A Tuned Active Receiving Loop

Charles Wenzel presents on his web site a simple and clever tuned receiving loop antenna, which works very well. The antenna consists of essentially three parts: a large wire loop, a varactor diode (voltage-controlled variable capacitor) that forms a parallel resonant circuit with the loop, and an amplifier that transforms the high impedance of the parallel tuned circuit to a low impedance for driving the coax and the receiver.

There are two clever ideas Charles’ design. One is the use of parallel resonance, rather than the series resonance that many active loop circuits use. The use of parallel resonance allows the design to maintain balance without transformers, and I think that it also reduces the significance of Ohmic losses, although they are probably less important in receiving antennas than for transmitting antennas, at least on HF. The other clever idea is the use of a differential video IC, a TL592, as the amplifier. The chip has high-impedance input, high enough bandwidth for HF, and is very easy to use. The amplifier part of the antenna consists of the chip, 5 resistors, and two capacitor. No transformers at all. You can set the voltage gain of the TL592 to values between about 15 and 400; the amplifier sets it to 15.

Charles not only put the design on his web site, he even sent me a few TL592 to try it out! Thanks Charles.

I built the loop out of the same polyethylene-coated aluminum tube that I used for my transmitting loop. I filed away the polyethylene at the ends of a 90cm-diameter loop and drilled holes for screws.

The amplifier is built on a piece of unetched PCB in which I scored pads with a knife. Two large pads with holes are used to connect the amplifier to the loop. I didn’t have the varacator that Charles used, so I used a pair of MV1404′s that I had in parallel, for increased capacitance. The flat ribbon cable that exists on the right carries power and control voltage for the varactors.

Charles’s amplifier includes a relay that can connect an inductor in parallel with the varactor, to cancel out some of the capacitance so that the antenna can tune higher in frequency. I didn’t include one yet.

To tune the antenna, I attached a 10-turn potentiometer to a piece of board to which I also attached the control wires and a 12V jack. Because the voltage limit of my varactors is 12V, I added a 2.7k resistor in series with the 10k potentiometer, to limit the control voltage to a bit over 9V. A 2k resistor is probably more appropriate, allowing the varactors to be biased at up to 10V.

That’s pretty much it, as far as construction goes. I didn’t put the amplifier in a weather proof box, just screwed the loop to it. This is good enough for experimentation, but not for extended use.

How well does it work? It works fantastically. Just outside the balcony, the 90cm loop pulls in lots and lots of signals. One evening I monitored the 7MHz, 10MHz, and 14MHz PSK31 frequencies, and was able to receive many stations on each one of them. I left the software running overnight monitoring 14MHz. The pskreporter.info map below shows the stations I received on all three bands (14MHz in orange, 10 in green, and 7 in blue). I was able to receive numerous European stations, some in the middle east, two in Africa, and several on the east cost of North America. I also logged one Japanese station now shown on the map.

The following night, I left the software running monitoring 7MHz, but with an even simpler receiver, a Softrock Lite II, a $10 kit. The results are just as good, again reaching the east coast of North and South America. The two African stations are spurious reports; they appear to be incomplete call signs that pskreporter.info incorrectly assigned to these countries (nobody else reported them, as opposed to the 14MHz African stations that many monitors reported the night before).

As Charles writes, tuning is not critical. He placed a 4.7k resistor across the parallel tuned circuit, which reduces the circuits Q. I’m curious as to whether the signal-to-noise ratio would improve if I increase this resistor (at the expense of more difficult tuning), but I have not yet tried that. With the 4.7k resistor, my 10-turn potentiometer is more of a liability than an asset.

I was a bit nervous about tuning a receiving antenna. Charles writes that you need to tune for maximum received noise. This is a bit more challenging than tuning a transmitting antenna, where you can tune by looking for a dip in a meter or a LED, but it works. It’s easy when the band is open and you can receive signals and harder when the band is almost dead. You may wonder why one would want to tune an antenna to a band that’s dead, but the point is that you don’t know it’s dead until you tune the antenna. When the tuning is completely off, you receive essentially nothing even when the receiver is tuned to a strong station.

The simplicity of the amplifier seems to come at a certain cost. At some positions of the tuning potentiometer, strong AM station appeared where they don’t actually transmit (that is, in the middle of an amateur band). As I continued to turn the tuning knob, the station would disappear. I think that this happened mostly when the tuning was a bit off, but not completely off. Maybe these were shortwave broadcast stations in the bands above or below the amateur band I was tuned to, but I’m not sure. In any case, it indicates distortion in the amplifier. This is not completely surprising in an amplifier that uses a chip that is not at all designed to sit at the front end of a radio. When I wrote about this to Charles, he wrote that he also experienced some problems, with strong FM stations in his case. It’s a bit annoying, but the antenna is still a joy to use.

I should also add that I forgot to include the 10u capacitor in the circuit. This might have contributed to the overloading; I’ll add it and revisit the issue soon. (Update: I added decoupling capacitors and they didn’t improve the overloading at all.)

There are several other designs on the web for receiving-loop amplifiers. About two years ago I built an untuned loop amplifier designed by John Hawes, which Des Kostryca published on the web. I’ve used it, but I’m not too impressed with its performance. To be honest, I didn’t really compare it to Charles Wenzel’s amplifier when both were using the same loop element, so maybe the loop I was using with Hawes’ amplifier was not good enough. I’ll have to investigate this in the future.

Chris Trask published two designs for receiving loops. Both use series tuning, not parallel tuning, so his designs need to transform the very low impedance of the loop to 50Ω, whereas Wenzel’s design transforms a high impedance to 50Ω. Trask first receiving loop design, published in QEX in July/August and September/October 2003, is an active loop with a 3-transistor amplifier. The circuit is pretty complicated for one using only 3 transistors, using 4 transformers for impedance transformations and for feedback. Trask’s second receiving loop is passive, using varactors for tuning and transformers for impedance transformation, but no transistors or integrated circuits. Chris writes that the designs has very good signal-to-noise ratio that eliminates the need for amplification. I assume that he means that this design is essentially better than his QEX amplifier.

Another example of parallel high-impedance loop tuning is Daniel Wissell’s SLR receiver (in QST, October 1997). The high-impedance tuned antenna is connected directly to the 1.5kΩ-impedance input of an SA602A mixer.

Interestingly, Chris Trask also presents a balanced, tuned, high-impedance active-antenna amplifier, but not for loops but for short dipoles. A short dipole is capacitive, requiring inductance to tune. Therefore, the input network in this amplifier is not suitable for loops. But I assume that the design can be adapted to parallel-tuned loops but simply replacing the input network. Such a design might perform better than Wenzel’s TL592 amplifier, but it is also quite a bit more complex.

In summary, varactor-tuned loops are excellent receiving antennas, and Charles Wenzel’s design is simple and effective. It does appear to get overloaded sometimes.