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How to fix rain attenuation with a bare 4G LOG periodic antenna
Most cellular LOG periodic antennae are enclosed in a plastic case to protect the elements from the weather, particularly rain.  Generally these and other plastic enclosed antennae do not suffer from rain fade, at least not with water droplets on the antennae. 

Longer LOG periodic antennae tend to be bare just like a LOG periodic antenna for TV reception.  The problem with enclosing a large antenna in plastic is that it catches wind, making them prone to strong gusts knocking them off alignment.  Bare antennae catch a lot less wind, but have the drawback of letting water droplets collect directly on the active components.

As the main bars are spaced close together near the tip of each antenna, rain droplets often wick between the two bars, effectively shorting out the antenna:


In the example above, this antenna usually gets a -100dB reading on both band 20 and band 3 with the distant Three mast it is aimed at.  With water trapped between the elements above, the antenna had no signal at the time and the other cross-polarised antenna with slightly less water between the elements had a -125dB reading on band 20 and no band 3 signal.

Preventing water wicking between the bars

So how did I solve this?  Well, I initially through of several ideas: Wink
  1. Cover each antenna with plastic - Technically this would solve the rain issue, but as we're exposed to strong wind, I was concerned about wind potentially warping the antenna or the elements.  Another problem is that the elements would potentially poke through the plastic, allowing water to run along them.
  2. Spray each antenna with rain repellent - I actually have no idea whether windscreen or fabric rain repellent would work on metal and was concerned about the chemicals affecting the antenna.
  3. Space the bars further apart - While again I was concerned about this affecting the performance, this is easily reversible by removing whatever object I squeeze between the bars.

The handiest object that came to mind is a plastic ballpoint pen.  These are generally rigid and the plastic wouldn't short the elements.  I always find myself unlucky for grabbing a pen that doesn't work and sure enough the first one I grabbed wouldn't write even after trying to scribble and shake it.  So it got repurposed. Big Grin 

I removed the ink tube from the pen and sawed the pen in half with a hacksaw.  I grabbed the ladder and shoved one half between the bars near the pole mount as it is wider here.  I then slid this piece about midway and repeated with the other half on the other antenna.  From checking the signal readings, they appear to be roughly the same as before (no more than 1dB variation), so the bar spacing is not as critical as I thought.

After the next spell of rain, which included misty rain that leaves heavy condensation on everything outdoors, the signal readings dropped by just 2dB to 3dB during the rain and recovered the moment the rain stopped.  The following shows one antenna with the cut pen used as a spacer and covered in rain droplets with no droplets bridging the bars:


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How to fix rain attenuation with a bare 4G LOG periodic antenna - by Seán - 25/05/2020, 05:45 PM

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