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MA-160V Phased Verticals

Ok, where do I start?

 

My need for a new 160m antenna began when I replaced my Cushcraft X-9/X-940 with a SteppIR DB36. Prior to the DB36 I used a quarter-wave sloper on 160m fed at the 40’ level on my 54’ tower. With the DB36 on the tower, the quarter-wave sloper no longer worked. In fact, the presence of the sloper changes the tuning of the DB36.

 

For several years, I wanted to put up a 60 ft vertical made of 3” aluminum irrigation pipe. The ideal location would have been in the ¼ acre pasture in front of our house. Unfortunately, I knew that would be an aesthetically non-starter, so I started looking for another solution. At that time, Bob Selbrede, K6ZZ, posted a for-sale ad for a Cushcraft MA-160V vertical on the SCCC reflector. I immediately bought it, and then set out to figure out where to mount it. After assembling the antenna, I realized I could mount it on a corral fence post. The antenna in that location would not be very objectionable to my better half. It was barely visible since in was hidden from the road in front of the house by a tree, and from the house it blended in with the trees.

 

Ok, so I now had a 160m antenna that had a VSWR of less than 2:1 from 1810-1860 kHz. The first contest after the installation was the 2017 CQ Worldwide CW. That was when I discovered I could hear and work CU, EA, EA8, and all over Zone 8, 9, and 11. The vertical was really working.

 

Prior to using this vertical, 160m was just a band to pick up mults during contests. I would get on 160m to work a few zones and countries, but it was usually somewhat of a struggle. For some reason, this MA-160V was really working. As my 160m experience progressed through December, January, and February, I was beginning to really enjoy getting on 160m. We had about 20 good openings to Europe during that period, and I was able to pick up several new countries in zones 14, 15, and 16. I was able to hear several European stations I wasn’t able to work. The equipment I was using to chase this DX was the Cushcraft MA-160V vertical, Yaesu FTDX-5000, SPE 2K-FA, and a MFJ-1026.

 

After the experience of working the European stations with relative ease compared to my previous 160m experiences, I decided to look for ways to work those station I could hear, but not work. Using antenna models I tried to come up with a way to improve my TX gain, while keeping the antenna(s) fairly well hidden in the trees. What I came up with was to add a second Cushcraft MA-160V. The antennas could be mounted on two of the fence posts on the pasture and corral in front of our house. I also decided to add a K9AY dual loop antenna to help with receive.

 

The first vertical I had installed had been resonant at 1830 kHz, and provided less than 2:1 VSWR from 1810 to 1860 kHz. The low VSWR was important, because the antenna tuner in my Expert 2K-FA amplifier will not tune an antenna with higher VSWR. I wanted to cover a little more of the band without adding any other method of antenna tuning. My first attempt had both antennas resonant at 1830, and I tried using the quarter-wave lengths of 75 ohm cable feed them from a coaxial T. This method did not work very well for me because the 2:1 VSWR points limited my band to 1820 to 1845 kHz.

 

I was really puzzled by the outcome of these first tests of the phased verticals. I tested each antenna alone, and the results were very similar to the original single antenna. What I found out was the combination of the two antennas narrowed the 2:1 VSWR bandwidth. The tuned 75 ohm feedlines also contributed to narrowing the bandwidth. What I did initially to improve the bandwidth tolerated by my amplifier, was to tune one antenna for 1830 kHz and the other for 1840 kHz. That gave me good coverage from 1815 kHz to 1850 kHz. The staggered tuning gave me slightly more usable bandwidth.

 

My next task was to try to understand what else I could do to cover more of 160m. Out of desperation, I eliminated the tuned 75 ohm coax sections and fed the antennas with equal shorter lengths of 50 ohm coax. I made a transformer using three stacked Amidon FT-240-61 cores, and 14 and 10 turns to give me a 2:1 transformer I used the transformer at the junction of the two 50 Ohm feeds, and found the usable 2:1 VSWR bandwidth was now 1810 kHz to 1865 kHz. Okay, I found a solution, but why is everyone using 75 ohm coax to phase their antennas instead of a transformer? I still do not know the answer to that question. Instead of two quarter-wave lengths of 75 ohm coax, it seems easier to run two eighth-wave lengths of 50 ohm coax. It takes less cable, and you get a wider usable bandwidth.

 

In addition to the equipment described, I am also using RF chokes at each antenna and at the input to the phase switching box. These chokes are homemade from two stacked Amidon FB-31-24001 cores and RG-400 coaxial cable.

 

I have had the two vertical antennas up in the air since July, but because of the experimentation and packaging of the necessary phasing/switching hardware, I have only been using one of the antennas. I am nearing the end of the project, so now I only have to mount the hardware, and have cables pulled through my attic.

 

I did not meet my deadline of having the phased verticals ready for the CQ Worldwide CW contest. Oh well, hopefully, we will have several good openings on 160m after the phased vertical project is completed. I did meet the my deadline for installing the K9AY loops.

 

I’m looking forward to really testing the antennas this 160m DX season.

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© William Shell