Frame Antenna & +40dB Amplifier

[HUGH]

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.

[WØTKX]

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

Unless it is SO huge

[WZ7U]

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

Please and thank you.

[PA5COR]

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

[HUGH]

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.

[HUGH]

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.

[PA5COR]

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

[HUGH]

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