This is a type of antenna known as a box loop that is very useful for AM broadcast DXing. It is a highly directional highly selective antenna. The directionality in particular has a very sharp strong deep null that is useful for rejecting interfering stations, and the selectivity minimizes receiver overload by strong local stations on other frequencies.
The antenna I built used 1×6 wood planks about 30 inches long, notched half-way through in the center with a notch as wide as the plank was thick to allow them to be fitted together in this “X” pattern. The rigidity of this structure is very important. Use a hard wood like walnut or oak, not a soft wood like pine. Make the notches fit is very tight and then glue them together with wood glue.
The variable capacitor mounted near the “X” is a type used in old tube radios, something on the order of 50pf-365pf. With these dimensions eight turns of #16 copper enameled wire was just right to allow the antenna to cover 530-1600 kHz. If you build it larger then fewer turns may be required, and conversely if you make it smaller, more turns will be required.
The wire used is a trade-off between “Q” (how selective the antenna is) and ability to actually wind the wire tight on the form. For example, #12 wire would result in an even more selective antenna but you’d break the form trying to wind it tight enough to be rigid. In my experience #16 is a good compromise.
The type used in most transistor radios with the plastic spacers provide neither adequate range nor the ability to easily fine tune, The best type is the type out of the old radios that has a built in planetary drive so that it takes three or four revolutions to open or close the capacitor completely.
The turns should be spaced about 1/2 inch apart. Make notches in the end of the planks to accommodate each turn so that the spacing remains fixed.
Connect one end of the loop to the “ground” (chassis) side of the capacitor. Connect the other side of the loop to the terminal on the capacitor that is the non-ground side.
On the first loop from the ground end of the winding, make a tap consisting of a wire leading back to the center of the “X”. Attach the ground shield of your coax to the ground side of the capacitor and the “tap” at the first loop to the center of the coax.
The idea of having the coax feed off a tap of just one turn is that it allows very little loading of the antenna and thus the “Q” remains high providing good selectivity and sensitivity at the resonant frequency.
A large wooden or plastic (something non-conductive) table is a good place to work with this antenna and your receiver. Connect the other end of the coax to your receivers antenna and ground connector. Having a physical ground on the receiver will improve the systems overall stability by making the receiver end of the arrangement insensitive to body capacitance and other stray effects.
Set your receiver to a weak station around the center of the dial and tune the variable capacitor on the antenna. You will encounter a point where the signal is enormously stronger and if you build the antenna to the specifications given here that point will be very sharp. The selectivity, if built as described here, will be sufficient that it will actually attenuate the sidebands of the station you are tuned to. This can be helpful in situations where you have adjacent channel interference and your receiver is not sufficiently selective.
As described, this should cover pretty much the entirety of the old AM band, it may not cover much above 1600 kHz because the old style capacitors had too great a minimum capacitance.
Most of the old broadcast radio capacitors had at least two sections, one that tuned the local oscillator and one that tuned the input to the mixer. On better receivers with one or more RF stages, there would be additional sections for each RF amplifier stage.
The section for the oscillator is smaller than the others. One thing you might try to extend the antenna to cover the 1600 – 1710 kHz section is to take one turn off the antenna which will raise the overall frequency range of the antenna, and then add a switch to cut in another capacitor section to cover the lower range.
By adding more turns and additional capacitance, this type of antenna can also be made to work efficiently down into the long wave portion of the radio spectrum. This is a good option for a long-wave antenna when you don’t have room for larger alternatives.
Back when I was using this antenna, I was in Seattle WA, and at the time there was nothing on either 1200 kHz or 1210 kHz locally. At night, the only station on 1200 kHz was WOAI from San Antonio TX. With just the built in ferrite rod antenna I would get maybe 1-2 on my S-meter, but with this antenna I could peg the S-meter with the same signal.
Being the somewhat insane type that I am I also tried a modification of this basic design where I took another tap at 1/2 turn from the ground end, and wired a one transistor amplifier as an oscillator using the ground, 1/2 turn tape, and 1 turn tap in a Hartley oscillator except the only capacitor was the variable across the entire coil. I put a potentiometer between the 1-turn tap and the base of the transistor to allow me to adjust the “gain” so that I could adjust it to be just on the verge of oscillation so instead of being an oscillator it became a regenerative amplifier.
I included a link to a site describing Hartley oscillators in case the reader is not familiar with them. The bottom turn of the coil with the half turn tap was used as the entire coil for the oscillator which wasn’t allowed to have quite enough gain to oscillate. I used a bipolar transistor with bias, but you might have better performance with an FET as shown on the site I linked to. Whichever method you use, you will want to insure that the transistor operates at very low power so that if it does break into an oscillation you do not damage your receivers front end.
This antenna didn’t need more gain, but the regenerative amplifier allowed me to narrow the bandwidth still more (to several hundred hertz if I got it right on the edge of oscillating) and I could adjust how narrow the bandwidth was by the adjusting the pot.
The reason I added this is that I was trying to receive some foreign stations that operated on frequencies other than the 10 kHz allotments used in the United States, things like 654 kHz, and the additional selectivity was desired. I was never successful though at receiving any of these, too much power line noise and other noise sources to receive very weak signals where I was.
Without the regenerative amplifier, I also tried using this antenna for transmission but because the impedance was so high across the full coil of antenna, the voltages across the capacitor exceeded the value required to arc it over at very low power levels so this is not useful for anything more than a few hundred milliwatts unless you got capacitors with a huge plate spacing.