Part 2 of..... Antenna solve one problem to only create another.

 

If you have not read Part 1 of this post, go HERE and then make your way back.

To recap, I was having an issue with my Hustler 4BTV vertical antenna, and I discovered that water was making its way into the 15m trap. Once I had the antenna reassembled and back in the upright position, I went into the house to check the SWR. I like to do this whenever I work on the antenna.

To my surprise, the minimum SWR was 2.4:1 on 15m, but10m, 20m, and 40m were all ok. I had taken my time putting the antenna back together, so I really wasn't sure what had happened.

My new SWR on 15M

I got my NanoVNA out and performed a sweep from below the 15m band through the entire band 15m band. The results showed that the antenna was resonant at around 20.540 MHz instead of in the 15m 

amateur band, so something was definitely wrong with the trap. I knew that on Hustler vertical antennas the outer trap tubing can be moved up or down to adjust the resonant frequency. I double-checked my trap adjustment marks, and they were exactly where they should have been.

I needed some time to think this one through, so for the time being I let the automatic antenna tuner (ATU) in my Icom 7610 took care of the high SWR.

The only thing I had changed on the antenna was adding silicone. That got me thinking about the old saying, "less is more." I had wanted to make absolutely sure the plastic cap on the 15m trap wouldn't allow water to get inside. I was convinced that I had applied too much silicone, and I decided to investigate the following day. My theory was that I had somehow changed the electrical characteristics of the 15m trap with the excess silicone.

When I took the trap apart the next day, my question was answered. I had been very generous with the silicone—so generous, in fact, that it had bridged the gap between the main antenna tube and the trap's adjustable outer tubing.

Issue found.

Silicone is not a conductor, but it also does not have the same dielectric properties as air. By filling that air gap, the silicone increased the capacitance of the trap and shifted its resonant frequency.

I disassembled the trap once again, carefully cleaned all of the excess silicone from inside the tubing, and this time remembered that less is more when applying silicone.

Once the antenna was reassembled and back upright, I checked it again. This time the SWR was exactly where it should be.

NOTE: New 15m sweep posted as other pic was from 10m. 

Lesson learned the hard way—but I'm glad I was able to figure it out.