Response of Coaxial Cable Over a Finite Ground Plane to an External Electromagnetic Field

The resonance p ro p e r t ie s o f a coaxial cable over a r e s t r i c t e d ground “plane" s t r u c tu re to an ex­ t e r n a l ly impressed f i e l d are d iscussed. Experimental data have been obtained as the s iz e o f the ground s t r u c tu re was var ied . The advantages o f m ult ipo in t grounding are ind ica ted in a q u a n t i t a t iv e manner. 1. In troduction Modern communication and data processing systems f requen t ly use coaxial cables fo r s ignal transm ission . These cables inhe ren t ly provide a s i g n i f i c a n t degree o f p ro tec t ion a g a in s t unwanted e x te rn a l ly impressed e lectromagnetic f i e l d s . With s in g le braided ca b le s , however, occasions arisewhere the p ro tec t ion i s inadequate. In such cases the condition of “grounding" o f such cables may s i g n i f i c a n t ly a f f e c t the sh ie ld ing e f fec t iv e n ess o f the cab le . This paper i s concerned with the e f f e c t s o f such grounding techniques. O rd in a r i ly , th e o re t ic a l considera t ions o f th i s problem are based on the assumed presence of an in ­ f i n i t e p e r f e c t ly conducting ground plane. With such a s t r u c tu re i t i s common to apply image theory as long as the cable i s not too f a r from the ground plane in terms o f the cable leng th . According to image theory , the s ing le -w ire transmission l in e over the i n f i n i t e p e r fe c t ly conducting ground can be t r e a te d as a twowire transm ission l i n e without ground as shown in Fig. 1 [ l ] . Direct field comparable with t h a t o f the cable i t s e l f . In t h i s s tudy , an at tempt i s made to determine the r e l a t i o n between the s iz e o f the "ground" con­ ductor and the in te r fe re n c e produced by an external e lectromagnetic f i e l d by p rac t ic a l measurement. In the measurements the leng th and he igh t o f the coaxial cable and ground conductor were kept co n s tan t while varying the width o f the ground conductor only. For the purpose o f t h i s s tudy , the sh ie ld ing e f fec t iv e n ess to ex ternal e lectromagnetic f i e l d s i s defined by the r a t i o V^/E, where Vt, i s the voltage across the i n t e r i o r load impedance and E i s the e l e c t r i c f i e l d s treng th o f the ex ternal e l e c t r o ­ magnetic f i e l d inducing V^. A 4-meter length o f RG-58C/U coaxial cable was used in the experimental se tup. The magnitude o f the surface t r a n s f e r impedance Zt was obtained from experimental data [2 ] as given below: Zt = 0.0286 f 0 -462 0 .5 < f < 10 .0.0074 f 1 ' 05 10 < f < 50 ( 1) where f is the frequency in MHz. 2. Experimental Arrangement The experimental arrangement i s shown in Fig. 2. The 4 meter length o f cable i s located a t a