Converting an FM Antenna to a 2m/70cm Transmitting Antenna

Question

I have a 2006 Honda Goldwing 1800 with an extra FM antenna that I would like to convert to a 2m (or 2m/70cm) transmitting antenna. Changing the connector to a PL-259 seems to be the easy part and I have a 2m70cm whip that I can use. What I am struggling with is determining if the base will work. That is, since the base is designed for the motorcycle, is there anything else I need to be concerned about? It seems that the whip and the base are electrically connected...there is no resistance between them. The radio is not attached the antenna when I check the ohms. Comparing my other dual band antennas, there is no connectivity between the center pin of the antenna and the base.

Answer 1

Any whip antenna installation on a vehicle must have a structure that is physically and electrically something like the following. The precise construction of the junction/mount is not important (as long as it is fairly small relative to the wavelength of the signal, i.e. small relative to the length of the antenna), but it must be electrically exactly like this:

                  | antenna element
                  |
                  |
                  |
                  |
                  |
                  |
                  |
                  |
----------------+ + +-------------------- vehicle chassis
                | | | feed point
                | | |
                | | |
                | | |
                | | |
                | | |
                | | |
                | | |
                | | |
                | | |
                | | | coaxial cable

That is, at the feed point (base of the whip),

• The center conductor of the coax must have electrical continuity with the antenna element (whip).
• The shield of the coax must have electrical continuity with the vehicle chassis.
• There must not be electrical continuity between the center and shield.

It doesn't matter whether any particular physical component of the feed point is connected one way or the other, as long as the parts extending away from the feed point are connected as specified. (If they are not, then the antenna may or may not function, but it will not function as designed; the impedance and/or radiation pattern will be wrong.)

For example, in a common “CB” antenna mount, the antenna itself has just a 3/8" threaded stud on its base, which screws into a socket on the mount mount; this part is connected to the coaxial center conductor. Then, the mount includes an insulator which keeps the threaded socket separate from the vehicle chassis, while the coaxial shield is joined to the chassis using the same metal hardware that mechanically secures the mount. Vehicle antennas for FM broadcast are often designed like this but will likely have a different thread on the antenna element.

On the other hand, some antennas may have coaxial connectors on their end — having a center and shield. In most cases, it does not matter what happens to the shield of the connector on a whip antenna, because it is so short that it could be connected wherever and it doesn't affect the system; it is just making the connector mechanically complete, not performing any electrical function. (However, this is not necessarily true if the antenna has a built-in matching network.) This is probably the situation with your “other dual-band antennas”.

In either case, the antenna is not responsible for making the shield-to-chassis connection; the mount is — but the electrical role of the threaded part differs.