We think of radio navigation and direction finding as something fairly modern. However, it might surprise you that direction finding is nearly as old as radio itself. In 1888, Heinrich Hertz noted that signals were strongest when in one orientation of a loop antenna and weakest 90 degrees rotated. By 1900, experimenters noted dipoles exhibit similar behavior and it wasn’t long before antennas were made to rotate to either maximize signal or locate the transmitter.
Of course, there is one problem. You can’t actually tell which side of the antenna is pointing to the signal with a loop or a dipole. So if the antenna is pointing north, the signal might be to the north but it could also be to the south. Still, in some cases that’s enough information.
John Stone patented a system like this in 1901. Well-known radio experimenter Lee De Forest also had a novel system in 1904. These systems all suffered from a variety of issues. At shortwave frequencies, multipath propagation can confuse the receiver and while longwave signals need very large antennas. Most of the antennas moved, but some — like one by Marconi — used multiple elements and a switch.
However, there are special cases where these limitations are acceptable. For example, when Pan Am needed to navigate airplanes over the ocean in the 1930s, Hugo Leuteritz who had worked at RCA before Pan Am, used a loop antenna at the airport to locate a transmitter on the plane. Since you knew which side of the antenna the airplane must be on, the bidirectional detection wasn’t a problem.
[Peter] points out that spies like the [Krogers] needed to report British Navy secrets like the plans for a nuclear boomer sub to Russia but didn’t want to attract the attention of their neighbors. In this case, the transmitter itself was so well-hidden that it took MI5 nine days to find the first of them. Clearly, then, there wasn’t a giant antenna on the roof. If there had been, the authorities could simply follow the feedline to find the radio. A concealed spy antenna might be just the ticket for a deed-restricted ham radio station.
The antenna the [Kroger’s] used was a 22-meter wire in the attic of their home. Keep in mind, the old tube transmitters were less finicky about SWR and by adjusting the loading circuits, you could transmit into almost anything. Paradoxically, older houses work better with indoor antennas because they lack things like solar cell panels, radiant barriers, and metallic insulation.
Like many people, [Peter] likes loop antennas for indoor use. He also shows other types of indoor antennas. They probably won’t do as much good as a proper outdoor antenna, but you can make quite a few contacts with some skill, some luck, and good propagation. [Peter] has some period spy radios, which are always interesting to see. By today’s standards, they aren’t especially small, but for their day they are positively tiny. Video after the break.
While we can’t argue that FM has superior audio quality and digital streaming allows even higher quality in addition to worldwide access, there’s still something magic about hearing a weak and fading AM signal from thousands of miles away with nothing between the broadcaster’s antenna and yours. If you can’t have a big antenna — or even if you can — a loop antenna can help your big antenna fit in less space. In the video after the break, [TheOffsetVolt] covers an AM loop and shows how it can pull in distant AM stations.
Continuing with the educational radio he’s talked about before, he adds a loop antenna that is two feet on each side of a square, making it four square feet in area. Although he calls it an amplifier, it’s really just a passive tuned circuit that couples to the radio’s built-in antenna. There’s no actual connection between the antenna and the radio.
We aren’t sure if the reradiation explanation is really what’s happening, or if it is just transformer coupled to the main antenna. But either way, it seems to work well. You can think of this as adding a preselector to the existing radio. Loop antennas are directional, so this design could work as a direction finder.
[K5ACL], aka [SignalSearch], recently brought his active receive loop antenna in off the roof to give it a checkup and perform any necessary maintenance. While it was in the shack, he took the opportunity to discuss how well it would perform indoors. The verdict? Not ideal. He’d mount it 50 feet away from the house if the HOA would let him.
Houses, and subsequently most ham shacks, are filled with noise sources that interfere badly with HF. So after spending a minute or so listening on an SDR, [K5ACL] demonstrates another use for this type of tightly-tuned antenna—as a noise detector.
The main culprit in [K5ACL]’s house is the ceiling light that’s right there in the shack. You can see the noise striping the waterfall as he turns it on and off. But the noise from the light is small potatoes compared to some other common household items, like those power line adapters that turn house wiring into networking cable. Those produce so much noise that even an active loop is really no match. Stay tuned after the break to watch [K5ACL] work the bands through the noise.
We don’t know if [OH8STN] has a military background, but we suspect he might since his recent post is about a “DIY Man Portable Magnetic Loop Antenna.” “Man-portable” is usually a military designation, and — we presume — he wouldn’t object to a woman transporting it either.
[OH8STN] started with a Chameleon antenna starter kit. This costs about $100 and is primarily a suitable variable capacitor with a 6:1 reduction drive premounted and soldered. Of course, you could source your own, but finding variable capacitors that can handle transmit duty (admittedly, these can apparently handle about 10 W continuous or 25 W on single sideband) can be tricky, especially these days. Although he started with a kit, he did modify the antenna to switch between two different sets of ham radio bands. You can see the antenna in the video below.
Loop antennas aren’t ideal–but neither is any other small antenna. Because the loop is tightly tuned to a particular frequency, it requires retuning for even relatively small frequency changes, even though it can operate on many different frequencies. If you want more technical details, you might enjoy this recent presentation from [W4RAX]. The links at the end are worth checking out, too.