the clamp, but that would create a fairly large and unwieldy package to be stored. Having the two components able to store separately is much neater. I do not have a picture of this mount to post to the site yet, but I think it is pretty self-explanatory.

Instead of having to repeatedly screw on and screw off the coax runs directly at the back of the radio, I now only have to throw two switch knobs to the alternate position and carry on with erecting the temporary masts. The effect is to shorten the setup time by several minutes, and just as importantly, lessen the wear and tear on the radio. The use of the telescopic masting raises the antennas from just above the roof of the SUV to the point where the VHF/UHF antenna is 21 feet above ground. The HF antenna (The "Octopus Antenna from QST, December 2007) winds up 22 feet above ground, on its own mast, supported by an "under the tire" type mounting base at the front end of the SUV.

Whether you are manning a checkpoint for a bike race or a charitable event, or are responding to the aftermath of a disaster, or maybe even just setting up for an afternoon of operating at a public park or beach, anything to make the set up easier and more efficient is generally a positive thing.