Hy-Gain TH7DX Antenna
The TH7DX is one of the most popular high-end 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 developed 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 amateur radio operator in a meaningful way.
The antenna is a heavy duty design, intended to cope with the ice and snow loading for those radio operators located in northern hemisphere, as well as extreme heat found in Africa and Australia. The often heard poignant phrase ‘built like a battleship’ rings true.
There were only two formal models produced, the TH7DXS which was supplied with all stainless steel hardware and the TH7DX which has a combination of stainless steel fasteners and some zinc plated clamps.
There was a TH7DXX that did appear in litererature and marketing, but this was never a model. A probable thow-back to the grandfathered TH6DXX antenna.
Intellectal Property / Ownership
sales story to come..
One of the widespread quirks of amateur radio operators is their desire to customise or optimise a piece of equipment to suit their own interests. The TH7DX antenna was not immune to such requests or fiddling. These included:
- Adding extra drain holes to the radiating elements and boom to avoid them filling with water;*Making the broadband antenna operate with higher gain, and lower VSWR in narrower (usually lower) parts of the 20, 15 and 10 meter frequency bands;
- Fitting stainless spring washers and other stainless steel hardware to minimise dissimilar metal corrosion;
- Stacking 2 or more of these antennas into a much larger array;
- Painting the antenna;
- Using NoOx or an aluminium grease to facilitate the numerous electrical joints, aluminium sleeve spalling and joint oxidisation;
- Fitting a larger 1:1 balun to allow the antenna when be used with radio transmitters up to 1,500 watts output.
- and other tweaks that will others may generously highlight or add to this list.