I’ve been very active on 6m over the past several years. I am closing in on DXCC and Worked All States on the magic band. I operate on 6m daily during Es season. We are also very active in VHF contesting on the 6m band and have worked just under 700 grids on 6m. This post is about our plans to develop an enhanced 6m antenna system for contesting and DX’ing.
6m LFA Portable Antenna System
We developed an updated 6m antenna system for Field Day and portable use a few years back. The portable setup is based upon a 3-element Loop Fed Array (LFA) antenna. I was impressed with the improvement in the ability to hear weak stations above the noise floor compared to our previous 3-element conventional yagi antenna. Subsequent conversations with Joel Harrison, W5ZN suggested that fixed direction stacks of 3-element antennas would make a very good setup for 6m contesting and grid chasing. This led to our plans for some significant 6m antenna upgrades at our station.
6m Antenna Plans
Our planned 6m antenna upgrade consists of the following elements:
- Install two fixed direction 4-stacks of 3-element LFA Yagi antennas on our 100 ft tower facing south and east
- Install one fixed direction 3-stack of 3-element LFA Yagi antennas facing west
- Install a new 7-element LFA Yagi on the mast at the top of the same tower
- Build a high-power capable, Low-Noise preAmp (LNA) system to support all of these antennas
- Install a remote antenna switch to select between the 4-stacks and the 7-element antenna
- Build and install a second high-power LNA preAmp system for use with our existing 7-element 6m antenna on our house bracketed tower
All of these antennas will use 7/8″ hardline coax cables for the main segments of their feedline system.
Advanced Receiver Research RF Switched LNA
I am in the process of building two high-power capable LNA systems for our 6m antennas. These systems will be based upon low noise factor (0.55 dB) GaAsFET RF switched preamps from Advanced Receiver Research. These LNAs should improve the overall noise-factor performance of the 6m receivers in our station by a noticeable amount. We choose the RF switched version of these preamps so that we could disable the preamps and maintain a direct receive path through the LNAs to our antennas. This is desirable for SWR testing and for situations where very strong signals may cause overloading. It also ensures that we can continue to use our antenna should we experience an LNA failure.
I plan to use the shared LNA sequencing capability of our microHam system to control the two LNA systems. All of the antennas for this project will come from InnoVAntennas. The 3-element LFA antennas will be custom-made for fixed direction rear mounting on our tower.
Why LFA Antennas?
The use of a loop-driven element has several advantages, which include:
- Better suppression of side lobes in the antenna pattern results in an antenna that hears better (lower noise temperature)
- The potential for an efficient direct feed design that does not require driven element matching
- Wider useful bandwidth
- Slightly high gain
The 7-element Yagi that I chose takes this one step further by employing a bent reflector to further improve the ability to suppress side and rear lobes in the antenna’s pattern and improve the antenna’s noise temperature.
6m Antenna Stack Designs
I performed several High-Frequency Terrain Analysis (HFTA) runs to determine the heights and directions for our 4-Stack Antenna Arrays.
4-Stack Facing the EU – Gain vs. Arrival Angles
The example above shows the projected performance of the 4-stack facing Europe. The 3-element LFA Yagi that we are using has a 3 dB azimuthal beamwidth of about 60 degrees. This gives each stack an effect range of azimuth angles approximately the same as the 3 dB beamwidth. The headings that I choose for the stacks are as follows:
- Europe facing 4-stack – 50 degrees
- Central/South America and the Caribbean facing 4-stack – 180 degrees
- The United States facing 3-stack – 260 degrees
I looked at both a 3 Yagi and a 4 Yagi configuration for the U.S. facing stack on the top half of our tower. It turned out that the 3 Yagi design did a better overall job of covering the range of arrival angles that we can expect. This situation is due to a combination of the high elevation of the stack above ground and the wide range of potential arrival angles encountered when working stations across the U.S.
The combination of the new and existing 7-element 6m rotatable Yagis that I am planning or already have installed should cover the remaining directions nicely.
Gain vs. Arrival Angles Towards Oceania – 7-Element Yagi and 3-Stack
The HFTA analysis illustrates the performance of the combination of the west-facing 3-stack and the new 7-element LFA Yagi towards Oceania (ex., Australia and New Zealand). The minimum gain achieved by switching between these two antenna systems is never less than 10 dBi. This part of the analysis also suggests good performance towards Hawaii.
7-Element LFA Yagi Gain vs. Arrival Angles for Japan and Asia
Finally, I looked at the projected performance of the 7-element LFA Yagi towards Japan and Asia. The height above ground for this antenna results in both good performance at low arrival angles and a good bit of gain variation across arrival angles. The low noise performance of this antenna combined with our planned use of high-performance LNAs in the receive path should provide some opportunities to work stations in Japan and Asia.
I also built a combined EZNEC model to look at possible interactions between these and other antennas on our tower. This analysis indicated that we should be fine if we remove the 6m passives from our SteppIR DB36 antennas. The combination of the stacks and the new 7-element LFA Yagi we are planning will replace the 6m capabilities that our SteppIR antennas have been providing.
The antennas will arrive in the next few weeks, and work is underway to build the high-power LFA housings. I will be posting additional articles about this project as we go.
Fred, AB1OC
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