[Antennas] Effect of surfactants in water on ladder line, also ice and snow dielectric properties

2002年12月17日 18:29:17 -0500

Dan Zimmerman, N7BHB, wrote:
> Interesting thread. The wetting agents and even tap water 
> contains impurities which makes it a much better conductor 
> than rain water which is distilled by nature. Not sure just 
> how much difference it would make but might be an 
> interesting test.

As I wrote earlier, the effect of dissolved ionic conductors 
in water has a reciprocal frequency relationship. Liquids 
that are quite conductive at power line frequencies are far 
less so at radio frequencies. The exact relationship depends 
on the size of the ion and its mobility in the solution. 
Wetting agents (soap, surfactant) have long organic ends 
attached to a small ionic end. The ionic end is attracted to 
water (hydrophilic) and the organic end is attracted to oils, 
greases, and the waxy polyethylene (hydrophobic). They tend 
to be large molecules with low mobility so they should not 
have much of an effect themselves. However, they do allow the 
water to thoroughly wet the ladder line and this does have a 
big effect. Other ingredients in detergents (like trisodium 
phosphate) do have more of an ionic effect, but I doubt they 
are important here. The difference between tap and distilled 
water is negligible at higher radio frequencies.
> Distilled water (snow) is a very good insulator. I have 
> personally seen 7KV power lines laying on about 5 inches of 
> snow and not tripping any of the protective devices or 
> melting into the snow. There were several dead weeds
> protruding up through the snow next to the wire that were 
> pretty well burned, but the lights were still on.

Snow is ice. Its purity is often less than rain as the high 
surface area provides lots of adsorption to atmospheric 
pollutants.
But ice has very different dielectric properties than liquid 
water anyway. In ice, the mobility of ions is virtually nil, 
so you see little conductivity. Likewise the polar water 
molecules cannot rotate in the alternating electric field so 
the dielectric losses are far less.
[Electrolytic capacitors use one aluminum foil as the anode 
which is covered by a thin coating of aluminum oxide which is 
the insulating dielectric. The other electrode is the 
conductive solution in contact with the anode film. The 
second foil serves only as an electrical contact to this 
solution. At low temperatures, the solution ceases to be an 
effective conductor and as the internal resistance builds up, 
the capacitor ceases to be effective.]
I presented the dielectric properties of water the other day. 
The table below shows the properties for ice (from distilled 
water, 10 F), freshly fallen snow (-4 F), and hard-packed snow 
followed by a light rain (21 F). The data is from Von 
Hipple's "Dielectric Materials and Applications." Temperature 
has a large effect, as the lower the temperature, the more 
rigid the atomic structure becomes.
Frequency Ice, 10F Fresh Snow, -4F Hard Wet Snow, 21F
 e/e0 tan d e/e0 tan d e/e0 tan d
--------------------------------------------------------------
 1 kHz 3.33 0.492
 10 kHz 1.82 0.342
100 kHz 4.8 0.8 1.24 0.140 1.9 1.530
 1 MHz 4.15 0.12 1.20 0.0215 1.55 0.290
 10 MHz 3.7 0.018 1.20 0.0040 
300 MHz 1.20 0.0012
 3 GHz 3.2 0.0009 1.20 0.0003 1.5 0.0009
 10 GHz 3.17 0.0007 1.26* 0.0042*
* Measured at 21 F.
Print the table in a fixed width font to read properly.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]