Thursday, December 13, 2018

160 Meter Shunt Fed Tower System - Vertical Antenna

I wanted to use my tower as a vertical on 160 meters. The tower is an LM-470D motorized 70 foot crank-up supporting a ten foot mast and the following antennas: KT34XA, 40-2CD, 3el 6m, and 80 meter quarter wave sloper This system is designed to allow the tower to be raised and lowered each day. The 3 foot metal stand-off arm is attached at the third level, approximately 50 feet fully extended, and the pipe is 15 ft long. The only purpose of the pipe is to stabilize the gamma wire so that it doesn't get tangled in the tower when it is raised or retracted. There are other ways to accomplish it, but this system is simple and has proven reliable over a 20 year period. If a fixed height non-retracting tower is used, the pipe is unnecessary. I mounted the omega match inside on the barn wall to get it out of the weather. Prior to moving it to the barn, I had it at the base of the tower, first in a tupperware style container and then in a heavy duty irrigation box. In both cases, I had to regularly deal with animals, insects, and moisture. It worked, but it was high maintenance compared to this system, which has been "no maintenance". There is about 100 sq feet of aluminum sheet tied to the base of the tower, which is earth grounded. Two 1/4 wave elevated radials are attached at the top of the first section, about 15 feet off the ground. These radials are not grounded and a tap wire runs from the radials to the top feed through insulator. This system has been in use in more or less this configuration for 20 years with about 120 DXCC countries confirmed through casual DXing on the Top Band. See photos of this setup at

20210427 Update: Melted Choke forces Conversion to L Match
Little is known about the risks of shunt feeding a telescoping tower for 160 but we have some anecdotal data indicating that care is needed. In my case, all was fine until RF decided it had to find a better way to the 40 meter yagi boom and or elements  at the top of the stack. This is just a guess, but perhaps the driven element of the yagi "wanted" to be integral part of the system? A problem then arises when large currents reach the choke, which is designed to block such current flow. You can see below what 1500 watts of shunt fed power on 160 meters did to the choke! Check out VE6WZ's you tube and blog posts and you'll find a similar story.  Apparently, the 40m choke added some inductive loading, making the tower electrically longer.
click to enlarge

My options at this point were to install a relay to ground the driven element during 160 operation or to replace the choke with a proper high power balun and hope that it wouldn't burn up. I decided to go for a new balun and chose the DxEngineering MC20 with the mounting hardware that's offered for it. After installation and roval of the coax choke, the electrical length of the tower got shorter and I could no longer match it.  The Omega match will only match an electrically long antenna!  (I have read posts on-line by hams wondering why they can't match a short tower on 160 with an omega match. If you're thinking of doing this, the omega match won't work.) The solution was to convert to the classic L match. This resulted in a perfect match. Six months of subsequent rigorous high power testing during contests has not shown any failure or deterioration of the system.

The L match mounted on the wall of the barn. 
The 2kw inductor is tapped at 7 uH and the capacitor is a 90-150 pF variable

My conclusion is that stacks under 80 feet in height will most likely require the L match and not the Omega match to resonate. I have gotten some inquires about how to build match.  The variable capacitor is frequently available on ebay used. The coil can be built on a form, air wound or on pvc. You might need to build the inductor. An inductance/capacitance meter is helpful These are available on-line at low cost. 

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