Ground in Vehicles
In the environment of the mobile antenna system many factors contribute to the radiation of an excellent RF signal, but none more than the quality of the RF ground. The RF ground represents the "unseen half" of your antenna system. The visible half is the whip or other radiating element. Failure to construct a good RF ground inhibits the efficiency of the system's radiation and can present danger to the operator through RF feedback.
In mobile installations, the chassis or body represents a ground plane: a common circuit return or reference point for your signal. The signal radiates outward from the radiating element and flows back to the radio via the ground plane. Then the polarity switches and this process reverses, back and forth, in synchronization with the transmitted sine wave.
In constructing an efficient antenna system for your vehicle always make sure that your frame or car body are at RF ground by connecting them electrically and physically with the engine block. The engine block acts like a terminal strip or "bus" for your car's electrical system: the negative terminal of the battery and all other electrical grounds are connected to it as the central meeting point. The engine, in turn, is bound to the vehicle chassis through the engine mounting bolts, though not necessarily grounded! In today's modern vehicles insulating elements, i.e., rubber motor mounts, are used to cushion vibration. At DC a solid path to ground exists, and even if this path should somehow fail, the car's body ground can act as a reserve. At RF frequencies however, an acceptable DC ground can sometimes present such a high impedance to your antenna system that it is, in effect, no ground at all!
To ensure your mobile antenna environment is at RF ground simply bind the block to the chassis with tinned C_O_P_P_E_R BRAID. (1" wide minimum, silver tinned) Use SHORT RUNS so as to AVOID introducing any INDUCTIVE REACTANCE WHICH will IMPEDE the flow of the RF CURRENT to ground. round conductors are extremely lossy and should NOT be used in any lengths, regardless of diameter.
Signs that the quality of RF ground may be lacking :
1. Difficulty or inability to tune to an acceptable SWR match with your manual or automatic antenna tuner. (Assumes you have confirmed in advance that the antenna is already resonant "off the system".)
2. Noticing a waving up and down of the SWR reading on the meter during transmitting while the vehicle is in motion.
3. Noticing that the radio is "kicking", (cutting out and turning itself off) during transmission, an indication of significant RF feedback.
4. Getting an RF "bite" on the radio equipment or code key during transmission, an indication that excessive RF energy is feeding back from the antenna system or that the system is floating above ground potential.
Remedies require a review of how your antenna system is mounted to the car, specifically :
1. Is there a good physical and electrical connection between your antenna's ground and the vehicle frame/body?
2. If utilizing the body as the ground plane is there isolation from RF ground which may be causing the RF return path to float above ground potential?
3. Do you have faulty shield connections along your transmission line?
4. Are ground loops present in your system?
Based upon your assessment of the above, take action as necessary to clean up any weak points you uncover.
Ground Loops in The Mobile Environment
In your vehicle ground loops are often unknowingly created as a consequence of frantic attempts to ground out noise sources by braiding "everything" one can think of to the nearest metal available. This is most likely when working under the hood to suppress noise in the antenna and power supply systems, but can also occur when your antenna is inadequately mounted to the frame/chassis. In reading the nature of the formation of ground loops above, you can see how, in braiding to the nearest available metal, series connections could very well have crept into your design.
Remember: if you are not at ground potential, RF currents may still be circulating within your ground system which may then be re-radiated as RF noise that can be received by the antenna system. To eliminate any loops in your ground, the concept of applying a centralized ground point as for a home station applies to the mobile configuration as well. In adopting a single point for your vehicle, use the frame/chassis as your "ground stake", if you will. It is the common metal to all other metal points of your vehicle, and provides an excellent ground plane for your vertical element to work against.
Automotive RFI
Large body sheet metal surfaces can float above RF ground if they are electrically insulated by rubber body and engine-cushioning devices designed to absorb road noise, rattles and vibration.
As a result, RF currents can be encouraged to circulate on your hood, trunk, quarter-panels or other body surfaces which can act as small antennas, re-radiating noise signals from the ignition system and associated components.
To solve the problem, follow the steps :
1. Bonding body sheet metal parts to ground by utilizing a 1" wide tinned flat C_O_P_P_E_R braid, connect the :
* Hood and trunk lid to car frame (chassis);
* Exhaust system (at several points along the length of the system) to the frame;
* Bumpers (metallic) to the frame;
* Engine block to the frame.
2. Shielding the spark plug wires.
3. Shielding the distributor and/or ignition module with aluminum foil.
