Seasonal Antenna Effects

Weather in our climate goes from one extreme to another on an annual cycle. The average daily temperature differs by more than 30° C between January and July, and the maximum range is 75° C. In my original home in VE4 these figures were 40° C and 85° C, respectively. When my parents immigrated to Canada and moved to Winnipeg they often laughingly quipped that they'd been exiled to Sibir (Siberia). Siberia (UA0) has an even greater temperature range than north central North America.

The temperature changes and freeze-thaw cycles are hard on people, machines and modern infrastructure. Our antennas are also affected. Beyond the mechanical effects there are the electrical changes that are evident when we turn on our radios. With my many towers and antennas I get to witness these changes firsthand. They are most evident in late fall and early spring when the seasonal shift accelerates.

In this first article of the new year I'll talk about some of these as a light-heartened start to blogging in 2021.

Verticals

The effect of radials on vertical behaviour is strongly dependent on the velocity factor of the ground beneath. This is most true of surface and buried radials, and elevated radials to a lesser degree. When there are too few radials to be an effective non-resonant ground plane the seasons will change the vertical's resonance. 

For a multi-element arrays, and especially for a yagi such as mine, those effects are very noticable. Single verticals are less affected, usually not beyond a small shift in resonance frequency and impedance. It is measurable.

Dry ground, wet ground and frozen ground have different velocity factors. The variability depends on wavelength, soil type and frost depth. My 160 meter shunt fed tower showed such a change recently. I set it to resonance at 1840 kHz when the ground was dry and not frozen. Then the cold came and the ground froze to several inches depth. The resonant frequency rose to 1855 kHz.

A week later the weather warmed and we had 50 mm of rain in one day. The ground saturated and thawed unevenly. The resonant frequency returned to 1840 kHz. The cold returned and the antenna resonance again rose to over 1850 kHz. I'll have to adjust that since the antenna bandwidth is only 75 kHz and that makes the SWR at the low end of the band higher than I'd like.

The 80 meter vertical yagi is also affected. There are 2× and 4× times the radials on the parasitic and driven elements, respectively, compared to the 160 meter antenna. The antenna is broadband enough that it is only slightly effected by the freeze-thaw cycle. However the shift in parasitic resonance moves the frequencies for optimum F/B. The bandwidth for F/B is narrower than that for gain and impedance match. Although not a serious problem it is noticable. In contrast, the pattern of a 4-square is less affected by these changes due to wider element spacing and current forcing.

Although Beverage antennas rely on the velocity factor of the ground beneath I have not noticed any changes with the seasons. I suspect there are a couple of reasons for this. First, low frequencies penetrate further into the ground and so are less sensitive to soil changes close to the surface. Second, the antenna is non-resonant. Any pattern shift will be small and the impedance shift barely noticed. I'm tempted to experiment with time spaced measurements to see how visible the change might be.

Rope

Tension of antenna support ropes does not appreciably change with temperature. The tension will change when ropes become water saturated, and the amount depends on the material and weave. Most of the dacron rope I've used is little affected. Nylon weave is affected more. From reports I've heard, natural fibres are affected a lot more.

When antenna wires sag due to these weather effects their performance is affected. For example, the impedance of inverted vees falls and the resonant frequency increases. For a typical single element antenna the change is rarely a concern. It is a problem for my 80 meter vertical yagi.

As the parastic element support ropes become dry or saturated I find it beneficial to spend a few minutes to adjust the tension of the ropes. When I have an otherwise unexplained degradation of F/B the rope tension is often to blame. Snugging the ropes restores the pattern. A yagi is very sensitive to the resonant frequency (reactance) of the parasitic elements. You should expect the same from horizontal wire yagis.

Last year I purchased a quantity of dacron rope. When I upgrade the 80 meter yagi, probably this year, I will replace the remaining sections of nylon rope. Hopefully that will reduce the periodical need to adjust rope tension. Another solution, where convenient, is to use a weight and pulley to automatically adjust rope tension as the rope reacts to the weather.

Rain, snow and ice

Any dielectric coating on a conductor alters its velocity factor and therefore its resonant frequency. Wire antennas are typically affected more by precipitation induced changes than antennas with tubular elements. However, I have noticed larger changes on tubular elements when water gets into, for example, gamma matches. Although there is little risk of damage to the gamma capacitor, keeping the water out with a weather seal is beneficial.

