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Loop antenna matching question
"John KD5YI" wrote in
: .... I seem to recall that a small loop has less inductance than a large loop. I also seem to recall that the transformation between them is a function of their inductances. So, a small loop would transform the larger loop's impedance to an even lower impedance. Is this not so? You can think of it in terms of inductance, but visit what inductance really means (ie the relationship between inductance, current and flux). I gave you an explanation in terms of area of the two loops. If your feed loop os 20% of the area of the main loop, it intercepts 20% of the flux, the voltage induced in the feed loop is 20% (1/5) of the main loop voltage, the impedance is transformed by 1/(1/5^2)=25, so a 2 ohm main loop is transformed to a 50 ohm load at the feed loop feed point. Now you can expand that and write it in terms of inductance... but Mike can measure the loop dimensions with a ruler and calculate area. Measuring inductance of the components is fraught with problems. So, if Mike works out what the radiation+loss resistance is of his main loop, divides it by 50, takes the square root, it gives him the portion of the main loop area to be occupied by the feed loop. Initially, one might make the area of the feed loop 20%-50% larger and squash it to fine tune the transformation ratio. Owen |
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