Hy-Gain TH7DX Antenna
The TH7DX is one of the most popular HF amateur radio antennas, originally designed and prototyped by Hy-Gain Electronics in 1981.
The TH7DX was designed as a broad band HF beam in response to compete with the new KLM KT-34 and KT-34XA. At the time, Hy-Gain's biggest tribander was known as the TH6DXX, designed by Howell "Howie" Pabian, W0QAN. Howie had actually let his ham radio license expire and was unlicensed for many years, but eventually got his original callsign back. The TH7DX evolved from the TH6DXX by adding a second driven element and feeding the pair of elements out of phase, similar to a log-periodic array cell of 2 elements. The VSWR was manually optimized to cover as much of each of the 20, 15 and 10 meter bands with under 2.0 VSWR. The azimuth pattern was also optimized at the same time to produce F/B higher than 20 dB across as much of each band as possible.
It is important to recognise this antenna was develped prior to the development of accessible antenna modelling and optimisation software! As a broadband antenna, the intent was to meet the needs for pretty much every anateur radio operator in a meaningful way.