Frame Antenna & +40dB Amplifier

This system was set up for LF, that is frequencies up to 138khz, but will probably work above that. It's simply a 20-turn square frame with twin flex loudspeaker cable feeding a balanced input preamplifier I've been working on. For a while I tried various circuits, some off the internet, but some didn't work and most were unsatisfactory. This one has received a 136khz signal using WSPR15 from a little under 5000km, considering it was received "through" a 200 ft limestone cliff from the NW (USA) direction I was pleasantly surprised.

The antenna is 1m/39 inches square and currently only about 2.5m/10 feet above ground with 20 turns of thin flexible wire, about 22swg/awg. The feeder cable is cheap twin and probably at least 20pF/metre capacitance. If the preamp is successful with a variety of applications I can post the circuit (jpg file) if anyone is interested.

Please post the .jpg as an image file, riiight here...

Unless it is SO huge

Yeah, I'm curious too. Can't wait.

Please and thank you.

Interested, had a balanced amp made here with BFW 16 A transistors, uHF 1.5 watt types in class A.
Loop single turn 3 meter loop, did well on the low frequencies as well.

Datasheet of the BFW 16 A http://www.alphacron.de/download/hardware/BFW16A.pdf

Nearly there, I'd just like to test it with a tuned ferrite rod (balanced) and a "white stick" (unbalanced). It is simply a high-speed, low noise, differential input op-amp followed by another op-amp as buffer/driver, largely direct coupled. As these things are, I just discovered a better op-amp but I'll post the prototype anyway.

Attachment 16036

This is absurdly simple, the reason for using a balanced input op-amp is that the common mode rejection for signal and power supply is high up to a given frequency, there is negative feedback on both inputs, it is designed to be stable and the input impedance is very high.

I have tried this with tuned and untuned loops, it will work with a tuned ferrite rod circuit as long as the Q is not too high. A feedback problem arises with a Hi-Z (Hi-Q) antenna. I will try a dipole and tinker with the feedback, in my area there is a very high noise level using an unbalanced antenna, though it's semi-rural I suspect it's on the overhead supply cables. Using a single "rod", actually a strip of SRBP about 18 inches long and 2 inches wide with self-adhesive copper tape, there is noise using a conventional up-converter but, by using two as a dipole, this is gteatly alleviated.

I've ordered some different op-amps, particularly balanced input to single output, just to try. You can use other op-amps as the buffer, I found some ancient stock Elantec EL2020s, it partly depends on the highest frequency required. Thus seems to be good to about 500kHz but I haven't tried lowering the gain to see what happens. Both op-amps are available here from $5 or so.

The filters are optional as long as they are dc-coupled, they will have some effect on feedback on IC2 but the voltage capability of this circuit means I don't need them in spite of having the national long-wave, 400kW, 198kHz transmitter not far away.

Reports appreciated, Hugh, M0DSZ.

I made the loop with INA 2185 amp.
Design here:http://www.pa0fri.com/ antenna's then loopantenna.

Thanks 'COR, I've bookmarked PA0FRI's site as he's devised some interesting items.