Newsletter Article

A Simple Topband Upgrade That Made a Big Difference

By Bill N6WS

So where do I start?

The need for a new 160-meter antenna began when I replaced my Cushcraft X-9 and X-940 with a SteppIR DB36. Prior to installing the DB36, I had been using a quarter-wave sloper for 160 meters fed at the 40-foot level on my 54-foot tower. Once the DB36 went up, the sloper was no longer practical.

My first idea was to install a 60-foot vertical made from 3-inch aluminum irrigation pipe. The ideal location would have been in the quarter-acre pasture in front of our house. Unfortunately, I knew that proposal would likely be a non-starter with my better half, so I began looking for a less visible solution.

About that time another member of the Southern California Contest Club posted a Cushcraft MA-160V vertical for sale. I bought it immediately and then began figuring out where I could install it without making it too obvious.

After assembling the antenna, I realized it could be mounted on a corral fence post where it would not be very noticeable. From the road it was hidden by a tree, and from the house it blended in with the surrounding trees.

Once installed, the antenna showed a VSWR of less than 2:1 from 1810 to 1860 kHz.

The first real test came during the 2018 CQ Worldwide CW Contest. That is when I discovered how well the antenna actually worked. I could hear and work stations in CU, EA, EA8, and throughout Zones 8, 9, and 11. Clearly the vertical was doing something right.

Prior to installing this antenna, 160 meters had mostly been a band where I picked up a few multipliers during contests. I would drop down to the band, work a few zones or countries, and move on. It was usually a struggle.

With the MA-160V things were different.

As December, January, and February progressed, I found myself spending more and more time on Topband. During that period we had roughly twenty good openings to Europe. I worked several new countries in Zones 14, 15, and 16, and I could hear many additional European stations that I simply could not quite work.

At the time my station consisted of the vertical antenna, a Yaesu FTDX-5000, and an MFJ-1026 noise canceller.

After experiencing the improvement the vertical provided, I began looking for ways to work those European stations that I could hear but not quite reach. Using antenna modeling software, I started exploring ways to improve transmit gain while still keeping the antennas reasonably hidden in the trees.

The solution was to add a second Cushcraft MA-160V vertical. Both antennas could be mounted on existing fence posts in the pasture and corral area in front of our house. At the same time, I decided to install a K9AY loop antenna to improve receive performance.

The first vertical had been tuned to resonance at 1830 kHz and provided less than a 2:1 VSWR from 1810 to 1860 kHz. Maintaining low VSWR was important because the antenna tuner in my Expert 2K-FA amplifier will not tune antennas with higher SWR. I wanted to cover as much of the band as possible without adding additional tuning networks.

My first attempt used both antennas tuned to 1830 kHz and fed through quarter-wave lengths of 75-ohm coax from a coaxial T-connector. Unfortunately this arrangement narrowed the usable bandwidth significantly. The 2:1 VSWR limits reduced the usable range to roughly 1820–1845 kHz.

Initially this puzzled me. Each antenna performed well individually, but together the bandwidth shrank. Eventually I realized the combination of the antennas plus the tuned 75-ohm phasing lines was narrowing the system bandwidth.

To improve coverage, I experimented with stagger-tuning the antennas. One antenna was tuned to 1830 kHz and the other to 1840 kHz. This increased the usable bandwidth to roughly 1815–1850 kHz.

Still wanting better coverage, I tried something different.

I removed the tuned 75-ohm phasing sections and fed both antennas with equal lengths of shorter 50-ohm coax. At the junction I installed a 2:1 transformer.

To my surprise, the usable 2:1 VSWR bandwidth expanded dramatically — from approximately 1810 kHz to 1865 kHz, easily covering the operating portion of the band.

This left me with an interesting question: why do so many phased vertical systems use quarter-wave 75-ohm phasing lines instead of a transformer approach? From a practical standpoint, using equal eighth-wave lengths of 50-ohm coax with a transformer requires less cable and appears to provide wider bandwidth.

Both verticals have been installed since July, although much of the time has been spent experimenting with phasing and switching hardware. Because of that, I have mostly been operating with only one antenna active.

The project is finally nearing completion. Once the phasing and switching hardware is permanently mounted, the full system will be ready for real operating tests.

And fortunately, the CQ Worldwide CW Contest is right around the corner — the perfect opportunity to see what these phased verticals can really do.