Mounting an antenna through the car roof is not a difficult project, but must be done carefully. Here are basic instructions, which you may adapt to whatever vehicle you will be working with.
Remember you don't have to do this to be a good foxhunter! Many hunters just get out of the vehicle once in a while to take bearings. Nor, for that matter, do you have to use yagi- or quad-type antennas. Many hunters use Doppler-type devices for hunting and are quite good with them.
Going to the "extreme" step of center-mounting an antenna through a hole in the roof is only used by crazed hunters who do use yagis and/or quads (or perhaps rotatable TDOA systems), and who wish to enjoy these advantages:
Note that you must have the base-mount completed before you proceed, as you will need it to align the hole that will be cut in the roof.
The base-mount should provide a bearing surface, which will support the weight of the mast and antenna and allow it to turn freely. Optionally, you may wish to provide an additional flat surface, on which you could affix a compass rosette, but this latter is not necessary.
Some vehicles have a console between the seats, which may allow installing a temporary base-mount either in or on top of the console compartment.
Bearings of all shapes and sizes can be found cheaply at Surplus stores. You will have to experiment and find something that will work for you and the base-mount you design.
We use 2-inch PVC FITTINGS , i.e., they have a 2" inside diameter. These will accomodate so-called 1-1/4" Schedule 40 PVC pipe. This pipe has a 1-1/4" inside diameter, and the outside diameter is 1-5/8". To increase the diameter of the pipe mast where it passes through these 2" fittings, a 1-1/4" Pipe Coupling is placed around the mast in this location. This makes a not-too-tight, not-too-loose fit. We like this size pipe because it stands up well to highway speeds and winds.
One note: we have found that PVC fittings may vary in size slightly by different manufacturers. We had to go to several hardware stores to find the ones that fit together the best.
At the TOP OF THE PIPE MAST, install a 1-1/4" Reducer Fitting, into which you can fit (with a little bit of file-work) the center leg of a 1/2" PVC "Tee". You may secure the tee into the reducer with PVC Glue, but some hunters don't glue it, on the premise that if you should run into a low-hanging tree branch this fitting will break off, perhaps preventing possible damage to the antenna or vehicle.
Do NOT glue the reducer into the mast, but instead clamp it in with a hose-clamp. This will allow you to easily remove and/or switch antennas.
The long dimension of the tee will support the boom of your antenna. The so-called 1/2" Tee has an inside diameter of 13/16". Some hams split the tee horizontally, lay the boom of the antenna in the bottom half, replace the top half, and use hose-clamps at each end of the tee to hold it all together. With the right amount of tension in the hose-clamps, you can even rotate the antenna from vertical to horizontal easily, yet it won't turn by itself.
In case you were wondering, the coax from the antenna runs down the inside of the pipe! You cut a hole near the top of the mast, install a grommet, and insert the coax. Near the bottom end cut another hole (somewhat elongated) for the coax to exit.
How long should the pipe (mast) be? We have not done any real testing, but have decided that the bottom of the antenna should be at least 1/4-wavelength (about 20.1") above the car roof. Try to keep that as a minimum. Actual experience using side-mounted antennas (e.g., through the window into the armrest) has shown that errors as much as 15-degrees can occur due to the body of the vehicle. The direction of the error changes when the antenna is moved to the opposite side of the vehicle, so we know that the vehicle body is a factor.
Consider using RG-223 coax, as it is double-shielded, and will accept crimp-on BNC connectors. The double shielding may help keep RF from the fox sneaking past the antenna into your radio in the car. Well, maybe.
Refer to the previous sketch of the 2" PVC Fittings.
Park the vehicle on level ground. Mark the location of the wheels on the ground, with chalk.
Install the base-mount in the vehicle.
Suspend the plumb-bob over the base. Run the string over the tip of a flat-bladed screwdriver, holding the tip of the blade up against the ceiling of the vehicle. Adjust the location of the screwdrive blade so that the tip of the plumb-bob is exactly over the center of the support bearing in the base.
Using the soft pencil, mark the spot on the ceiling (headliner) of the vehicle where the string goes over the screwdriver blade.
Reverse the location of the vehicle, so that it points in the opposite direction. Make sure that the wheels are exactly over the chalk marks.
Repeat the steps with the plumb-bob. Mark the location of the string, as before. The second mark will be the same as the first mark, IF the vehicle was truly level. If you end up with two separate marks, make a new mark exactly half-way between them. This will be the correct location.
With a small drill-bit, drill up through the center mark, out through the roof of the vehicle. Keep the drill-bit as vertical as possible, so that the hole in the metal roof will be aligned over the center mark in the headliner. This purpose of this small hole is to center your compass in the next step.
Using the compass, scribe or mark a circle on the roof of the vehicle. The diameter of this circle should be slightly less than the outer diameter of the threads of the bottom fitting. This will allow the bottom fitting to grip the metal of the roof.
Using the compass, scribe or mark a circle on the ceiling of the vehicle. The diameter of this circle should be the same as the outer diameter of the threads of the bottom fitting.
Use the Exacto knife to cut through the headliner, following the marked circle. Remove the headliner plug.
Drill a 3/8-inch hole in the roof, just inside the scribed circle. Do not let the hole extend beyond the scribed circle.
Insert the nibbling-tool into the drilled hole, and proceed to cut out the circle of metal in the roof. Use the scribed circle as a guide, and do not cut beyond the circle.
Dress up the hole, if necessary, to smooth the edge.
You may now install the fittings. Insert the bottom fitting up through the headliner and into the hole in the roof. Have someone hold this bottom fitting while you place the gasket in place, and screw the top fitting onto it from outside the vehicle.
The headliner material will compress as you tighten the fittings; tighten them as necessary to get a watertight seal. This depends on the type of material, you will have to judge. You may wish to use a small bead of RTV around the hole (under the top fitting) to get a better seal.
Use the PVC cap to cover the hole when not in use. The inside of the cap is tapered, so you can make make it fit as tight as you wish, just with hand pressure.
Lastly, you may wish to paint the top fitting and cap to match the color of the vehicle. We have found these fittings in both black and white varieties. The white ones tend to discolor after being exposed to the sun for a few weeks.
