Sunday, April 12, 2009

Common-Mode Current Choke Disaster!

It is Spirng time and Spring means rain - lots of rain! Yesterday I wanted to use my SteppIR vertical antenna after a couple of months rest (I've been using my dipoles meanwhile). When I did, the SWR was very high on all bands regardless of the SteppIR tuning. It was 4.3:1 even when the element was completely retracted and the same thing with completely extended element - 40/80 meters tuning - a sure sign of trouble. No matter what I was doing the SWR was not changing and very strong signals were extremely attenuated - about 1/3 of what the dipole was receiving! MFJ-259B was showing the same thing - the impedance was changing a bit on different bands but the SWR stayed very high. Next thing to try was the DVM - aha!!! - DC resistance between ground and antenna was 6-7 kOhm and constantly changing up and down (ideally it should be just a few ohms of DC feedline resistance as the 80m coil has a built-in balun showing as 0 ohm for DC)! After going to the antenna site, everything looked normal until I started the troubleshooting by inspecting first the common-mode current choke/balun in the base of the antenna!
Here is what I have found upon removing the cover of the balun enclosure! Totally flooded - just missing a few toads inside! Not only that! Note the left antenna connector, between the two top screws of the flange connector, where the coaxial is soldered to the center pin receptacle of the female N connector - the pin receptacle, together with the white dielectric insulator (visible between the bolts) were (!) pulled out of the N connector housing and were almost entirely inside the enclosure! As a matter of fact - they were so far out of the connector's barrel - the center pin of the male antenna connector was not making contact - not even close! The solder connection of the coaxial shield was broken off too! I have no idea what happened there but looks like some serious force was in play - I can't do such a thing with my fingers and even with a tool will be a chore! One speculation is ice. In a solid block of ice this might be possible during freezing / melting cycles during the winter but none of  the ferrite toroids were broken! Another possibility is a miniature steam explosion. When I used the antenna last time, was with 1kW according to my logbook, water inside the tightly coupled male-female N connectors could have vaporized and the resulting steam could have pushed out the dielectric inside the box and ripped off the shield's solder joint- the middle insulator is the only part that can easily relief such pressure. Who knows... Bottom line - the whole balun is badly damaged and I'll be making Version 2 once the parts arrive!
The reason for this disaster is very simple - having too much faith in labels such as "Weather resistant electrical box" (aka the balun enclosure) from Home Depot. By "Weather resistant" they actually mean - "water resistant", forgetting that Sun exposure is part of the "Weather". The seal around the cover failed. It is a ring type gasket and either the gasket material changed over time or it was no good in first place. After removing the gasket, the seemingly dry gasket produced a lot of water just by squeezing it with my fingers. Seems to me that the gasket is made of some sort rubberized open-cell foam material (?!? huh?) and it was SOAKED with water. It is possible that the UV exposure broke down the rubberized coating and changed the properties of the material - the gasket no longer functioned as gasket but as a wick letting water to fill the box.
Looks like Carlon - the manufacturer of the so-called "Weather Resistant Electrical Box" needs to learn more about gaskets and materials or revise the description. (Needles to say - the cover was screwed very tightly so the only thing really to blame is the gasket!). There was also a very small discoloration and deformation of the cover (due to the sun exposure) but that was well within the tolerance of the gasket. I should have removed the original gasket when I built this choke and use some silicon sealant instead but back then I didn't even think they will sell electrical boxes with such poor choice of gasket material - now I know - they do! Lesson learned!
Construction notes for the original choke (Ver. 1.0) are on my antenna site. Before the damage, the choke worked just great! The new one, I am planing to build will be of open type design, with no enclosure.
If anyone decides to built this version - my recommendation is to have a small drain hole on the bottom or use a bead of black RTV sealant around the edge of the gasket, once the box is closed to protect the material from UV damage.

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