It is
Spirng time and Spring means rain - lots of rain! Yesterday I decided to use my
SteppIR vertical antenna after a couple of months of rest (I've been using my dipole). 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! Same thing with completely extended element - 40/80 meters tuning. No matter what I was doing the
SWR was not changing and the signals were very attenuated - about 1/3 of what the dipole was receiving!
MFJ-259B was showing the same thing - the impedance was changing 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! After going to the antenna site, everything looked normal until I decided to start 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! Not only that! Note on the left antenna connector, between the two top bolts of the left N 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/ripped 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! One speculation is ice. In a solid block of ice this could be possible during freezing/melting of the ice but no 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 this 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. 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 while squeezing it with my fingers. Seems that the gasket is made of some sort rubberized open-cell foam material (?!? huh?) and it was SOAKED with water. Maybe the UV rays broke down the rubberized coating... anyway...the gasket was not functioning as gasket but as a wick letting watter 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. (Needles to say - the cover was screwed very tight so the only thing really to blame is the gasket!) I should have removed the original gasket when I put together this choke and use some silicon sealant instead but back then I didn't even think they will sell electrical boxes with bad 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 great! The new one I am planing will be open type design with no enclosure.
The second change is an electrical one - since there is no limit in space, now the choke has more ferrite rings and less coaxial cable turns (2.5 vs 3.5) through the binocular core. This improves the overall impedance! More ferrite rings - more inductance to make up for the lower number of turns. Less turns on the other hand means a decreased capacitive coupling between the turns on higher frequencies. My MFJ-259B is limited in measuring the complex impedance (Z) up to 650 Ohms. The choke was measuring >650 Ohms up to about 20 Mhz. The impedance is around 500 Ohms at 30 Mhz mainly due to the capacitive coupling between the turns. I have installed an additional plastic "jacket" over the coaxial in attempt to increase the space/dielectric between the turns and decrease the capacitive coupling but can't really tell if this has any effect! I am using the same LMR-240 coaxial (RG-8X type with solid center conductor). The choke should be fine for up to 1.5 kW but I am not planing to use it with more than 1 - 1.1 kW (ACOM 1000 amplifier). For steady 1.5 kW use and with antennas with higher SWR I'll recommend a high-temp cable such as RG-142 (silver/teflon).
The new choke takes a little more space but this is not a big deal. I am planing to put extra mechanical support (another stake) for the choke itself.
