Shunt Feed

The best and most complete source of information on shunt feeding a tower is the ON4UN’s book, Low Band DXing. It contains a wealth of information, several shunt feed matching methods, and verifies that a vertical needs to be at least an electrical quarter wave in length at the lowest frequency being used.

After contemplating these requirements, I decided to take the easy way out. I added a drop wire to a cross spacer at the bottom of the rotator plate. You can see the cross spacer, drop wire, and extra halyard in the picture of my antenna installation. The drop wire is actually three lengths of RG62 coax twisted together. This drop wire is cut about 15 feet above ground and attached to 450 ohm ladder line. The ladder line is routed to a homemade tuner.

The system works well on all bands and is more effective than the inverted vee I was using before.

MORE ON THE SHUNT FEED FIASCO
EZNEC is a neat program. It allows you to compare antenna performance on a computer. It also requires you know what you are doing. Looking at radiation patterns alone can be misleading. I thought that I had the perfect vertical radiation pattern for my shunt feed tower. It was a perfect pattern with launch angle at about 27 degrees but on closer examination the gain was down to -2.5dbi! Maybe I just wanted to believe that it worked better than the old inverted vee.

Luckily I installed three drop wires which were all bundled together for the first shunt feed attempt. Using EZNEC I remodeled the system with one of the drop wires acting as a top load. The top load wire is attached to the tower at the 45 foot level. I lengthened the top load wire to be 69 feet long and ran it to a tree in the side yard. The end of the wire was about 12 feet off the ground. With this setup EZNEC reported a takeoff angle of 90 degrees, but at 27 degrees the gain had gone from -2.5dbi to 1.5dbi. A 4dbi improvement!

I always thought that if you could get power into an antenna it would radiate regardless of what you were using for an antenna. That is still true but some radiators are more efficient than others. Getting the antenna to take power is just half the battle. The rest of the problem is making sure that the antenna makes effcient use of the power that is fed to it. Oh sure, the original shunt fed tower worked. Signal reports were as good or better than any I had received while using the inverted vee. The problem was that it was not working a well as it could. The new top loading wire brings the system to resonance at 3.9mHz. With an antenna tuner you don’t need a system to be in resonance to take power and radiate, but if you want maximum efficiency, a resonant condition is the best solution.

True the new system with top load wire is a cloud burner with 5dbi launched at 90 degrees. However, it has 1.5dbi to offer at 27 degree launch angle where the original setup only delivered -2.5dbi.

So for transmitting it should be better than the previous setup. For receiving it may not be as good because now it is going to much more sensitive to noise coming in at 90 degrees with a gain of 5dbi.

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