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HUGH
01-20-2019, 03:34 PM
16069

I have already posted this one which has the obvious disadvantage of requiring feedback on both inputs so it is unlikely to be much good except with an untuned loop where it functions well. It is not necessary to use both outputs from op-amp 1 but there was more voltage out using both.

The reason for pursuing balanced inputs is to avoid any coax or other sources. I know there are lots of circuits using loop antennas but they all seem to have an unbalance en route to the receiver. I'm running a loop 30ft up in a tree with 200 ohm ribbon feeder with this circuit.

16070

This was the next attempt using an opamp usually used for microphone preamps. Both inputs require tieing to ground via not less than 1.2k but if you can create a midpoint in the antenna that will do the trick and the two resistors R1 and R2 are not required. Creating the centre-tap though is not always possible. This is good with a single turn loop anyway.

16071

This is the best so far, input impedance is high so the Q of a tuned antenna, ferrite rod for example, is high. There is only a single output from the op-amp and if you use the gain resistor values here it may well require a filter between the two op-amps. I've reduced the gain of op-amp 2 to unity because even my good communcations receiver wasn't entirely happy even with 40dB attenuation. The wrong supply polarity will probably kill the AD8130. If the gain of op-amp 1 is reduced to ten it's happy up to 10MHz. Many devices will work at op-amp 2 but I like the idea of one which works as a line driver.

N8YX
02-07-2019, 03:09 PM
I'm looking at using a pair of loops at quadrature for an ELF-VLF sferics receiver array - maybe a steerable loop for AMBCB work. What frequency ranges have you tested your designs with, Hugh?