160 Meter Shunt Fed Tower System - Top Band 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 QRZ.com.

April 27, 2021 Update: Choke Failure and Conversion to an L Matching Network

Lesson learned: a choke on an antenna mounted on a shunt-fed tower can experience substantial RF stress during operation on the top band.

During operation on 160 meters at 1,500 watts, the coiled-coax choke feeding my 40-meter Yagi failed. The photograph shows the damaged choke and burn marks on the boom.

I suspect that coupling among the tower, Yagi, and feed line subjected the choke to excessive RF voltage or current. I was unable to establish the exact failure mechanism.

VE6WZ documented a similar balun failure and suggested that shunt feeding his tower on 160 meters might have been responsible. His experience and mine illustrate a potential problem in these installations, but they do not establish that every coiled-coax choke will fail. 

I replaced the damaged choke with a DX Engineering Maxi-Core 20 1:1 balun and its associated mounting hardware. After the replacement, the tower system’s impedance changed enough that I could no longer obtain a match with the existing omega network. My interpretation is that the original choke and its interaction with the Yagi and feed line had contributed to the loading of the tower system. Without before-and-after impedance measurements, however, I cannot say precisely how that loading changed.

A conventional two-capacitor omega network requires a suitable inductive load at its connection point. Whether it can match a particular tower depends on the impedance presented by the complete installation, including the shunt conductor and tap location. Physical tower height alone does not determine whether it will work. However, the tower must be electrically long to achieve a match. 

 

click to enlarge

Boom Burn

In my installation, replacing the choke shortened the electrical lenghth of the system such that the Omega network could not produce a match. I replaced the two-capacitor omega network with an inductor–capacitor L network (below) which resulted in an indicated SWR of 1:1 at 1.825 MHz. Subsequent high-power contest operation has shown no observed failure or deterioration.

The original work described here preceded my ownership of a vector network analyzer (VNA). A properly calibrated VNA can measure the resistance and reactance presented at the matching-network connection point. Those measurements can then be used to calculate suitable matching-network configurations and component values.

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 may require the L match and not the Omega match to resonate. I have gotten some inquires about how to build the L network.  The variable capacitor is frequently available on eBay used. The coil can be purchased, built on a form, air wound or on PVC. If you need to build the coil, an inductance/capacitance meter is helpful These are available on-line at low cost.