These tests show that the signal strength of a local station can be many tens of decibels weaker than from a DX station.
For the last several years I have been operating WSPR from N6GN on the 600 m through 70 cm Amateur Radio bands. I’ve found the large number of geographically spaced participants combined with the round-the-clock worldwide reporting make the WSPRnet.org web page and database an excellent tool for investigating propagation, and also for measuring and analyzing Amateur Radio station performance. Among several local WSPR stations we noticed database entries that showed stations 1,000 to 2,000 miles away spotting our transmissions with significantly larger signal to noise ratio (SNR) than did local stations as close as four miles away
. Assuming that the HF ionospheric propagation path follows an inverse-square law, one where the wave front expands spherically, it would seem at first glance that there should be a greater penalty for the greater distances. Distant signals (DX) should be much weaker than local signals. From previous propagation experiments at UHF I knew that foliage and slight terrain variations in the propagation path could be responsible for extremely large attenuation at shorter wavelengths.
It’s also clear that HF antennas are usually closer to the ground, in terms of wavelength, than they are at UHF. Perhaps the peak of the main lobe of our HF antenna patterns getting pushed well above the horizon. This increases the signal incident on the ionosphere at the expense of signal levels measured at the local horizon. The question became, “How much of this greatly increased attenuation ofthe local signal revealed by WSPR was due to foliage attenuation and how much was due to the HF antenna elevation pattern?” Our measurements using a quadcopter, see Figure 1, were designed to help answer that question. …..READ FULL ARTICLE