Heavy ice cover on antennas has a larger effect. That is rarely a preeminent worry while we're worrying whether the antenna will survive! Just be aware of the various precipitation effects and you'll be less surprised and, hopefully, worry less.

Transmission lines

Open wire line is as susceptible to dielectric effects as wire antennas. A coating of precipitation will alter the line's impedance and therefore the match. Ice on open wire line can be destructive because most of the span is susceptible to wind and ice. Coax is typically bonded to a structure or lying on a surface and therefore the risk is lower.

With one reversible Beverage antenna made from open wire line I am on the watch for these problems. My main concern is with the latter: destructive ice loading. Change in velocity factor appears to a largely negligible effect.

Those who use open wire transmission lines need to be aware of these effects. Tuners may need to be tweaked when it rains. Line supports and wire tensile strength must be sufficient to withstand ice loads. Coax cable is affected by ice loading but its velocity factor is unaffected by weather.

One exception is that the nominal impedance of coax does change with temperature. At a lower temperature the distance between the inner and outer conductors increases a small amount. The nominal impedance increases. The effect is rarely if ever measurable, especially in light of the greater weather effects on antenna impedance. But it is there.

Direct buried coaxial cable should be placed below the frost line (depth to which the ground freezes). Water expands when it freezes and increases the pressure on the encased cable. Hardline can withstand the pressure in most cases and I, like many others, have had no problems with buried Heliax. However, if there is a cut in the cable the soil pressure can push melt water into the break. The result is more rapid corrosion of the outer conductor. 

If the outer conductor is breached water will be driven inside the cable and that can cause serious damage. Heliax and other cables with closed cell foam dielectric will resist contamination longer than others. It's a better choice for direct burial to protect against freeze-thaw cycles.

Tower guys

The tension of guys is certainly affected by the wind. It decreases on the lee side and increases on the windward side. Temperature also has an effect. For example, let's say that in mid-summer you adjust the pre-load tension of guys to 1000 lb (450 kg) -- typical for 5/16" EHS. The tension will not be the same in depths of a cold winter.

Materials expand when they are warmed and contract when they are cooled. For a guy this is most evident along their length. Guys are shorter in cold weather than in warm weather. The tension therefore increases. Had the guys been adjusted in winter the pre-load tension would be lower in summer.

For optimum safety and performance the pre-load tension should be 10% of breaking strength. Add a little tension when you adjust the guys in cold weather and it will remain high enough when the weather warms and the tension falls. That said, as long as you're close it isn't critical to strive for perfection. Most tensiometers used by hams are not very accurate anyway.

Animals

Animals change their behaviour in concert with the seasons. Food becomes scarce for herbivores in winter when nothing grows. The abundant deer scour the bushes for edible tidbits such as shrivelled and frozen berries. They paw the ground for edible plants and fallen apples. People in this rural area call these wild fruits deer apples.

The problem is that they are likely to taste everything in reach when they are hungry. There is little that they pass over. While I was taking the pictures for this article I checked on the 80 meter vertical yagi since it showed a deterioration of its directivity in the northeast and southwest directions. To my great alarm the deer had done their worst.

There was no question of the culprit's identity. The deer tracks in the snow were clear, as was their wandering path to the damaged areas. This has happened once before, but in fall when there were no tracks to be seen, which left me wondering whether I did it while mowing with the lawn tractor.

The deer chewed right through a nylon support rope for one of the parasitic wire elements. I retied the rope using its extra length to connect the severed ends. The repair is seen in the picture above. Luckily the improved driven element stinger withstood the tension imbalance.

For the second time the deer chewed and severed the Cat5 cable to the northeast element. The lack of control voltage kept that element offline and explained (along with the wire's collapse!) the directivity problem. The severed ends had to be spliced. An otherwise easy job is unpleasant in an open field with the cold north wind blowing. The repair had to be done before the NAQP this weekend.

The burial rated Cat5 cable is flooded with gel which, supposedly, deters curious critters in addition to keeping water out. So much for that theory! Curiously none of my on-ground cables have ever been molested by animals, not even sharp toothed rodents.

Now that the antenna is repaired I have to consider protection around vulnerable antenna sites. I made the rounds of other antennas and towers to look for more tracks and tooth marks. It's bound to happen again.

2021 begins

I have been busy since the start of the new year. There are a number of projects that will eventually make their way into the blog. With everything I have going on it will be a challenge to write articles for the next several week. But I may surprise myself.

Happy New Year.