Yesterday, a heavy rainstorm caused some high SWR readings to appear on my station’s power meter. I thought things would return to normal after the storm passed, which they eventually did. In the meantime, tweaking the tuner helped keep the radio happy. But I was still curious about what really caused the problem.
Rain, snow, and other forms of precipitation affect antenna systems and propagation. We should be aware of what happens and what—if anything—can be done about it.
Raindrops Keep Falling on My Dipole
Is rainwater a dielectric (insulator) or conductor? It could be either one.
When it rains, there will be some water clinging to the antenna. Pure water is actually a poor conductor of electricity, and more like an insulator. But when rainwater comes into contact with air pollution or dissolved substances, such as minerals or pollutants, it can become a conductor.
Rain can shift the frequency as a result of an increase in dielectric loading of the antenna. So when it rains, the antenna becomes electrically longer. This explains why the resonant frequency and SWR frequency of a wet antenna typically drops.Because RF flows over the surface of a conductor and not through it, anything that affects that surface will alter how electrically long the radiator appears to RF.
Most antennas don’t show huge changes when it rains. The effects are small enough that you may not notice—or feel they’re not significant. You will most likely find that the SWR went up on higher frequencies and down on lower ones. This is true whether it’s a basic dipole or a multi-element Yagi.
Icing is one of the most serious problems for antenna installations. Icing not only increases antenna wind load but can also cause more significant changes in antenna resonance. The ice buildup affects the dielectric constant of insulated wire (now layers of plastic/ice/air), changing the impedance and bumping up the SWR. Usually we’re more concerned with the antenna staying up in one piece rather than making attempts to knock the ice off. I saw a post on a message board detailing one ham’s solution: filling a Super Soaker with washer fluid and spraying the antenna. At least he stayed at ground level and avoided using a ladder.
Also, realize that near field objects like metal roofs, aluminum siding, and deciduous trees covered by precipitation can add to the problems. These items are already there and coexist with your antenna—for better or worse.
Ground
I used to think hams were joking when they told me their antenna SWRs changed during periods of heavy rain, but now I’m not so skeptical. There’s another piece to the puzzle. An antenna originally tuned over dry ground could also be detuned by rain, which increases the moisture content of the soil.
We know that ground conductivity can affect the performance of your antenna system. If the soil is moist‚ it has more conductivity than dry‚ hard‚ or sandy soil. Increased soil conductivity can cause a small change in resonant frequency, raising or lowering the SWR somewhat. But it often requires really soggy soil to make a significant change.
Michael, KB9VBR, performed some testing on ground covered with snow in a YouTube video. The premise was that the snow will cause the tuning to go long, and that appeared to be the case. Comparing SWR scans ofdry and snow-covered ground, the curve shifted a bit up in frequency and the SWR increased a little. But the SWR increase was slight and still less than 1.5:1. He also mentioned the possibility of improved ground wave propagation with snow blanketing the ground.
Other Scenarios
Up to this point, we’ve just been scratching the surface, so to speak. If rain is changing your antenna’s resonant point significantly, I’m willing to bet water seeping into the coax or antenna system is the problem. Water leaking into feedlines significantly alters the impedance of the line, causing all kinds of problems.
First, do a physical inspection, preferably after a hard rain. Run your hands along the length of cable, looking for any breaks in the outer jacket that could let water in—take your time. Then disconnect it at the antenna, take a tissue, and wipe down the connector. If it’s wet, you’ll know the connector wasn’t properly waterproofed where it meets the antenna.
Don’t bother attempting to dry it out because water has probably begun to oxidize the braid, center conductor, and connectors. Even small amounts of water ingress can quickly degrade feedline performance. Cut the cable back a few feet until it feels dry—or until the shield shows no corrosion. Applying a new connector is a simple and inexpensive solution.
If you use coaxial cable, it’s essential that the top end is sealed and that you form a drip loop to help shed water. Many commercial antennas with a coaxial cable running from outside to inside have heat shrink tubing around any external joints (antenna to base, or base to coax) for this reason. Another solution is to use Scotch Temflex tape. For both these methods, following up with a layer of Scotch 33+ electrical tape helps prevent UV exposure from breaking things down. Electrical tape alone is not sufficient.
Falling Into the Trap
Verticals like the Hustler BTV series and Yagis such as the Cushcraft A3S use traps to reduce antenna size and add more bands. They also add another location where water can seep in. Water collecting in the traps will change the inductance and capacitance values and detune your antenna.
Start by inspecting the antenna right after it rains and when it’s acting up. You’ll probably find that water is pooling inside one or more of the traps. Drain and dry the traps, and make sure any drain holes are not blocked and pointed downward. Inspect trap caps/seals for cracks or damage and wear—replace if needed. Tighten trap clamps and add Permatex Ultra Black as needed to seal gaps.
The Ends
Ends of wire antennas are high voltage points. Insulators are needed to isolate the voltage from coming into contact with other objects. Insulators can also prevent antennas from being detuned if the element comes into electrical contact with wet ropes, etc. Antenna insulators are typically made from plastic, ceramic, porcelain, or glass. Ceramic is a good choice for durability and voltage handling.
Center insulators isolate the two elements in a common dipole, act as a feed point connection, and may also be part of a balun/unun. These also need close inspection for cracks, broken or corroded connections, balun eye-bolt damage, or water inside the balun enclosure. Repair, drain, or replace as necessary and weatherproof any electrical connections.
Weather or Not
As the old saying goes, climate is what we expect and weather is what we get. The best strategy is to practice preventive measures and deal promptly with conditions as they happen.
No comments:
Post a Comment