Baluns - Bal uns: What They Do And How They Do lt By Roy...

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Unformatted text preview: Bal uns: What They Do And How They Do lt By Roy lV. Lewallen,* IVTEL * 5 4 7 0 S W 1 5 2 n d A v e . , B e a v e r t o n , O R 9 7 0 0 7 I ' v e a l w a y s b e e n a b i t b o t h e r e d b y b a l u n s t since I was never sure what they are supposed to dot let alone hotv they might go about doing it. The m a j o r i t y o f a r t i c l e s d e a l w i t h v a r i o u s w a y s o f building and testing baluns, or the advantages of one type over another, but almost never a word about when or why a balun is neeessaryr if at all. Like most amateurs, there have been few occasions when I have been able to tell if a balun has any effect on a n a n t e n n a s y s t e m , a n d w h e n i t h a s , t h e e f f e c t hasn't always been good! The turning point came when I was trying to measure the resonant frequency of a folded dipole through a one-vravelength coaxial line. The bridge null varied a great deal as I moved my h a n d a r o u n d t h e c o a x i a l c a b l e , o r i f t h e l i n e o r bridge was moved. A hastily eonstructed balun in- stalled at the center of the dipole eliminated the p r o b l e m . B u t w h y ? I f o u n d a b r i e f , b u t c l e a r e x p l a n a t i o n o f o n e phenomenon involved in a paper by lllaxwell' W2DU.U However, many questions remained. This led me to an i n v e s t i g a t i o n o f j u s t h o w b a l u n s a r e s u p p o s e d t o work, and what problems they are supposed to cure. One surprising conclusion I found from my research is that one popular type of balun' when properly designed and used in an antenna system, may not s o l v e t h e p r o b i e m s t h a t b a l u n s & r e e x p e c t e d t o solve. Other results indicated that the type of feed Iine (balanced or unbalanced) has little to do with how well a system is balanced. ln order to verifyt o r r e f u t e , t h e t h e o r e t i c a l r e s u l t s , s e v e r a l experiments were earefully set up and run, and the data analyzed. The result is a much elearer view of the operation of baluns in antenna systems, and some definite t'dostt and t'donttst' regarding their use. What Prpblems ale Baluns fupposed to Solve? B a l u n s u s u a l l y s o l v e p r o b l e m s c a u s e d b y a n imbalance. An imbalance of what? To answer this q u e s t i o n , w e n e e d t o l o o k a t c u r r e n t f l o w i n transmission lines. I n a c o a x i a l c a b l e , t h e c u r e n t s o n t h e i n n e r conductor and the inside of the shield are equal and opposite. This is because the fields from the two eurrents are confined to the same spaee.\2 With the presence of skin effeet, a different current flows on the outside of the shield than on the inside.\] T h e c u r r e n t o n t h e o u t s i d e , i f s i g n i f i c a n t , c a u s e s the feed Iine to act like an antenna, radiating a field that is proportional to this current....
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This note was uploaded on 10/21/2011 for the course EE 4101 taught by Professor Yeotatsoon during the Spring '11 term at National University of Singapore.

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Baluns - Bal uns: What They Do And How They Do lt By Roy...

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