Topband: Water as a Dielectric in Capacitors

[EP Swynar]

Good Day All,

It's amazing how what may seem, at first, to be having a rather narrow focus of interest, can ultimately expand into something much larger...

A case in point: over the past couple of days, with temperatures above 32F after sunrise here, I noticed that the reflected power from my phased array suddenly showed visible discrepancies between certain directions whenever I'd "steer" it (I make it a habit to always transmit through my HB transmatch. Normally, the reflected indicator is  essentially flat, no matter which direction I might choose).

The rise in this reflected power was evident primarily when the east inverted "L" of the 3-element array was the "lead" (or "lagging") element, either by itself, or in conjunction with a second element (I can phase either one, or two elements, in order to be able to steer the configuration to any one of 8 different directions). It was also most pronounced when the temperatures were close to exceeding the freezing point.

The especially warm weather here to-day compelled me to go outside and have a look at all of the enclosures which encase each tuning capacitor at the base of each 170- "L": sure enough, a branch had fallen off a tree, striking the enclosure of the tuning capacitor for the eastern "L"...and to my horror, the blow from this branch had popped the lid off of the enclosure, which itself was half full of water, engulfing most of the air variable capacitor inside!

Needless-to-say, I quickly evacuated all of the water the best I could, and towel-dried what little that I saw remaining. I then poured some uncooked rice onto the inner base of the enclosure --- to help sop up any residual moisture --- and re-sealed the works. 

Some research on different dielectric constants for capacitors proved interesting: a vacuum displays "unity" (or one) dielectric constant, with air just a fraction more than this. Water on the other hand --- in its frozen, solid ice state --- has a dielectric constant of about 3.2. Liquid water, however, has a whopping dielectric constant of 80! In layman's terms (that is, MINE!), that means that if, say,100-pfd. of my air variable capacitor was under water, its effective capacitance would be 80 times that value, or, 8000-pfd.!

That would explain, I suppose, why the reflected power was considerably higher in warmer weather, than in sub-freezing temperatures: the effective capacitance of the air variable was much higher --- hence the reflected power --- when the water was in liquid state. However, I'm still trying to come to grips with why ANY such effect would have been far more pronounced ONLY when the affected element was the "lead" (or part of the pair) element, as opposed to simply being on the "tail end"...

~73~ Eddy VE3CUI - VE3XZ