Determine the received strength of a transmitted signal
I’ve been an SWL for 30+ years now. In that time, I’ve had probably dozens of HF antennas. My first was a simple Random Wire (aka longwire), about 75 ft long, going from the shack (my second story bedroom as a kid) to a tree. Fed with single conductor wire to the antenna input of my Radio Shack DX-160. It worked reasonably well, I heard lots of stations, and back then there were far fewer sources of QRM in the house. We didn’t have computers or plasma TVs to deal with, nor dozens of switching power supplies. Just horizontal sweep harmonics from TV sets and the occasional dimmer switch.
Eventually, I discovered dipole antennas. By this time I was in to listening to pirate radio stations, so I put one up cut for about 7400 kHz, since that is where most pirates were operating. Dipoles are inherently narrow band antennas, and I eventually had several including one cut for around 6200 kHz for Europirates, since that is where they tended to operate. This was a folded dipole made from standard 300 ohm TV twinlead. The ends were shorted, and a 4:1 balun was connected to the center of the lower conductor, since the antenna was theoretically 300 ohms impedance. From memory, this antenna worked very well.
At one point, when I was more involved with ham radio, I put up a G5RV antenna. I don’t recall spectacular results with it. I spent most of my time on 15 meters CW, so I ended up putting up a 15 meter band dipole, which worked quite well as expected. I had a lot of contacts with Latin America.
Several years ago, I discovered the terminated, tilted, folded dipole (T2FD) antenna. This is a very broadband antenna, with a typical claimed bandwidth ratio between highest and lowest frequency of about 5:1. In my case, I put up a 132 ft long T2FD, which was designed for about 2.5 to 15 MHz. This was at the bottom of the sunspot cycle, so higher frequencies were not of much interest. (Of course, several years later, we still seem to be at the bottom of the solar cycle) This antenna was fed with 75 ohm coax into a 9:1 balun. I got very good results with it for HF, and it worked reasonably well down to the upper end of the MW broadcast band. Performance was very poor, as expected for the rest of the MW band and for longwave.
One thing I immediately noticed about this antenna, vs the various dipoles I had, was that the very low noise. It did not seem to pick up QRM as much as the dipoles. Signal levels of stations were also lower, but, more important, signal to noise ratios were higher. There has been a lot of speculation and claims about the low noise characteristics of various forms of loop antennas. This may explain the excellent results I had years ago with my folded dipole for 6200 kHz.
I finally had an antenna that worked well over most of HF, which meant that rather than switching in various dipoles depending on where I wanted to listen, I could just leave the one antenna connected. Plus I had generally lower noise levels. But I did not have something that worked well down into the MW band.
The next antenna I discovered was the sky loop antenna. The sky loop is a giant loop antenna in the horizontal plane. You run it as high as you can, often around the perimeter of your yard. The exact shape is not important, in my case there are about a dozen or more supports around the various sides of the antenna, and it most closely resembles a trapezoid in shape, with a perimeter of about 635 feet. Yes, it’s a huge antenna. MW reception is excellent as expected, semi-local MW stations are in the S9+60 dB range.
My T2FD was damaged in a storm around the time the sky loop was installed. I hope to get it back up shortly, to run some better comparisons between the two antennas. Also, the T2FD may be better for the higher bands. Should solar activity return to reasonable levels, I may install a shorter T2FD for 19 to 10 meters.
Based on my recent experience, I am certainly sold on loop antennas in their various forms. The lower noise pickup characteristics are reason enough to consider building one the next time you’re considering putting up a new antenna.