02-03-2016, 08:07 AM
(This post was last modified: 02-03-2016, 08:27 AM by Radio Fixer.)
Well Jeffrey the theory of magnetic loop aerial is here and for making a single turn loop probably gives the best information.
http://sidstation.loudet.org/antenna-theory-en.xhtml
Now no where have I read this so it maybe a Gary myth but I think more interference is in the electric field than in the magnetic (mainly picked up by the loop) and so by using that the signal to noise ratio of the loop is better than a long wire and in my case more convenient.
Yes! the signal goes down and that's where the amplifier comes in. Typically these loops are made for receivers with 50 Ohm inputs and hence the choice of 100 Ohm resistors for the amplifier collectors. It might not look as if it has much gain, and I'm sure I'm about to write stuff that many already know but bear with me for those that don't. The emitter of the long tail pair doesn't move to differential inputs. As the current goes up in one transistor it goes down in the other and in the ideal balanced set up the emitter has no signal across the 'tail'. The emitters are said to be at AC ground. Thus the gain of each half is approximately re (the internal transistor emitter resistance) divided into the collector R. There is an ancient rule of thumb that re =25 / I where I is in mA. Each transistor is passing about 30 mA but lets keep it simple and say re = 1 Ohm. So the gain of each side of the pair is 100 /1 or about 40 dB and for the pair add another 6 maybe (well un terminated). This is substantial and the loop performance without the amp would be of little use.
My window loop is not going to be as good as my Wellbrook that according to , I believe Trevor, is ferrite filled ? and is also on a rotator( and way down the garden and far from Main)s but for normal MW and LW stations I don't find that too much of an advantage, perhaps because they are mainly omni directional. I will try rotating the window aerial and see what difference that makes, if I can do it with breaking something; very little room in the Den.
Will add more as I play or try suggestions, thanks Gary
http://sidstation.loudet.org/antenna-theory-en.xhtml
Now no where have I read this so it maybe a Gary myth but I think more interference is in the electric field than in the magnetic (mainly picked up by the loop) and so by using that the signal to noise ratio of the loop is better than a long wire and in my case more convenient.
Yes! the signal goes down and that's where the amplifier comes in. Typically these loops are made for receivers with 50 Ohm inputs and hence the choice of 100 Ohm resistors for the amplifier collectors. It might not look as if it has much gain, and I'm sure I'm about to write stuff that many already know but bear with me for those that don't. The emitter of the long tail pair doesn't move to differential inputs. As the current goes up in one transistor it goes down in the other and in the ideal balanced set up the emitter has no signal across the 'tail'. The emitters are said to be at AC ground. Thus the gain of each half is approximately re (the internal transistor emitter resistance) divided into the collector R. There is an ancient rule of thumb that re =25 / I where I is in mA. Each transistor is passing about 30 mA but lets keep it simple and say re = 1 Ohm. So the gain of each side of the pair is 100 /1 or about 40 dB and for the pair add another 6 maybe (well un terminated). This is substantial and the loop performance without the amp would be of little use.
My window loop is not going to be as good as my Wellbrook that according to , I believe Trevor, is ferrite filled ? and is also on a rotator( and way down the garden and far from Main)s but for normal MW and LW stations I don't find that too much of an advantage, perhaps because they are mainly omni directional. I will try rotating the window aerial and see what difference that makes, if I can do it with breaking something; very little room in the Den.
Will add more as I play or try suggestions, thanks Gary
