Let's face it: Not many of us can erect a full-sized directive array for 160 meters, if for no other reason than the lack of a sufficiently tall support system. Nonetheless, in advance of the 2006 Peter I DXpedition, Bob Allphin, K4UEE, asked me to help design a 160 meter beam for 3YØX. He explained that Ralph, KØIR, had used a 3-element 160 meter beam lying on the surface during the 1994 DXpedition to Peter I, and he wanted to employ the same concept. Several weeks before the team had regrouped and was set to leave for the 2006 3YØ DXpedition I started looking into whether a 160-meter beam was feasible.
"The 3YØX Peter I Island 160 Meter Beam" in March 2009 QST recounts the tale of how this "beam without a tower" came to be and how it contributed to the DXpedition team's Top Band success from Peter I. This companion article will focus on concept and design considerations for such an antenna.
Antenna on Snow and Ice
To assure a successful design, the first thing we needed to know was the approximate conductivity and relative dielectric constant of the ice and snow in the glacier on Peter I. Our inability to find definitive data on polar ice cap electrical parameters dictated making a great deal of assumptions to come up with reasonable estimates.
I got in touch with my old friend, Pete Gaddie, W6XX, who had spent his career working for Stanford Research Institute (SRI) in the field of propagation. Pete had prior experience at measuring ground conductivity and dielectric constant. He did a search of the literature and other sources in an effort to determine the likely properties of the Peter I ice cap. He came up with figures that seemed to match the values I had determined would be needed if the antenna was to work.
While Pete was checking into the values for ice and snow, I had been modeling a 3-element antenna in free space and started modeling it above ground using EZNEC. For the antenna to be useful, it appeared that the relative dielectric constant could not exceed 2 and the ground conductivity could not exceed 100 uS/m (that's microsiemens, as opposed to the millisiemens we're used to seeing in the conductivity tables).
In checking his sources, Pete further determined that there was a difference between Arctic and Antarctic ice - as well as for most of the ice between. It seemed, however, that the values for conductivity ranged in the low uS/m range - as low as 10 uS/m - while the value for the relative dielectric constant was around 1.5 to 3.2. So, it looked like we might be in business. With the relative dielectric constant and conductivity at these values the surface begins looking like an insulator.
It's said that in polar regions covered by an ice cap and snow, wire antenna elements laid directly on the surface will behave as if they were in free space. In fact, while the antenna may indeed work, it's unlikely to perform as if in free space. Its tuning and elevation pattern also will differ from its free space value due to the snow and ice cap beneath.
Polar icecaps over land typically consist of a top layer of snow that increases in density with age and depth, gradually transitioning into ice. The surface may vary from lightly to densely packed snow. In areas where it rains, there may be alternating layers of snow and ice. During warm or rainy periods the surface may even be slush.
The relative dielectric constant of lightly packed snow is about 1.2, and it increases with density. The relative dielectric constant of ice is about 3.2, while fresh water is about 78. The conductivity of pure snow and ice is very low, but if contaminated by salt spray or other pollutants the conductivity value may increase significantly.
A 160 or 80 meter Yagi built with small wire elements is inherently a narrowband device. For good performance, elements must be accurately tuned. The gain and pattern of antennas such as V beams, rhombics or extended double Zepps would be less affected by such detuning effects. Such antennas may be designed within certain limitations by using the NEC2 or NEC4 modeling engines. A basic limitation of these modeling programs is their assumption that the antenna will be installed in or over a homogeneous medium. That's obviously not the case here, as the relative dielectric constant almost certainly will vary with depth. Therefore, we'll estimate the medium's composite parameters.
For the complete version of this article as published in the NCJ, view the pdf version.