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There must be at least two variable components on every antenna tuner.
This is because every impedance has two components, R and jX. With a magloop one variable is the loop tuning capacitor. The other variable is the diameter of the small coupling loop. It so happens that the setting of the coupling loop diameter does not change very much from one band to another. Users who never have to touch it don't realise how lucky they are compared with other types of tuner which have at least two knobs. The main loop and coupling loop are like the primary and secondary windings on an impedance matching transformer. The turns ratio cannot be changed but the diameter of the coupling loop can be. The greater its diameter the greater the coupling. It so happens that for loops of typical dimensions the ratio of the two loop diameters is approximately 5-to-1. Optimum setting does change a little from one band to another as can be seen using program MAGLOOP4 from website below. The coupling loop is often screened inside a short length of coax line because the designer saw it done that way in a magazine. Or he just copied somebody else's without having any reason. If there's any noise to be picked up it will be 25 times stronger on the main loop and nobody bothers to screen THAT. What might be picked up on the very small coupling loop is quite insignificant in comparison. And it's much easier to adjust the coupling between the two loops if one of them is just a self-supporting length of wire no thicker in principle than the diameter of the coaxial inner conductor. Instead of reducing its diameter it can be rotated a little or bent out of the plane of the main loop to have a similar effect. Or squashed flatter to reduce the area enclosed. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
#12
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Reg,
Thanks for the confirming information. I will be making a trip to the attic soon. I presently have the 20m loop fed with a Gamma feed which is not too critical but de-mystifying the "plain" loop coupling seems to be the way to go with more variables in geometry. Hams like lots of things to twiddle. Dick N3HKN Reg Edwards wrote: There must be at least two variable components on every antenna tuner. This is because every impedance has two components, R and jX. With a magloop one variable is the loop tuning capacitor. The other variable is the diameter of the small coupling loop. It so happens that the setting of the coupling loop diameter does not change very much from one band to another. Users who never have to touch it don't realise how lucky they are compared with other types of tuner which have at least two knobs. The main loop and coupling loop are like the primary and secondary windings on an impedance matching transformer. The turns ratio cannot be changed but the diameter of the coupling loop can be. The greater its diameter the greater the coupling. It so happens that for loops of typical dimensions the ratio of the two loop diameters is approximately 5-to-1. Optimum setting does change a little from one band to another as can be seen using program MAGLOOP4 from website below. The coupling loop is often screened inside a short length of coax line because the designer saw it done that way in a magazine. Or he just copied somebody else's without having any reason. If there's any noise to be picked up it will be 25 times stronger on the main loop and nobody bothers to screen THAT. What might be picked up on the very small coupling loop is quite insignificant in comparison. And it's much easier to adjust the coupling between the two loops if one of them is just a self-supporting length of wire no thicker in principle than the diameter of the coaxial inner conductor. Instead of reducing its diameter it can be rotated a little or bent out of the plane of the main loop to have a similar effect. Or squashed flatter to reduce the area enclosed. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
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