The effects of moisture in co-axial cables.

The use of air spacing air spacing in co-axial cables is to drop the capacitance per unit length and thus produce a cable of better characteristics to perform its function of carrying power from point A to point B with minimum loss and phase distortion.

The dielectric constant of dry air is about one. The dielectric constant of water is around 80, but even worse the loss tangent is high enough to cause considerable problems. Loss tangent is the resistive component which dissipates power and causes heating and attenuation. The additional dielectric constant will change the velocity factor of the cable and spoil any tuned lines causing further losses.

the effective dielectric constant of the cable is

efective e = eair x (1-m) + ewater x m

where m is the percentage of water a function of the RH (relative Humidity) but not RH

Practical case example only: eair say 1 ewater say 80 let m be 3%

ecable = 1 x ( 1 - 0.03) + 80 x 0.03 = 0.97 + 2.4 = 3.37

Of course to suggest that moisture represent 3% is extravagant, but for illustration you can see that such a small amount of water will have a large effect upon the velocity factor of the cable.

trying again for 0.3% water

ecable = 1 x 0.997 + 80 x 0.003 = 0.997 + 0.24 =1.237

The dielectric constant for the cable has increased by 24% for an increase in moisture of only 0.3% (this is NOT RH)

This is a classic case of the tail wagging the dog, and emphasises the importance of keeping those lines dry.

A practical example.

Imagine for a moment that you have spent some time phasing up an antenna by cut and try. You have managed to get two components with nasty VSWR in just the right phase relationship at the frequency of operation to cancel out and give you a good match. Now water gets into your carefully cut length of matching cable. The velocity factor changes and just like a trombone slider producing a glissando electrically the cable is getting longer. The two mismatches start to move out of cancellation into addition there goes that good system. Of course the worst scenario is when the cable breaths near the balance point where results come and go along with that PA.

The effects of heat on the cable.

If there are substantial losses in the cable then this will cause extra heating. Unfortunately hot air can carry far more moisture than cold air. The tendency is then for a hot cable to have the internal pressure rise above atmosphere and some gas and moisture will egress. More gas than moisture because of the large difference in molecular weight. Now as the cable cools the internal pressure will fall and some external gas is sucked in along with more moisture in the atmosphere. The gas moisture mixture will sit there as the cable temperature cycles each time the cable heats the capacity of the internal air to carry water will increase and the moisture that has accumulated at the entry point will defuse along the length of the cable. The previously described cycle will repeat over and over and the net result is that the moisture in the cable will increase.

Credited to Gary G7SLL